Low voltage negative resistance oscillator design, lessons l

[piglet]

On 03/09/2019 12:15 am, Clifford Heath wrote:

On 2/9/19 8:55 pm, Phil Hobbs wrote:
When I was an undergrad, circa 1980-81, I had a research assistantship
with Professor Bill Shuter (radio astronomer and all-round good guy).
He had a millimeter-wave radio telescope that looked for carbon-12 and
carbon-13 monoxide emission at 115 and 110 GHz, respectively, for
probing the structure of interstellar giant molecular clouds.

The front end was a cooled parametric downconverter, followed by an
X-band GaAs FET IF amp, another downconverter, and a filter bank.
(FFTs were still too slow in those days.)

Wow. But the question remains, does a TD/Varactor paramp need to be
pumped, or can it operate just by being biased into a negative
resistance region?

Varactors don't have a negative resistance region.

If pumped, wtf was that thing pumped with?

Clifford Heath.

In the early 1960s radio astronomers used MASERs as the pump source.

piglet

 

[piglet]

On 02/09/2019 9:25 pm, j.ponte@student.utwente.nl wrote:

On Monday, September 2, 2019 at 8:33:48 PM UTC+2, Steve Wilson wrote:
whit3rd <whit3rd@gmail.com> wrote:

On Friday, August 30, 2019 at 4:39:30 PM UTC-7, Clifford Heath wrote:
On 9/8/19 2:52 am, bitrex wrote: ather than varactors.

It's a curious thought though, using a varactor as an amplifier. Does
anyone have an example circuit?

The Philbrick (P2?) was earlier, but this one is well documented

https://archive.org/details/AnalogDevicesDataAcquisitionProductsCatalog1
979/page/n97

The AD310 usually sat in a teflon-bushed socket and might have to be
hand-wired, but it gave the vacuum-tube vibrating reed electrometers
some real solid state competition on leakage current. After all, if
you bias a varactor at zero volts, what IS the expected DC current?

I am trying to understand how this worked. There is a diagram of the input
circuit on page 3 of the file. Do you have any idea of the frequency and
amplitude of the signal fed to the bridge?

I did a teardown of one and reverse-engineered it:
https://imgur.com/jRxgMy4

Haven't bothered to figure out the color coding on the resistors. The feedback to the "front-end" (post-bridge) JFET still somewhat mystifies me.

I'll hang my scope on it tomorrow to check the frequency. I seem recall the Philbrick P2 operating around 5MHz.

Thanks, very interesting. Looks like the "varactors" are the E-BC
junctions of the 2N5962.

I am puzzled how the oscillator works (those 2 pnps) - that is going to
be my coffee-time challenge.

The color code is a good thing to learn. Ignore the yellow band after
the gold or silver tolerance band, the yellow band is a quality
indicator of established reliability for aerospace use and nothing about
the electrical characteristics.

piglet

 

[piglet]

On 03/09/2019 9:51 am, piglet wrote:

I am puzzled how the oscillator works (those 2 pnps) - that is going to
be my coffee-time challenge.

I think those three things that like resistors "brown, gray, brown,
white" and "brown, black, brown, white" are not resistors but capacitors
of bzw 180pF and 100pF.

The curious looking oscillator then is nothing more than a usual astable
multivibrator.

piglet

 

[John Larkin]

On Tue, 3 Sep 2019 09:14:47 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 03/09/2019 12:15 am, Clifford Heath wrote:
On 2/9/19 8:55 pm, Phil Hobbs wrote:
When I was an undergrad, circa 1980-81, I had a research assistantship
with Professor Bill Shuter (radio astronomer and all-round good guy).
He had a millimeter-wave radio telescope that looked for carbon-12 and
carbon-13 monoxide emission at 115 and 110 GHz, respectively, for
probing the structure of interstellar giant molecular clouds.

The front end was a cooled parametric downconverter, followed by an
X-band GaAs FET IF amp, another downconverter, and a filter bank.
(FFTs were still too slow in those days.)

Wow. But the question remains, does a TD/Varactor paramp need to be
pumped, or can it operate just by being biased into a negative
resistance region?

Varactors don't have a negative resistance region.

They do have a negative slope of C vs V. Sort of a negative reactance.
Sort of.

I bet a step-recovery diode could be an amplifier. A PIN diode
certainly can.

If pumped, wtf was that thing pumped with?

Clifford Heath.

In the early 1960s radio astronomers used MASERs as the pump source.

piglet

A MASER is a pumped amplifier.


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Phil Hobbs]

On 9/2/19 7:15 PM, Clifford Heath wrote:

On 2/9/19 8:55 pm, Phil Hobbs wrote:
On 9/1/19 6:25 PM, Clifford Heath wrote:
On 31/8/19 4:37 pm, whit3rd wrote:
On Friday, August 30, 2019 at 4:39:30 PM UTC-7, Clifford Heath
wrote:
On 9/8/19 2:52 am, bitrex wrote:
ather than varactors.

It's a curious thought though, using a varactor as an amplifier.
Does anyone have an example circuit?

The Philbrick (P2?) was earlier, but this one is well documented

https://archive.org/details/AnalogDevicesDataAcquisitionProductsCatalog1979/page/n97

The AD310 usually sat in a teflon-bushed socket and might have to
be hand-wired, but it gave the vacuum-tube vibrating reed
electrometers some real solid state competition on leakage current.
After all, if you bias a varactor at zero volts, what IS the
expected DC current?


That uses a pump signal at well above the signal frequency. It's
pretty hard to see how you could build an s-band amplifier that way.

When I was an undergrad, circa 1980-81, I had a research assistantship
with Professor Bill Shuter (radio astronomer and all-round good guy).
He had a millimeter-wave radio telescope that looked for carbon-12 and
carbon-13 monoxide emission at 115 and 110 GHz, respectively, for
probing the structure of interstellar giant molecular clouds.

The front end was a cooled parametric downconverter, followed by an
X-band GaAs FET IF amp, another downconverter, and a filter bank.
(FFTs were still too slow in those days.)

Wow. But the question remains, does a TD/Varactor paramp need to be
pumped, or can it operate just by being biased into a negative
resistance region?

If pumped, wtf was that thing pumped with?

Clifford Heath.

Paramps have to be pumped. I vaguely recall that this one was pumped
with a klystron, but it would have had to be stabilized somehow--the
fifth harmonic of an ammonia maser (120 GHz) would have been in the
right ballpark. A TD could probably be run as a self-oscillating
converter as John says, but I expect it would be even rattier than a
hexagrid converter tube.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Steve Wilson]

Blyat Cheker <anusaarshaar@gmail.com> wrote:

I've appended the imgur post with a scope capture of the oscillator. It
runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says they are
general-purpose amplifier transistors. The ones in the varactor amp are
marked with red paint, too (a bit hard to see in the pictures), so AD
must've handselected them. Maybe they were just cheap? I guess the main
important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised" as
resistors, I didn't know about that.

 

[Blyat Cheker]

On Tuesday, September 3, 2019 at 2:36:52 PM UTC+2, piglet wrote:

On 03/09/2019 9:51 am, piglet wrote:
I am puzzled how the oscillator works (those 2 pnps) - that is going to
be my coffee-time challenge.


I think those three things that like resistors "brown, gray, brown,
white" and "brown, black, brown, white" are not resistors but capacitors
of bzw 180pF and 100pF.

The curious looking oscillator then is nothing more than a usual astable
multivibrator.

piglet

I've appended the imgur post with a scope capture of the oscillator. It runs at about 200kHz.

I do still wonder how they found the 2N5962. The datasheet says they are general-purpose amplifier transistors. The ones in the varactor amp are marked with red paint, too (a bit hard to see in the pictures), so AD must've handselected them.
Maybe they were just cheap? I guess the main important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised" as resistors, I didn't know about that.

 

[Jeroen]

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson wrote:

Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the oscillator. It
runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says they are
general-purpose amplifier transistors. The ones in the varactor amp are
marked with red paint, too (a bit hard to see in the pictures), so AD
must've handselected them. Maybe they were just cheap? I guess the main
important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised" as
resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

 

On Tuesday, September 3, 2019 at 10:47:36 PM UTC+2, Steve Wilson wrote:

Jeroen <j.ponte@student.utwente.nl> wrote:

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson wrote:
Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the oscillator.
It runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says they
are general-purpose amplifier transistors. The ones in the varactor
amp are marked with red paint, too (a bit hard to see in the
pictures), so AD must've handselected them. Maybe they were just
cheap? I guess the main important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised" as
resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

I found the waveform. Thanks. It's a plain multivibrator. I guess that
makes sense. It's about the simplest oscillator you could make at these
frequencies.

I'm a bit confused by the amplitude reading. The photo shows the 10X scope
probe is on, and the readout along the bottom shows Vpp=6.56 V. You give
the amplitude as 80mVpk-pk. None of these numbers agree!

The datasheet for the 2N5962 says see the 2N5088 for characteristics. The
sN5088 has the SPICE model. Here's the links:

http://datasheet.octopart.com/2N5962-Fairchild-datasheet-43296.pdf

http://datasheet.octopart.com/2N5088BU-ON-Semiconductor-datasheet-6029.pdf

Thanks for your help.

Ah yes I see it now. Geez, I drew it pretty much in the worst way possible

The scope capture is somwhere in the oscillator. Must've been the collector of the side not going into the bridge.
The signal into the bridge is tapped off on the 33 Ohm resistor that together with the 3300 Ohm resistor forms the load in the other collector.

The 80mVpk-pk was measured across the junctions/bridge windings. That's pretty close to the 2 * 7/9 * 1/100 of the two 7-turn bridge windings in series on the core with the 9-turn primary winding after a 1/100 divider.

 

Thanks for the spice model, by the way. I might model it sometime.

I drew the schematic pretty quickly, there might be some errors. The blended picture should help pick those out.

> Thanks for your help.

I'm glad to share it.

 

[Steve Wilson]

Jeroen <j.ponte@student.utwente.nl> wrote:

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson wrote:
Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the oscillator.
It runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says they
are general-purpose amplifier transistors. The ones in the varactor
amp are marked with red paint, too (a bit hard to see in the
pictures), so AD must've handselected them. Maybe they were just
cheap? I guess the main important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised" as
resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

I found the waveform. Thanks. It's a plain multivibrator. I guess that
makes sense. It's about the simplest oscillator you could make at these
frequencies.

I'm a bit confused by the amplitude reading. The photo shows the 10X scope
probe is on, and the readout along the bottom shows Vpp=6.56 V. You give
the amplitude as 80mVpk-pk. None of these numbers agree!

The datasheet for the 2N5962 says see the 2N5088 for characteristics. The
sN5088 has the SPICE model. Here's the links:

http://datasheet.octopart.com/2N5962-Fairchild-datasheet-43296.pdf

http://datasheet.octopart.com/2N5088BU-ON-Semiconductor-datasheet-6029.pdf

Thanks for your help.

 

[Steve Wilson]

j.ponte@student.utwente.nl wrote:

On Tuesday, September 3, 2019 at 10:47:36 PM UTC+2, Steve Wilson wrote:
Jeroen <j.ponte@student.utwente.nl> wrote:

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson
wrote:
Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the
oscillator. It runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says
they are general-purpose amplifier transistors. The ones in the
varactor amp are marked with red paint, too (a bit hard to see in
the pictures), so AD must've handselected them. Maybe they were
just cheap? I guess the main important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised"
as resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

I found the waveform. Thanks. It's a plain multivibrator. I guess that
makes sense. It's about the simplest oscillator you could make at these
frequencies.

I'm a bit confused by the amplitude reading. The photo shows the 10X
scope probe is on, and the readout along the bottom shows Vpp=6.56 V.
You give the amplitude as 80mVpk-pk. None of these numbers agree!

The datasheet for the 2N5962 says see the 2N5088 for characteristics.
The sN5088 has the SPICE model. Here's the links:

http://datasheet.octopart.com/2N5962-Fairchild-datasheet-43296.pdf

http://datasheet.octopart.com/2N5088BU-ON-Semiconductor-datasheet-6029.p
df

Thanks for your help.

Ah yes I see it now. Geez, I drew it pretty much in the worst way
possible

The scope capture is somwhere in the oscillator. Must've been the
collector of the side not going into the bridge. The signal into the
bridge is tapped off on the 33 Ohm resistor that together with the 3300
Ohm resistor forms the load in the other collector.

The 80mVpk-pk was measured across the junctions/bridge windings. That's
pretty close to the 2 * 7/9 * 1/100 of the two 7-turn bridge windings in
series on the core with the 9-turn primary winding after a 1/100
divider.

Thanks for the readings. That is a strange bridge. The 2N5088 datasheet
shows the Cob is around 4.8pf at 0 Volts. I had a hard time finding the
meaning of Cob, but it shows up in the Toshiba Bipolar Transistors
Application Note at

https://toshiba.semicon-storage.com/info/docget.jsp?did=63512

Cob is the capacitance between collector and base at the specified
collector-base voltage and frequency when the emitter terminal is
open-circuited.

80mV seems very small to have much effect. However, Page 3 of whit3rd's
link shows the input diagram:

<https://archive.org/details/AnalogDevicesDataAcquisitionProductsCatalog197
9/page/n97>

The description says "Amplifier Ein varies varactor capacitances,
unbalances bridge and develops pump voltage output proportional to bridge
unbalance. [...] Matched low leakage varactors inherently eliminate 1/f
noise, give excellent offset and drift specs, and provide 3 x 10^11 Ohms
differential input impedance."

So we are getting some more pieces to the puzzle.

Thanks!

 

On Wednesday, September 4, 2019 at 12:13:25 AM UTC+2, Steve Wilson wrote:

j.ponte@student.utwente.nl wrote:

On Tuesday, September 3, 2019 at 10:47:36 PM UTC+2, Steve Wilson wrote:
Jeroen <j.ponte@student.utwente.nl> wrote:

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson
wrote:
Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the
oscillator. It runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says
they are general-purpose amplifier transistors. The ones in the
varactor amp are marked with red paint, too (a bit hard to see in
the pictures), so AD must've handselected them. Maybe they were
just cheap? I guess the main important thing is the matching.

Nice info regarding the color codes and the capacitors "disguised"
as resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

I found the waveform. Thanks. It's a plain multivibrator. I guess that
makes sense. It's about the simplest oscillator you could make at these
frequencies.

I'm a bit confused by the amplitude reading. The photo shows the 10X
scope probe is on, and the readout along the bottom shows Vpp=6.56 V.
You give the amplitude as 80mVpk-pk. None of these numbers agree!

The datasheet for the 2N5962 says see the 2N5088 for characteristics.
The sN5088 has the SPICE model. Here's the links:

http://datasheet.octopart.com/2N5962-Fairchild-datasheet-43296.pdf

http://datasheet.octopart.com/2N5088BU-ON-Semiconductor-datasheet-6029.p
df

Thanks for your help.

Ah yes I see it now. Geez, I drew it pretty much in the worst way
possible

The scope capture is somwhere in the oscillator. Must've been the
collector of the side not going into the bridge. The signal into the
bridge is tapped off on the 33 Ohm resistor that together with the 3300
Ohm resistor forms the load in the other collector.

The 80mVpk-pk was measured across the junctions/bridge windings. That's
pretty close to the 2 * 7/9 * 1/100 of the two 7-turn bridge windings in
series on the core with the 9-turn primary winding after a 1/100
divider.

Thanks for the readings. That is a strange bridge. The 2N5088 datasheet
shows the Cob is around 4.8pf at 0 Volts. I had a hard time finding the
meaning of Cob, but it shows up in the Toshiba Bipolar Transistors
Application Note at

https://toshiba.semicon-storage.com/info/docget.jsp?did=63512

Cob is the capacitance between collector and base at the specified
collector-base voltage and frequency when the emitter terminal is
open-circuited.

80mV seems very small to have much effect. However, Page 3 of whit3rd's
link shows the input diagram:

https://archive.org/details/AnalogDevicesDataAcquisitionProductsCatalog197
9/page/n97

The description says "Amplifier Ein varies varactor capacitances,
unbalances bridge and develops pump voltage output proportional to bridge
unbalance. [...] Matched low leakage varactors inherently eliminate 1/f
noise, give excellent offset and drift specs, and provide 3 x 10^11 Ohms
differential input impedance."

So we are getting some more pieces to the puzzle.

Thanks!

No problem, this is fun

Interesting. Nothing in the datasheet jumped out at me as a reason for this device. Perhaps it has/had a high dC/dV, low leakage or both.
I always thought the reason for this bridge is that only 2 "varactors" need to be matched, unlike for the P2s fully differential bridge.

There's an AD patent on the concept: https://patents.google.com/patent/US3530390A/en?oq=US3530390

I think I have an AD article on the design somewhere, but I haven't been able to find it yet. It might have been an analog dialog issue.

What do you think of the feedback to the JFET source? My best guess is that it's there to bootstrap the capacitance.

There are teardowns and schematics available of the Philbrick P2 over at the Philbrick archive.
Pease describes the design in "Analog Circuit Design Art, Science and Personalities" by J. Williams et al. He mentions its bridge getting "(...) perhaps 100mV of RF drive (...)".

 

[piglet]

On 03/09/2019 11:13 pm, Steve Wilson wrote:

80mV seems very small to have much effect. However, Page 3 of whit3rd's
link shows the input diagram:

Yes, 60-80mV seems about right. The RF modulation has to be very small
to keep well away from the onset of picoamps forward conduction at
higher temperatures.

piglet

 

[piglet]

On 03/09/2019 11:35 pm, j.ponte@student.utwente.nl wrote:

What do you think of the feedback to the JFET source? My best guess is that it's there to bootstrap the capacitance.

True, it will bootstrap capacitance somewhat but I think more simply it
is there to stabilize the open-loop gain.

piglet

 

[Steve Wilson]

j.ponte@student.utwente.nl wrote:

On Wednesday, September 4, 2019 at 12:13:25 AM UTC+2, Steve Wilson
wrote:
j.ponte@student.utwente.nl wrote:

On Tuesday, September 3, 2019 at 10:47:36 PM UTC+2, Steve Wilson
wrote:
Jeroen <j.ponte@student.utwente.nl> wrote:

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson
wrote:
Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the
oscillator. It runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says
they are general-purpose amplifier transistors. The ones in the
varactor amp are marked with red paint, too (a bit hard to see
in the pictures), so AD must've handselected them. Maybe they
were just cheap? I guess the main important thing is the
matching.

Nice info regarding the color codes and the capacitors
"disguised" as resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

I found the waveform. Thanks. It's a plain multivibrator. I guess
that makes sense. It's about the simplest oscillator you could make
at these frequencies.
[...]

So we are getting some more pieces to the puzzle.

Thanks!

No problem, this is fun

Interesting. Nothing in the datasheet jumped out at me as a reason for
this device. Perhaps it has/had a high dC/dV, low leakage or both. I
always thought the reason for this bridge is that only 2 "varactors"
need to be matched, unlike for the P2s fully differential bridge.

There's an AD patent on the concept:
https://patents.google.com/patent/US3530390A/en?oq=US3530390

Wow! Schematics! Description! Priceless! Thanks!

I think I have an AD article on the design somewhere, but I haven't been
able to find it yet. It might have been an analog dialog issue.

What do you think of the feedback to the JFET source? My best guess is
that it's there to bootstrap the capacitance.

I can't find the jfet source you are talking about. Which file is it in?

There are teardowns and schematics available of the Philbrick P2 over at
the Philbrick archive. Pease describes the design in "Analog Circuit
Design Art, Science and Personalities" by J. Williams et al. He mentions
its bridge getting "(...) perhaps 100mV of RF drive (...)".

Thanks. The paper is at

<http://s1.nonlinear.ir/epublish/book/Analog_Circuit_Design_Art_Science_and
_Personalities_0750696400.pdf>

Your help has been invaluable. The patent especially. Now we have enough
information to spend hours studying this amazing design. It still has merit
for the unmatched common mode range.

 

[piglet]

On 04/09/2019 8:52 am, Steve Wilson wrote:

j.ponte@student.utwente.nl wrote:

On Wednesday, September 4, 2019 at 12:13:25 AM UTC+2, Steve Wilson
wrote:
j.ponte@student.utwente.nl wrote:

On Tuesday, September 3, 2019 at 10:47:36 PM UTC+2, Steve Wilson
wrote:
Jeroen <j.ponte@student.utwente.nl> wrote:

On Tuesday, September 3, 2019 at 9:56:14 PM UTC+2, Steve Wilson
wrote:
Blyat Cheker wrote:

I've appended the imgur post with a scope capture of the
oscillator. It runs at about 200kHz.

Can't find the scope photo. So you have a url?

What is the waveform and amplitude into the bridge?

Thanks

I do still wonder how they found the 2N5962. The datasheet says
they are general-purpose amplifier transistors. The ones in the
varactor amp are marked with red paint, too (a bit hard to see
in the pictures), so AD must've handselected them. Maybe they
were just cheap? I guess the main important thing is the
matching.

Nice info regarding the color codes and the capacitors
"disguised" as resistors, I didn't know about that.

Sure, here's the image: https://imgur.com/gATUoJL
I'm a bit surprised by the bridge drive, it's about 80mVpk-pk.

I found the waveform. Thanks. It's a plain multivibrator. I guess
that makes sense. It's about the simplest oscillator you could make
at these frequencies.
[...]

So we are getting some more pieces to the puzzle.

Thanks!

No problem, this is fun

Interesting. Nothing in the datasheet jumped out at me as a reason for
this device. Perhaps it has/had a high dC/dV, low leakage or both. I
always thought the reason for this bridge is that only 2 "varactors"
need to be matched, unlike for the P2s fully differential bridge.

There's an AD patent on the concept:
https://patents.google.com/patent/US3530390A/en?oq=US3530390

Wow! Schematics! Description! Priceless! Thanks!

I think I have an AD article on the design somewhere, but I haven't been
able to find it yet. It might have been an analog dialog issue.

What do you think of the feedback to the JFET source? My best guess is
that it's there to bootstrap the capacitance.

I can't find the jfet source you are talking about. Which file is it in?

It is the input FET in the OP's handrawn schematic, feedback is from the
emitter of the second BJT stage.

<https://imgur.com/a/EPRpTgq>

There are teardowns and schematics available of the Philbrick P2 over at
the Philbrick archive. Pease describes the design in "Analog Circuit
Design Art, Science and Personalities" by J. Williams et al. He mentions
its bridge getting "(...) perhaps 100mV of RF drive (...)".

Thanks. The paper is at

http://s1.nonlinear.ir/epublish/book/Analog_Circuit_Design_Art_Science_and
_Personalities_0750696400.pdf

Your help has been invaluable. The patent especially. Now we have enough
information to spend hours studying this amazing design. It still has merit
for the unmatched common mode range.

The AD310 common mode range is only good at DC/low-AF since they
penny-pinched and used capacitive coupling from the bridge to the AC
amplifier. A fully transformer isolated design like the Philbrick P2
would be good to much higher volatges and/or frequencies.

piglet

 

[Steve Wilson]

piglet <erichpwagner@hotmail.com> wrote:

On 04/09/2019 8:52 am, Steve Wilson wrote:
I can't find the jfet source you are talking about. Which file is it
in?

It is the input FET in the OP's handrawn schematic, feedback is from the
emitter of the second BJT stage.

https://imgur.com/a/EPRpTgq

Your help has been invaluable. The patent especially. Now we have
enough information to spend hours studying this amazing design. It
still has merit for the unmatched common mode range.

The AD310 common mode range is only good at DC/low-AF since they
penny-pinched and used capacitive coupling from the bridge to the AC
amplifier. A fully transformer isolated design like the Philbrick P2
would be good to much higher volatges and/or frequencies.

piglet

The 310 bridge is transformer coupled. Essentially unlimited common mode
range.

Why do you need wide bandwidth in a circuit intended for pA applications?

 

[piglet]

On 04/09/2019 10:03 am, Steve Wilson wrote:

piglet <erichpwagner@hotmail.com> wrote:

On 04/09/2019 8:52 am, Steve Wilson wrote:
I can't find the jfet source you are talking about. Which file is it
in?

It is the input FET in the OP's handrawn schematic, feedback is from the
emitter of the second BJT stage.

https://imgur.com/a/EPRpTgq


Your help has been invaluable. The patent especially. Now we have
enough information to spend hours studying this amazing design. It
still has merit for the unmatched common mode range.


The AD310 common mode range is only good at DC/low-AF since they
penny-pinched and used capacitive coupling from the bridge to the AC
amplifier. A fully transformer isolated design like the Philbrick P2
would be good to much higher volatges and/or frequencies.

piglet

The 310 bridge is transformer coupled. Essentially unlimited common mode
range.

Why do you need wide bandwidth in a circuit intended for pA applications?

Hi Steve,

The AD310 has transformer coupling from carrier oscillator to bridge,
but from bridge to AC amplifier is capacitor coupled. A glass 22pF ultra
low leakage cap in one leg and a garden variety ceramic 1nF in the other
(cold) leg. I suggest that a balanced transformer design like P2 or P2A
or the AD patent would have better AC CMRR.

Wide signal bandwidth is not the issue but AC common mode interference
could be a problem. Imagine the AC CMRR needed for a worst case dc-like
pA signal riding on 120V 60Hz !

piglet

 

On Thursday, 8 August 2019 20:59:26 UTC+1, Phil Hobbs wrote:

Unfortunately negative resistors don't have imaginary noise.

Cheers

Phil Hobbs

No they have negative noise, it's the negative of the problematic noise you'd expect . Shame that doesn't help any.


NT