Matching a monolithic xtal filter

[Joerg]

On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.

That's where the old concept of the Q-multiplier comes in. After that it
only boils down to how good you are able to control the CF of a resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.

Ouch! Not around my designs, thank you. :-(

[For the uninitiated: just stay away from them.]

What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

Baaackk iiinnnn theeee oolldddd ddaaaaysss wwhhhhheenn IIII
wwwwwwwaaaaaaaaasssssss yyoouuuunnnggg, I had the bright idea to add
negative resistance to improve the Q of an otherwise passive bandpass
circuit, since the coil was limiting the performance due to its Q. An
experienced engineer told me not to bother trying for commercial grade
equipment that had to operate across a wide temperature range. I never
revisited it.

So you've had luck with this technique at RF? Seems more a ham/garage
thing to me.

Yes, I did. Can't release much in details. Suffice it to say that you
need to continually test where you are on the "real" feedback slope.
This has become tremendously easier these days since you can get a uC
for a Dollar and neat serial ADCs for not much more. Temperature is what
makes it drift and it is slow enough that you can get away with a loop
BW in the tens of Hertz. We've even done something similar in the THz
domain but that one I can't talk about at all.

Don't start with a lousy inductor. A Q-multiplier isn't the tool to
paper over a bad LC design. Also, at some point microphonics will become
an issue and you don't really want to be regulating those out.

 

[Tom Bruhns]

On Sep 11, 1:51 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:
On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:
Tom Bruhns wrote:
...
how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.
That's where the old concept of the Q-multiplier comes in. After that it
only boils down to how good you are able to control the CF of a resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.
Ouch! Not around my designs, thank you. :-(
[For the uninitiated: just stay away from them.]
What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom

Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside the
speaker cabinet (early 60's pricing, gets me drooling). This puppy has a
dynamic range from here to the Klondike.

And a 2b's IIP3 (with no pad at the front end) is how much? Under the
same conditions, the NF is how much?

Cheers,
Tom

 

[Joerg]

On Sep 11, 1:51 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.

That's where the old concept of the Q-multiplier comes in. After that it
only boils down to how good you are able to control the CF of a resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.

Ouch! Not around my designs, thank you. :-(

[For the uninitiated: just stay away from them.]

What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom

Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside the
speaker cabinet (early 60's pricing, gets me drooling). This puppy has a
dynamic range from here to the Klondike.

And a 2b's IIP3 (with no pad at the front end) is how much? Under the
same conditions, the NF is how much?
[/QUOTE]

I don't remember. Let's put it like the horsepower of a Rolls Royce:
Adequate. Even 30 years later that receiver outperformed new gear in
terms of operation in the vicinity of strong transmitters. Ok, it's got
tubes which gives it a head start.

Why do you think a modest Q-multiplication would worsen that, provided
there is plenty of supply voltage? If you just want to reign in the LC
before a filter you don't need to reach Qs of a gazillion.

 

[Joerg]

On Sep 11, 1:51 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.

That's where the old concept of the Q-multiplier comes in. After that it
only boils down to how good you are able to control the CF of a resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.

Ouch! Not around my designs, thank you. :-(

[For the uninitiated: just stay away from them.]

What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom

Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside the
speaker cabinet (early 60's pricing, gets me drooling). This puppy has a
dynamic range from here to the Klondike.

And a 2b's IIP3 (with no pad at the front end) is how much? Under the
same conditions, the NF is how much?
[/QUOTE]

Other question: Where do you have +55dBm right now? And why do you need
that much?

 

[Tom Bruhns]

On Sep 11, 4:29 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:
On Sep 11, 1:51 pm, Joerg <[email protected]>
wrote:
Tom Bruhns wrote:
On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:
Tom Bruhns wrote:
...
how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.
That's where the old concept of the Q-multiplier comes in. After that it
only boils down to how good you are able to control the CF of a resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.
Ouch! Not around my designs, thank you. :-(
[For the uninitiated: just stay away from them.]
What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.
--
Regards, Joerg

Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.
Cheers,
Tom
Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:
http://home.earthlink.net/~kf6gk/
One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside the
speaker cabinet (early 60's pricing, gets me drooling). This puppy has a
dynamic range from here to the Klondike.

And a 2b's IIP3 (with no pad at the front end) is how much? Under the
same conditions, the NF is how much?

Other question: Where do you have +55dBm right now? And why do you need
that much?

Why: because at that level, a couple 0dBm signals from 7MHz broadcast
transmitters will yield only -110dBm third-order product that will
make unintelligible a microvolt-level signal.

Where: look at what people are doing with H-mode mixers, for example;
but that's not the only place.

Cheers,
Tom

 

[Phil Hobbs]

Tom Bruhns wrote:

On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

...

how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.

That's where the old concept of the Q-multiplier comes in. After
that it
only boils down to how good you are able to control the CF of a
resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.

Ouch! Not around my designs, thank you. :-(

[For the uninitiated: just stay away from them.]

What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom

Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside the
speaker cabinet (early 60's pricing, gets me drooling). This puppy has a
dynamic range from here to the Klondike.

Besides the nonlinearity problem, Q-multipliers aren't the quietest
things on the planet either. That isn't usually a big problem at HF,
but (repeat after me) Negative Resistors Don't Have Imaginary Noise.

Cheers,

Phil Hobbs

 

[Joerg]

On Sep 11, 1:51 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:


Tom Bruhns wrote:


...


how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.


That's where the old concept of the Q-multiplier comes in. After
that it
only boils down to how good you are able to control the CF of a
resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.


Ouch! Not around my designs, thank you. :-(


[For the uninitiated: just stay away from them.]


What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

--
Regards, Joerg





Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom

Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside
the speaker cabinet (early 60's pricing, gets me drooling). This puppy
has a dynamic range from here to the Klondike.

Besides the nonlinearity problem, Q-multipliers aren't the quietest
things on the planet either. That isn't usually a big problem at HF,
but (repeat after me) Negative Resistors Don't Have Imaginary Noise.

Sure. However, Q-multipliers are usually placed after there has already
been some gain. Also, often noise is less important than selectivity.
It's like pulling a trailer up a hill where your car won't run very
quiet and not the cleanest but you've got to get up there ;-)

 

[Joerg]

On Sep 11, 4:29 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

On Sep 11, 1:51 pm, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

On Sep 11, 11:15 am, Joerg <[email protected]>
wrote:

Tom Bruhns wrote:

how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.

That's where the old concept of the Q-multiplier comes in. After that it
only boils down to how good you are able to control the CF of a resonant
circuit up front. But shhht, don't tell anyone. The younger lads out
there don't have the foggiest idea what that is.

Ouch! Not around my designs, thank you. :-(

[For the uninitiated: just stay away from them.]

What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom

Just as an example which you can still occasionally buy but someone
would almost have to die first because they tend not to part with it,
scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For
something like $40 extra you could upgrade to a Q-multiplier inside the
speaker cabinet (early 60's pricing, gets me drooling). This puppy has a
dynamic range from here to the Klondike.

And a 2b's IIP3 (with no pad at the front end) is how much? Under the
same conditions, the NF is how much?

Other question: Where do you have +55dBm right now? And why do you need
that much?
[/QUOTE]


Why: because at that level, a couple 0dBm signals from 7MHz broadcast
transmitters will yield only -110dBm third-order product that will
make unintelligible a microvolt-level signal.

Where: look at what people are doing with H-mode mixers, for example;
but that's not the only place.
[/QUOTE]

Ok, but at some point it's "good enough". At 7MHz you'll rarely deal
with uV level signals. At 20-30MHz that can happen though. Point is, the
old Drake 2B performed marvelously in such situations. In ham radio the
field days and the contests were the major challenges. Not multiband
because you'd be running the 2nd transmitter at another band and the
input filters take care of most of it but it was the guys on the next
hill half a mile away, competing against your group with a huge antenna
and lots of power. Often they were just 20-30kHz away. All the older
generation transistor rigs fell off the rocker in that situation. BTDT,
many times.

In CW mode (morse code) a steep enough mechanical or crystal filter was
not affordable in those days unless you had one of the Rockefellers on
the team. So the Q-multiplier saved the day.

 

[Tom Bruhns]

On Sep 12, 8:57 am, Joerg <[email protected]>
wrote:
....

Ok, but at some point it's "good enough".

The best currently available is still not quite good enough, I'm
afraid. There are those who have valid needs for better. Though you
sing the praises of the Drake 2B, I've seen other reports on it that
are not nearly so glowing, and we still have no actual measure of its
performance. I seriously doubt that it's anywhere near as good at the
good modern designs that have IIP3-NF > 40dB, and phase noise well
below -100dBc/Hz at 100Hz offset. There's certainly no _need_ for a Q
multiplier in front of a first crystal filter to achieve this
performance, and I doubt that you could add one there while
maintaining the same level of noise and dynamic range performance. I
presume the 2B's Q multiplier was at or near the end of the IF strip,
not at the RF amplifier or even at the mixer output.

Cheers,
Tom

 

[Joerg]

On Sep 12, 8:57 am, Joerg <[email protected]>
wrote:
...



The best currently available is still not quite good enough, I'm
afraid. There are those who have valid needs for better. Though you
sing the praises of the Drake 2B, I've seen other reports on it that
are not nearly so glowing, and we still have no actual measure of its
performance. I seriously doubt that it's anywhere near as good at the
good modern designs that have IIP3-NF > 40dB, and phase noise well
below -100dBc/Hz at 100Hz offset. There's certainly no _need_ for a Q
multiplier in front of a first crystal filter to achieve this
performance, and I doubt that you could add one there while
maintaining the same level of noise and dynamic range performance. I
presume the 2B's Q multiplier was at or near the end of the IF strip,
not at the RF amplifier or even at the mixer output.

After filtering AFAIR. Again, I agree that xtal filters generally do not
need a pre-filter, that was brought up by another poster. In the case of
a large ham radio contest that is very different. While you may have a
nice 2.3kHz filter for SSB and if you splurged maybe another 600Hz
filter that often wasn't enough when the narrow CW portions of a band
where crammed with others who also wanted to win that contest. I now own
a 300Hz filter here but as a young lad back then I certainly did not
have $150 for such luxury. Same for the others. Now, the $100 or so for
the booze during the contest, that was not a problem ;-)

The other thing you could do with a Q-muliplier was often even more
valuable. You could use it as a very sharp notch filter. All in all I
find these things rather useful.