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Narrow pulse generation

  • Thread starter Anthony Fremont
  • Start date
A

Anthony Fremont

Jan 1, 1970
0
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

thanks
 
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

thanks

Seems to me any logic gate can drive faster than that, but you might
not get the voltage swing you want. Raises the question of why you
want such a high voltage? With such a slow rise, you might want to
test very long cables?
TDRing nearby typically uses 25pS rise-times with about 100mV swing.
But I think with a typical logic gate you can see all the classic TDR
cases with a few feet of coax for sure.
 
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

thanks

You can use the output of some waveform generator (or a astable
multivibrator) to feed a high pass filter (a simple C-R with a diode
in anti parallel to R to avoid negative pulses). Then the filter
output feed to a schmitt trigger like 74hc14. The pulse wide is
controled by the values of R and C. Easy and cheap!!

Good luck
 
A

Anthony Fremont

Jan 1, 1970
0
Thanks for your reply.
Seems to me any logic gate can drive faster than that, but you might
not get the voltage swing you want. Raises the question of why you
want such a high voltage? With such a slow rise, you might want to
test very long cables?

The voltage I chose was arbitrary, I was just thinking of something battery
powered.
TDRing nearby typically uses 25pS rise-times with about 100mV swing.
But I think with a typical logic gate you can see all the classic TDR
cases with a few feet of coax for sure.

Those rise-times would be nice, but not really necessary for me AIUI. I'll
just be using an average scope for the monitoring so anything should work
for tinkering purposes.

Since I'm not very clever with analog stuff, how can I make the logic gate
switch on, wait a few (adjustable would be cool) nanoseconds and then switch
back off on command?
 
A

Andrew Holme

Jan 1, 1970
0
Anthony Fremont said:
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but
the load can be anything from an open circuit to a dead short. 5 to 10
Volts would be great. It's not a beauty contest on the shape so I want to
do this cheaply using generic parts I have laying around if at all
possible. I would want to be able to trigger these repetively at up to
say a few hundred Hz. It's for some TDR tinkering.

thanks

http://www.holmea.demon.co.uk/Avalanche/Avalanche.htm
 
J

Joerg

Jan 1, 1970
0
Anthony said:
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

Figure 4 shows the basic method of how it's done:
http://www.onsemi.com/pub/Collateral/MC14106B-D.PDF

Of course you have to use much faster logic and then amplify it. Or use
tons of parallel buffers and step up via a stripline transformer.
Sourcing 5-10V into a dead short might turn out to be a challenge
though. At least I'd ask the local uitility to spool up their peaker
plant and warn the fire district before moving that big switch. A notice
to the FAA would also be good so they can alert pilots that this big
bright flash is not a UFO or meteorite that has crashed ;-)
 
R

Rich Grise

Jan 1, 1970
0
Anyone have good ideas on generating 5nS pulses into a 100ohm load, but
the load can be anything from an open circuit to a dead short. 5 to 10
Volts would be great. It's not a beauty contest on the shape so I want to
do this cheaply using generic parts I have laying around if at all
possible. I would want to be able to trigger these repetively at up to
say a few hundred Hz. It's for some TDR tinkering.

You don't do TDR with a pulse - you do it with a step; you reset the step
when the last reflection gets displayed (which will depend on the length
of the line). That gives you a nice graph of the cable's impedance along
its length. A pulse will just differentiate that and smear out all of the
pips.

Generating the step is left as an exercise for the reader? Heck, it's
just a step generator - you just go from 0 to 1, and the risetime will
depend on the chip, when a trace is done, reset to 0 until all of the
reflections are back, and give another step. At 1' per nanosecond, you
probably won't have to wait long. :)

Good Luck!
Rich
 
J

John Larkin

Jan 1, 1970
0
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

thanks

Electrical TDR usually uses a step, not a pulse. My TDR scopes all
seem to use a square wave, 100 KHz ballpark, to generate a repetitive
step, although you might go slower to shoot out really long cables.

An AC-family logic gate should work.

John
 
A

Anthony Fremont

Jan 1, 1970
0
Joerg said:
Figure 4 shows the basic method of how it's done:
http://www.onsemi.com/pub/Collateral/MC14106B-D.PDF

Of course you have to use much faster logic and then amplify it. Or
use tons of parallel buffers and step up via a stripline transformer.
Sourcing 5-10V into a dead short might turn out to be a challenge
though. At least I'd ask the local uitility to spool up their peaker
plant and warn the fire district before moving that big switch. A
notice to the FAA would also be good so they can alert pilots that
this big bright flash is not a UFO or meteorite that has crashed ;-)

LOL You don't think I'll have trouble keeping the voltage up do you? ;-)
I guess I better allow for that then. I would like the output impedance of
the generator to be somewhere in the vicinity of 75 Ohms, adjustable from 50
to 100 would be ideal. Thanks for the link. :)
 
J

Joerg

Jan 1, 1970
0
Anthony said:
LOL You don't think I'll have trouble keeping the voltage up do you? ;-)
I guess I better allow for that then. I would like the output impedance of
the generator to be somewhere in the vicinity of 75 Ohms, adjustable from 50
to 100 would be ideal. Thanks for the link. :)

That would certainly mean some paralleling or transformer-based
combining so you can have fast logic and boost up the voltage. But as
Rich already mentioned TDR is usually done with a transition, not a pulse.
 
J

Joerg

Jan 1, 1970
0
John said:
Electrical TDR usually uses a step, not a pulse. My TDR scopes all
seem to use a square wave, 100 KHz ballpark, to generate a repetitive
step, although you might go slower to shoot out really long cables.

An AC-family logic gate should work.

I'd go all out and use PECL or something like that.
 
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

thanks

I don't know the details, but when I was doing video speed analog, we
used HP ATE. Anyway, it tuned itself up using a pulse that they
generated with a diode specifically designed for flat pulse
generation. If you do a patent search, Tek and HP have many pulse
generation circuits.

This particular HP circuit was designed into every DUT board.
 
A

Anthony Fremont

Jan 1, 1970
0
John said:
Electrical TDR usually uses a step, not a pulse. My TDR scopes all
seem to use a square wave, 100 KHz ballpark, to generate a repetitive
step, although you might go slower to shoot out really long cables.

An AC-family logic gate should work.

That certainly sounds easy enough and I happen to have some 74AHC04s. Will
those work ok?

And now, how do cheap micro based TDRs measure those nanosecond response
times? They obviously aren't using a CCP module to time them. There must
be a cheezy trick of some sort. ;-)
 
J

John Larkin

Jan 1, 1970
0
That certainly sounds easy enough and I happen to have some 74AHC04s. Will
those work ok?

Should. The source impedance of the tdr step should be 50 ohms. Put
several gate sections in parallel to get a stiff source, and then put
a series resistor, 47 ohms maybe, to make a net 50 ohm source. Run
that to a tee connector at the scope input (assuming a hi-z scope) and
the other tee port is the tdr thing.
And now, how do cheap micro based TDRs measure those nanosecond response
times? They obviously aren't using a CCP module to time them. There must
be a cheezy trick of some sort. ;-)

Dunno. The old Tek 1503 (?) handheld used a classic diode-bridge
sampler. Nowadays you could just trigger a fast adc, once a tdr step,
with the adc trigger slowly walked in time to make the range sweep.

John
 
J

Jonathan Kirwan

Jan 1, 1970
0
Dunno. The old Tek 1503 (?) handheld used a classic diode-bridge
sampler. Nowadays you could just trigger a fast adc, once a tdr step,
with the adc trigger slowly walked in time to make the range sweep.

By 'fast adc,' I think you are addressing yourself to the sampling
window (which needs to settle quickly) and not so much to the length
of the actual conversion time. That may need to be made clear to
Anthony.

Jon
 
J

John Larkin

Jan 1, 1970
0
By 'fast adc,' I think you are addressing yourself to the sampling
window (which needs to settle quickly) and not so much to the length
of the actual conversion time. That may need to be made clear to
Anthony.

Jon


Right. You only need to take one sample each TDR edge, and slowly walk
the sample delay to build the waveform. You can even take multiple
samples at the same delay, over several shots, to average out noise.

No rush.

John
 
A

Anthony Fremont

Jan 1, 1970
0
John said:
Right. You only need to take one sample each TDR edge, and slowly walk
the sample delay to build the waveform. You can even take multiple
samples at the same delay, over several shots, to average out noise.

No rush.

I can understand that, you just build up the information over many pulses
one piece at a time. My question then becomes, how are you "walking" your
samples so precisely? What is generating the time delays?
 
Hello,

Anyone have good ideas on generating 5nS pulses into a 100ohm load, but the
load can be anything from an open circuit to a dead short. 5 to 10 Volts
would be great. It's not a beauty contest on the shape so I want to do this
cheaply using generic parts I have laying around if at all possible. I
would want to be able to trigger these repetively at up to say a few hundred
Hz. It's for some TDR tinkering.

5nsec pulses don't sound too hard - we got 0.5nsec FWHM at 5V into 50R
and 7.5V for anything longer.

We did use a fairly expensive HP RF transistor as the output stage.

On the way there I got 800psec FWHM out of a bunch of dead cheap BFR96
wide-band transistors.

Farnell doesn't stock them any more, but you might like to look at the
Philips BFR106 or the BFG590, which are listed as limited stock in my
Farnell catalogue. The BFQ68 looks like a keeper and is a lot sexier,
but contains berylium oxide and costs a lot more.

We drove our transistors in long-tailed pairs from 100k ECL using
BFT92 or BFT93 PNP parts as level shifters - modern CMOS would be fast
enough for your purposes, but note that the thin base-emitter
junctions in wide-band transisors often break down when reversed
biased by more than 2V (check the data sheet!).
 
J

Joerg

Jan 1, 1970
0
Anthony said:
I can understand that, you just build up the information over many pulses
one piece at a time. My question then becomes, how are you "walking" your
samples so precisely? What is generating the time delays?

For example, you can build an LC or RC section that scoots your bridge
trigger time sliver by time sliver. You can use a varicap in the C or
make the R variable. The latter is what I usually do. Early on with the
SD5400 but when that got expensive I used dual gate FETs for TV tuners.
Those cost only pennies but have very low inherent capacitances. Low
stray capacitance is very important here. You can easily scoot a few
hundred psec. Personally I avoid digitally adjustable delay lines for
noise reasons.

That SD5400 was very handy. Since contains four matched devices I could
servo it an literally steer resistance. Very smooth. But all good things
will some day end <sigh>.

BTW the scope connection in Hi-Z that John mentioned needs one word of
caution added: Some of the glitzy "modern" digital scopes have the nasty
habit of reducing their bandwidth when cranking up the channel gain.
 
A

Anthony Fremont

Jan 1, 1970
0
John said:
Should. The source impedance of the tdr step should be 50 ohms. Put
several gate sections in parallel to get a stiff source, and then put
a series resistor, 47 ohms maybe, to make a net 50 ohm source. Run
that to a tee connector at the scope input (assuming a hi-z scope) and
the other tee port is the tdr thing.

_Sounds_ like a piece of cake, thanks. I should have most of what I need
laying around here somewhere. ;-) I'm not sure my old Hitachi scope is
going to be up for it, but I should see something with 100' piece of Cat-5.
 
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