# Step recovery diodes and SPICE

D

#### Darwin

Jan 1, 1970
0
Hello,
I am trying to build a fast pulse generator for an UWB transmitter.
My goal is to obtain a few V peak pulses in a 50Ohm load with a
10%-90% rise time < 100ps. The repetition rate should be at least a
few tenths of MHz. I have seen that step recovery diodes can be very
useful for this application and I read the interesting HP application
with SPICE.
Among all producers, I found Aeroflex/Metelics which provides SPICE
models for their diodes:

http://www.metelics.com/spiceStepRecovery.htm

This sound very interesting, but SPICE does not include the step
recovery diode's transition time.
I am trying to run a few simulations now but I would like to know how
to expect from SPICE results. SPICE calculates a transient response,
but how should I expect this would be degraded with a certain recovery
time?

Part of my SPICE file follows; since I plan to use a SOT-23 package, I
included a 1.5nH series inductance and a 0.2pF shunt capacitance to
the diode to have a rough idea of the parasitics' influence.

Thanks to everybody

* A simple pulse sharpener

..TRAN 1ps 20ns

VPLS 1 0 PULSE(0 10 5ns 1ns 1ns 20ns 50ns)
Rg 1 2 50Ohm
C1 2 3 1nF
Lp 3 5 1.5nH
Cp 0 5 0.2pF
D1 0 5 D_840
C2 3 4 1pF
Rl 4 0 50Ohm
If 3 0 10mA

D

#### Darwin

Jan 1, 1970
0
Part of my SPICE file follows

.... I forgot the model... sorry!

* A simple pulse sharpener

..TRAN 1ps 20ns

VPLS 1 0 PULSE(0 10 5ns 1ns 1ns 20ns 50ns)
Rg 1 2 50Ohm
C1 2 3 1nF
Lp 3 5 1.5nH
Cp 0 5 0.2pF
D1 0 5 D_840
C2 3 4 1pF
Rl 4 0 50Ohm
If 3 0 10mA

..MODEL D_840 D (IS=500.0E-15 N=1.3 RS=0.22 CJ0=0.545p VJ=0.5 M=0.235
XTI=3.0 EG=1.12 BV=60 IBV=10E-6 TT=10E-9)

..END

J

#### John Larkin

Jan 1, 1970
0
Hello,
I am trying to build a fast pulse generator for an UWB transmitter.
My goal is to obtain a few V peak pulses in a 50Ohm load with a
10%-90% rise time < 100ps. The repetition rate should be at least a
few tenths of MHz. I have seen that step recovery diodes can be very
useful for this application and I read the interesting HP application
with SPICE.
Among all producers, I found Aeroflex/Metelics which provides SPICE
models for their diodes:

http://www.metelics.com/spiceStepRecovery.htm

This sound very interesting, but SPICE does not include the step
recovery diode's transition time.
I am trying to run a few simulations now but I would like to know how
to expect from SPICE results. SPICE calculates a transient response,
but how should I expect this would be degraded with a certain recovery
time?

Part of my SPICE file follows; since I plan to use a SOT-23 package, I
included a 1.5nH series inductance and a 0.2pF shunt capacitance to
the diode to have a rough idea of the parasitics' influence.

Thanks to everybody

* A simple pulse sharpener

.TRAN 1ps 20ns

VPLS 1 0 PULSE(0 10 5ns 1ns 1ns 20ns 50ns)
Rg 1 2 50Ohm
C1 2 3 1nF
Lp 3 5 1.5nH
Cp 0 5 0.2pF
D1 0 5 D_840
C2 3 4 1pF
Rl 4 0 50Ohm
If 3 0 10mA

I'd doubt that spice modeling will be worth the effort. You'd do
better to experiment and measure. Getting below 100 ps Tr is hard,
because package and pcb parasitics start to really matter, and because
the faster srd's are hard to drive.

Faster srd's don't store much charge, so the problem is to reverse
bias them fast enough, to get enough reverse current through them,
before they snap. The driver becomes non-trivial, and it's sometimes
better to buy a "slower" diode just because you can slam so much more
reverse current into it.

The other way to get edges like this are to start with a fast ecl or
cml edge - 40 ps is easy these days - and run it through a cheap 5 or
10 GHz mmic.

John

J

Jan 1, 1970
0
Say John,

John Larkin said:
I'd doubt that spice modeling will be worth the effort. You'd do
better to experiment and measure. Getting below 100 ps Tr is hard,
because package and pcb parasitics start to really matter, and because
the faster srd's are hard to drive.

When you're designing this stuff, what's your technique for prototyping?
"Real" circuit boards (full layout and etched)? Copper clad (blank) PCBs that
you Dremel away the bits you don't need from? Some of the fancier "universal"
PCBs specifically meant for prototyping?

Thanks,
---Joel

J

#### John Larkin

Jan 1, 1970
0
Say John,

When you're designing this stuff, what's your technique for prototyping?
"Real" circuit boards (full layout and etched)? Copper clad (blank) PCBs that
you Dremel away the bits you don't need from? Some of the fancier "universal"
PCBs specifically meant for prototyping?

I just solder stuff to a piece of double-side copperclad, midair or
onto crude traces cut with an xacto knife, all impedance matched of
course. I usually use flanged sma connectors soldered to the board
edge-mount style, or solder hardline coax straight onto the parts.
There's tons of sma hardline cables on ebay.

For more complex stuff, we lay out a board and have AP Circuits or
somebody make us a few boards.

The critical thing is to have a good, 12 GHz or so, sampling scope. An
11801+SD22 can be had on ebay for $2K or less. Older 7000 series stuff isn't as good, but is a lot cheaper. John D #### Darwin Jan 1, 1970 0 I'd doubt that spice modeling will be worth the effort. You'd do better to experiment and measure. Getting below 100 ps Tr is hard, because package and pcb parasitics start to really matter, and because the faster srd's are hard to drive. I agree with you; I am waiting for a few samples to play with and I wanted to simulate a little them while I am waiting... I was thinking to drive the SRD with a logic gate such as a 74AC14, paralleling a few gates in order to obtain enough current. I hope it is fast enough... The other way to get edges like this are to start with a fast ecl or cml edge - 40 ps is easy these days - and run it through a cheap 5 or 10 GHz mmic. I have had a look at some very nice SiGe ultrafast comparators. I will try them if I do not have good results with SRDs. I was thinking to prepare a few boards with matched 50Ohm striplines to prototyping a little. I have access to a nice 40 GHz scope, but it is 100km away from where I am working, and we have a 20GHz network analyzer in house. J #### John Larkin Jan 1, 1970 0 I agree with you; I am waiting for a few samples to play with and I wanted to simulate a little them while I am waiting... I was thinking to drive the SRD with a logic gate such as a 74AC14, paralleling a few gates in order to obtain enough current. I hope it is fast enough... That's one place where simulation or calculation is useful. But you have to know the approximate equivalent stored charge of the srd, to calculate what the reverse current will be at the instant of snap. If you drive the srd through a small inductor, fairly common, the risetime of the driver pumping the inductor is the thing that has to be integrated. You'll need roughly 60-70 mA at snap time. An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts into 50 ohms in about 600 ps. I have had a look at some very nice SiGe ultrafast comparators. I will try them if I do not have good results with SRDs. You probably refer to the Analog Devices parts. One of them makes a nice 35 ps edge or some such. You can differentiate that with 0603-size r/l/c parts, into anybody's fast mmic, and get about a 100 ps blip. We're doing that to get a 90 ps, 6-volt fiducial pulse into a laser modulator, but we're using$180 Hittite distributed amps to get
that much swing.

But for a few volts at 100 ps, the srd is probably easier.

Oh, try M-pulse too. Thay have some nice parts.

John

D

#### Darwin

Jan 1, 1970
0
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.

If I have understood correctly, the transition time of the diode
depends on the stored charge. Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that the diode should snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diode snaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.

Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.

You probably refer to the Analog Devices parts.
[zip]

Yes, I do. Something like the ADCMP580.

Thanks a lot!

J

#### John Larkin

Jan 1, 1970
0
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.

If I have understood correctly, the transition time of the diode
depends on the stored charge.

No, or at least only indirectly. Diodes that store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that the diode should snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diode snaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?

Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpy step. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.

Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.

You probably refer to the Analog Devices parts.
[zip]

Yes, I do. Something like the ADCMP580.

Thanks a lot!

John

J

#### [email protected]

Jan 1, 1970
0
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
If I have understood correctly, the transition time of thediode
depends on the stored charge.

No, or at least only indirectly. Diodes that store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?

Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.

Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!

John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.

First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...

So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!

Thanks.
Johan

D

#### Darwin

Jan 1, 1970
0
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...

If you put a small transistor in the avalanche region, you should
easily get a sub-nanosecond rise time.
You can find a good example in Jim Williams' AN94, from Linear
Technology:
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!

I found very interesting the old AN918 from HP:
http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/an918.pdf

If you need a 1ns pulse rise time at a few volts, keep in mind that
this can be obtained with a reasonably fast logic gate, such as the
NC7NZ34 John suggested.

J

#### John Larkin

Jan 1, 1970
0
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.

No, or at least only indirectly. Diodes that store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?

Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.

An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!

John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.

First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...

So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!

Thanks.
Johan

If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.

Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.

John

D

#### Darwin

Jan 1, 1970
0
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...

Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.

@ Johan: I played a little with the transistors I have in my junkbox.
I did not see avalanching on a 2n2222, but I tried a few 2n2907 (they
are PNP transistors, so one needs to change the schematics
accordingly), from several brands and they all worked nicely. Of
course, they are not specified for avalanche use, but a lot of cheap
transistors can be made avalanching well. I did not run them for a
very long time, but they did not vanish in a puff of smoke. You have
to try. Of course, you need at least a 250V max DC adjustable power
supply. I used a flyback inverter I made a few years ago. Be careful,
of course.

@ John: I built several SRD pulse stretcher, but unfortunately I have
to wait until I have access to a fast scope to test them. Thank you
again.

J

#### [email protected]

Jan 1, 1970
0
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
[zip]
Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.

@ Johan: I played a little with the transistors I have in my junkbox.
I did not see avalanching on a 2n2222, but I tried a few 2n2907 (they
are PNP transistors, so one needs to change the schematics
accordingly), from several brands and they all worked nicely. Of
course, they are not specified for avalanche use, but a lot of cheap
transistors can be made avalanching well. I did not run them for a
very long time, but they did not vanish in a puff of smoke. You have
to try. Of course, you need at least a 250V max DC adjustable power
supply. I used a flyback inverter I made a few years ago. Be careful,
of course.

@ John: I built several SRD pulse stretcher, but unfortunately I have
to wait until I have access to a fast scope to test them. Thank you
again.

@ Darwin: That sounds great!! Which schematics did you use to try the
2n2222 and the 2n2907? What sort of input trigger did you use (and
specially the input trigger rise time)?? Because in a book that I
have, they say that it could work with a 2n918, 2n2222 and 2n3904.
Here is the schematics I would like to use for the transistor in
avalanche area:http://img156.imageshack.us/img156/9371/uwbrz4.jpg and
this one is the one I would like to use with a SRD:
http://img133.imageshack.us/img133/4029/uwbsrdcs3.jpg . For you, which
ones could give me the best result? I mean the smaller width impulse?

Thanks!!

J

#### [email protected]

Jan 1, 1970
0
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.
No, or at least only indirectly. Diodes that store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!
John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!
Thanks.
Johan

If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.

Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.

John- Hide quoted text -

- Show quoted text -

Hello,

if I understand, I can put any rectangular trigger on my schematics'
input, it will not influence the width of the output impulse (Even if
the rise time is more than 5 ns)? In fact, I would like to have an
impluse of maximum 1ns of duration... if it's smaller, it better!!

What do you mean by the fire or not? Does it mean that the transistor
died? Here is the schematics I would like to use:
http://img156.imageshack.us/img156/9371/uwbrz4.jpg But I don't know
what to put for Vbb ... Do you have any idea??

Johan

J

#### John Larkin

Jan 1, 1970
0
On 6 mar, 17:01, John Larkin
On 6 Mar 2007 00:37:11 -0800, "Darwin" <[email protected]> wrote:
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.
No, or at least only indirectly. Diodes that store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!
John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!
Thanks.
Johan

If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.

Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.

John- Hide quoted text -

- Show quoted text -

Hello,

if I understand, I can put any rectangular trigger on my schematics'
input, it will not influence the width of the output impulse (Even if
the rise time is more than 5 ns)? In fact, I would like to have an
impluse of maximum 1ns of duration... if it's smaller, it better!!

What do you mean by the fire or not? Does it mean that the transistor
died? Here is the schematics I would like to use:
http://img156.imageshack.us/img156/9371/uwbrz4.jpg But I don't know
what to put for Vbb ... Do you have any idea??

Johan

That looks OK, but the avalanche will kick a lot of pulse back into
the trigger input. I like to use a small trigger transformer, with the
secondary connected directly b-e on the transistor. A little RF
ferrite core with a few turns on each side, bifalar maybe, works fine
for a ttl-level drive.

You might also add a bit of inductance in series with the output 50
ohm resistor, or even reduce its value, to speed things up a tad. Keep
all lead lengths zero, of course, and build it on a solid ground
plane.

I've never damaged a transistor by avalanching it. Many generations of
Tek sampling heads used avalanche transistors, to either drive the
sampling gate directly or to drive an SRD. There must be billions of
device-hours there, and as far as I nkow, it's not a particular
failure mode. The dual-channel 7S14 uses one avalanche transistor to
drive both sampling bridges. It's spec'd at 350 ps risetime but is

Picosecond Pulse Labs and Kentec sell commercial avalanche-based
pulsers. We may do one soon too, for a custom application.

John

J

#### [email protected]

Jan 1, 1970
0
On 4 Apr 2007 14:18:44 -0700, "[email protected]"
On 6 mar, 17:01, John Larkin
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.
No, or at least only indirectly.Diodesthat store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!
John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!
Thanks.
Johan
If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.
Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.
John- Hide quoted text -
- Show quoted text -

if I understand, I can put any rectangular trigger on my schematics'
input, it will not influence the width of the output impulse (Even if
the rise time is more than 5 ns)? In fact, I would like to have an
impluse of maximum 1ns of duration... if it's smaller, it better!!
What do you mean by the fire or not? Does it mean that the transistor
died? Here is the schematics I would like to use:
http://img156.imageshack.us/img156/9371/uwbrz4.jpgBut I don't know
what to put for Vbb ... Do you have any idea??

That looks OK, but the avalanche will kick a lot of pulse back into
the trigger input. I like to use a small trigger transformer, with the
secondary connected directly b-e on the transistor. A little RF
ferrite core with a few turns on each side, bifalar maybe, works fine
for a ttl-level drive.

You might also add a bit of inductance in series with the output 50
ohm resistor, or even reduce its value, to speed things up a tad. Keep
all lead lengths zero, of course, and build it on a solid ground
plane.

I've never damaged a transistor by avalanching it. Many generations of
Tek sampling heads used avalanche transistors, to either drive the
sampling gate directly or to drive an SRD. There must be billions of
device-hours there, and as far as I nkow, it's not a particular
failure mode. The dual-channel 7S14 uses one avalanche transistor to
drive both sampling bridges. It's spec'd at 350 ps risetime but is

Picosecond Pulse Labs and Kentec sell commercial avalanche-based
pulsers. We may do one soon too, for a custom application.

John

Thank you for all these advices!!
I've try to simulate the circuit with the avalanche transistor with
PSpice but I think the simulation device don't know the avalanche
mode...
Do you think there is a way to simulate that? May be with ADS? I've
got it but I've never use it.

I will use a 2N3904 because that's what we have in the laboratory. But
to fixe -Vbb and Rb, I don't really know what to put there. Will it
depend on my transistor breakdown voltage?if I put Rb = 1Mohm and -Vbb
= -60V will it be ok?

Johan

J

#### John Larkin

Jan 1, 1970
0
On Apr 5, 11:43 pm, John Larkin
On 4 Apr 2007 14:18:44 -0700, "[email protected]"
On 6 mar, 17:01, John Larkin
On 6 Mar 2007 00:37:11 -0800, "Darwin" <[email protected]> wrote:
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.
No, or at least only indirectly.Diodesthat store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!
John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!

If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.
Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.
John- Hide quoted text -
- Show quoted text -

if I understand, I can put any rectangular trigger on my schematics'
input, it will not influence the width of the output impulse (Even if
the rise time is more than 5 ns)? In fact, I would like to have an
impluse of maximum 1ns of duration... if it's smaller, it better!!
What do you mean by the fire or not? Does it mean that the transistor
died? Here is the schematics I would like to use:
http://img156.imageshack.us/img156/9371/uwbrz4.jpgBut I don't know
what to put for Vbb ... Do you have any idea??

That looks OK, but the avalanche will kick a lot of pulse back into
the trigger input. I like to use a small trigger transformer, with the
secondary connected directly b-e on the transistor. A little RF
ferrite core with a few turns on each side, bifalar maybe, works fine
for a ttl-level drive.

You might also add a bit of inductance in series with the output 50
ohm resistor, or even reduce its value, to speed things up a tad. Keep
all lead lengths zero, of course, and build it on a solid ground
plane.

I've never damaged a transistor by avalanching it. Many generations of
Tek sampling heads used avalanche transistors, to either drive the
sampling gate directly or to drive an SRD. There must be billions of
device-hours there, and as far as I nkow, it's not a particular
failure mode. The dual-channel 7S14 uses one avalanche transistor to
drive both sampling bridges. It's spec'd at 350 ps risetime but is

Picosecond Pulse Labs and Kentec sell commercial avalanche-based
pulsers. We may do one soon too, for a custom application.

John

Thank you for all these advices!!
I've try to simulate the circuit with the avalanche transistor with
PSpice but I think the simulation device don't know the avalanche
mode...
Do you think there is a way to simulate that? May be with ADS? I've
got it but I've never use it.

Simulation is useless here. Use solder.
I will use a 2N3904 because that's what we have in the laboratory. But
to fixe -Vbb and Rb, I don't really know what to put there. Will it
depend on my transistor breakdown voltage?if I put Rb = 1Mohm and -Vbb
= -60V will it be ok?

To test transistors for avalanche behavior, try...

+300
|
|
100k
|
|
+---------+--- <-- 10M scope probe
| |
| |
| 47 pF
| |
| |
| gnd
|
c
+----b NPN
| e
10k |
| |
+-----+-----------> } 50r coax to attanuator
| gnd--> } and fast scope
|
|
50
|
|
|
gnd

This will self-oscillate and tell you the avalanche voltage and
risetime. Invert supply for PNP of course. Most transistors,
especially modern ones, won't avalanche at all.

I've noticed that some transistors will dump essentially all of the
cap voltage into the load, and some just a fraction. The Zetex parts
seem to turn on hard.

John

J

#### [email protected]

Jan 1, 1970
0
On 6 Apr 2007 07:15:29 -0700, "[email protected]"
On Apr 5, 11:43 pm, John Larkin
On 4 Apr 2007 14:18:44 -0700, "[email protected]"
On 6 mar, 17:01, John Larkin
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.
No, or at least only indirectly.Diodesthat store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!
John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!
Thanks.
Johan
If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.
Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.
John- Hide quoted text -
- Show quoted text -
Hello,
if I understand, I can put any rectangular trigger on my schematics'
input, it will not influence the width of the output impulse (Even if
the rise time is more than 5 ns)? In fact, I would like to have an
impluse of maximum 1ns of duration... if it's smaller, it better!!
What do you mean by the fire or not? Does it mean that the transistor
died? Here is the schematics I would like to use:
http://img156.imageshack.us/img156/9371/uwbrz4.jpgButI don't know
what to put for Vbb ... Do you have any idea??
Johan
That looks OK, but the avalanche will kick a lot of pulse back into
the trigger input. I like to use a small trigger transformer, with the
secondary connected directly b-e on the transistor. A little RF
ferrite core with a few turns on each side, bifalar maybe, works fine
for a ttl-level drive.
You might also add a bit of inductance in series with the output 50
ohm resistor, or even reduce its value, to speed things up a tad. Keep
all lead lengths zero, of course, and build it on a solid ground
plane.
I've never damaged a transistor by avalanching it. Many generations of
Tek sampling heads used avalanche transistors, to either drive the
sampling gate directly or to drive an SRD. There must be billions of
device-hours there, and as far as I nkow, it's not a particular
failure mode. The dual-channel 7S14 uses one avalanche transistor to
drive both sampling bridges. It's spec'd at 350 ps risetime but is
Picosecond Pulse Labs and Kentec sell commercial avalanche-based
pulsers. We may do one soon too, for a custom application.
John
Thank you for all these advices!!
I've try to simulate the circuit with the avalanche transistor with
PSpice but I think the simulation device don't know the avalanche
mode...
Do you think there is a way to simulate that? May be with ADS? I've
got it but I've never use it.

Simulation is useless here. Use solder.

I will use a 2N3904 because that's what we have in the laboratory. But
to fixe -Vbb and Rb, I don't really know what to put there. Will it
depend on my transistor breakdown voltage?if I put Rb = 1Mohm and -Vbb
= -60V will it be ok?

To test transistors for avalanche behavior, try...

+300
|
|
100k
|
|
+---------+--- <-- 10M scope probe
| |
| |
| 47 pF
| |
| |
| gnd
|
c
+----b NPN
| e
10k |
| |
+-----+-----------> } 50r coax to attanuator
| gnd--> } and fast scope
|
|
50
|
|
|
gnd

This will self-oscillate and tell you the avalanche voltage and
risetime. Invert supply for PNP of course. Most transistors,
especially modern ones, won't avalanche at all.

I've noticed that some transistors will dump essentially all of the
cap voltage into the load, and some just a fraction. The Zetex parts
seem to turn on hard.

John

Just to be sure...

50r coax to attanuator
gnd--> } and fast scope

are connected to b of the NPN? So I have b ---10k --- gnd and before
gnd I connect fast scope??
Is that correct??

thx!

F

#### Fred Bartoli

Jan 1, 1970
0
[email protected] a écrit :
On Apr 6, 6:20 pm, John Larkin
On 6 Apr 2007 07:15:29 -0700, "[email protected]"
On Apr 5, 11:43 pm, John Larkin
On 4 Apr 2007 14:18:44 -0700, "[email protected]"
On 6 mar, 17:01, John Larkin
Thank you John, your posts are very informative; I have seen that that
SRD stuff has been debated also a few months ago and I read the old
posts.
That's one place where simulation or calculation is useful. But you
have to know the approximate equivalent stored charge of the srd, to
calculate what the reverse current will be at the instant of snap. If
you drive the srd through a small inductor, fairly common, the
risetime of the driver pumping the inductor is the thing that has to
be integrated. You'll need roughly 60-70 mA at snap time.
If I have understood correctly, the transition time of thediode
depends on the stored charge.
No, or at least only indirectly.Diodesthat store a lot of charge
tend to have slower transition times.
Should I minimize that charge? The the
current risetime is mainly fixed by the inductor, if the driver has a
sufficiently small risetime. How should I choose the inductor then? I
guess that thediodeshould snap when the current equals the supply
voltage divided by the 50 Ohm load resistance. In this case, I should
get a clean output pulse. The inductor should be sufficiently small to
have a fast dI/dt, but big enough to isolate the generator when the
diodesnaps. I can then choose the forward current in order to make
the SRD snap at the right time. Is that correct?
Yes. The output voltage will be a bit less than (inductor current *
load resistance) at the instant of snap. The problem is to get that
much current into the srd before you use up all the stored charge. If
you lose that race, it snaps when the current is too low, and you get
a wimpystep. It's easier to pump a lot of current into an srd that
stores a lot if charge, but those are the same ones that tend to have
slow transition times.
An NC7NZ34, all sections in parallel, powered by +6, will put 5 volts
into 50 ohms in about 600 ps.
Thanks. I will have a look at that part. These tiny logic devices are
truly amazing.
I have had a look at some very nice SiGe ultrafast comparators. I will
try them if I do not have good results with SRDs.
You probably refer to the Analog Devices parts.
[zip]
Yes, I do. Something like the ADCMP580.
Thanks a lot!
John
Hi everybody,
I'm doing my study in Belgium and for my final work I have to do a
pulse generator too.
First I've planned to use a transistor (2N3904) in avalanche area. But
with my circuit, I need a very fast signal generator because the width
of my pulse is proportionnal to the rise time of the signal generator
and I want a 1 ns width pulse, so I need a 1 ns rise time for the
generator...
So I would like to use a SRD instead or inside my circuit. Could you
the pulse generator? Please email me and we can share
informations!!
Thanks.
Johan
If the transistor avalanches, the output rise time is unrelated to the
input trigger speed. It fires or it doesn't.
Zetex sells, as far as I know, the only transistors specified for
avalanche use. They are an old diffused process, fairly low Ft, made
in Russia. Newer epitaxials don't seem to avalanche.
John- Hide quoted text -
- Show quoted text -
Hello,
if I understand, I can put any rectangular trigger on my schematics'
input, it will not influence the width of the output impulse (Even if
the rise time is more than 5 ns)? In fact, I would like to have an
impluse of maximum 1ns of duration... if it's smaller, it better!!
What do you mean by the fire or not? Does it mean that the transistor
died? Here is the schematics I would like to use:
http://img156.imageshack.us/img156/9371/uwbrz4.jpgButI don't know
what to put for Vbb ... Do you have any idea??
Johan
That looks OK, but the avalanche will kick a lot of pulse back into
the trigger input. I like to use a small trigger transformer, with the
secondary connected directly b-e on the transistor. A little RF
ferrite core with a few turns on each side, bifalar maybe, works fine
for a ttl-level drive.
You might also add a bit of inductance in series with the output 50
ohm resistor, or even reduce its value, to speed things up a tad. Keep
all lead lengths zero, of course, and build it on a solid ground
plane.
I've never damaged a transistor by avalanching it. Many generations of
Tek sampling heads used avalanche transistors, to either drive the
sampling gate directly or to drive an SRD. There must be billions of
device-hours there, and as far as I nkow, it's not a particular
failure mode. The dual-channel 7S14 uses one avalanche transistor to
drive both sampling bridges. It's spec'd at 350 ps risetime but is
Picosecond Pulse Labs and Kentec sell commercial avalanche-based
pulsers. We may do one soon too, for a custom application.
John
Thank you for all these advices!!
I've try to simulate the circuit with the avalanche transistor with
PSpice but I think the simulation device don't know the avalanche
mode...
Do you think there is a way to simulate that? May be with ADS? I've
got it but I've never use it.
Simulation is useless here. Use solder.

I will use a 2N3904 because that's what we have in the laboratory. But
to fixe -Vbb and Rb, I don't really know what to put there. Will it
depend on my transistor breakdown voltage?if I put Rb = 1Mohm and -Vbb
= -60V will it be ok?
To test transistors for avalanche behavior, try...

+300
|
|
100k
|
|
+---------+--- <-- 10M scope probe
| |
| |
| 47 pF
| |
| |
| gnd
|
c
+----b NPN
| e
10k |
| |
+-----+-----------> } 50r coax to attanuator
| gnd--> } and fast scope
|
|
50
|
|
|
gnd

This will self-oscillate and tell you the avalanche voltage and
risetime. Invert supply for PNP of course. Most transistors,
especially modern ones, won't avalanche at all.

I've noticed that some transistors will dump essentially all of the
cap voltage into the load, and some just a fraction. The Zetex parts
seem to turn on hard.

John

Just to be sure...

50r coax to attanuator
gnd--> } and fast scope

are connected to b of the NPN? So I have b ---10k --- gnd and before
gnd I connect fast scope??
Is that correct??

Nope. The 10k base resistor, the 50R resistor and the 50R coax/scope are
all connected to the transistor emitter.

Replies
0
Views
1K
A
Replies
12
Views
2K
Joerg
J
Replies
0
Views
297
Replies
30
Views
2K
Replies
2
Views
419