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.
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
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Dr Philip C D Hobbs
Principal Consultant
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Optics, Electro-optics, Photonics, Analog Electronics
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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.
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:
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
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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:
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."
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:
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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 (...)".
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.
I can't find the jfet source you are talking about. Which file is it in?
Thanks. The paper is at
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.
It is the input FET in the OP's handrawn schematic, feedback is from the emitter of the second BJT stage.
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.
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 !
Thanks that's interesting. I made this 'audio' double balanced mixer with four diodes in a ring. I found that diode connected transistors (B-C shorted) worked 'better' than regular signal diodes.
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