Low voltage negative resistance oscillator design, lessons learned

The Apollo Unified S-band communications (USB) used parametric amplifiers on receive, I think using TDs rather than varactors.

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

Clifford Heath.

ather than varactors.

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

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?

Offset voltages could wreak havoc:

This drawing shows one way:

Here are several quirky things all going together:

piglet

But the inverting node was the sensitive one; the noninverting node was usually grounded with a nice low impedance wire. That left the inverting node for making a transimpedance amplifier, converting current to voltage... with an unamplified DC offset added at the output terminal. No havoc in that configuration.

Where did you find that information? I tried to find a datasheet without success. If you have one, can you post it?

The old data catalog here has the info, pages 97ff. I think they called it the '310' with no letters prefixed, and that'd be a hard string to search on.

Thanks. I didn't know you could page through by left-clicking.

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.

AFAIK they did, using X, Ku, or K band pumps. Then downconverting using conventional noisy devices.

To get gain in a para-amp requires the pump frequency to be higher than the input signal?

piglet

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.)

Cheers

Phil Hobbs

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:

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.

Seems I screwed up the imgur post. Anyhow, here's the whole thing:

Thanks. Much appreciated.

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.

There were some self-oscillating TD parametric amplifiers and downconverters. TDs are basically gone, except for some back-diode RF detectors.

A varactor amp needs a separate pump oscillator.

Varactors don't have a negative resistance region.

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

piglet

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