Avionics reverse-engineering: spacecraft equipment

[D Straney]

The last couple years I've been keeping an eye out for interesting electronics to take apart in gold-scrap auctions or from industrial surplus sellers.  I've been lucky enough to find some space electronics, and thought other people might be interesting in seeing these too.

Hughes mystery RF box

The seller listed this as coming from Hughes Aircraft, which means it was likely from their Space & Communications Group, which is now Boeing.  From this and other sources (a 1992 promotional photo showing the Hughes Space & Comms group's spacecraft), their main business seemed to be communications satellites.

So, we can expect this to be most likely communications-related.  This particular module has a couple DE-15 connectors for power & control, and generates an RF output of some kind, as well as having another coax that according to the "T.P." label serves as an internal testpoint(??):


DC control/power half
After removing many screws and taking off the top lid, you can see the low-frequency half:


The outer section contains relays, diodes, and filtering components, which probably switch & filter bias power to the RF section, via the many feed-throughs visible in holes in the board.
The inner section contains an op-amp (DIP package, JM38510/13503 = OP27A) and possibly a second op-amp (metal can labeled "OP37034J", might be an OP37 according to one obscure reference).  This inner section, judging from the feed-through connections, appears to provide the final amplification and/or complementary bipolar drive voltages for biasing a varactor in the RF portion (we'll get to that in a bit).


You can see the thick layer of conformal coating, and the very nicely-bundled-and-epoxied wires:


RF half
The other lid, with even more screws for all the separate compartments, holds a wonderful variety of microwave magic:

There's a lot of additional internal shields, but after removing those, the circuitry becomes more clear:


Starting in the top-right corner...
1. Oscillator #1 & amplification, filtering

The metal can seems to be an oscillator: it's labeled "122.542338 Mhz".  This drives a series of amplifier stages, in the form of RF transistors in interesting glass-lid packages.  It's possible, depending on the biasing, that some of these are being driven into saturation to create harmonics for frequency multiplication, but I have no way of knowing.

A filter follows, which has an interesting combination of distributed elements (the rectangular traces) and lumped elements (the spring-looking inductors):

A couple more amplifier stages follow the filter:


2. SRD comb generator & filtering
After this, some frequency multiplication almost definitely happens: the signal goes through a lumped pi filter, to a diode connected in a shunt configuration to ground.  See the middle of this photo:

This is probably a Step Recovery Diode (SRD) or something similar.  SRDs have a reverse-recovery current when reverse-biased which stops very suddenly; if you excite it correctly with an AC current you can create a train of very short pulses, which due to their sharpness contain many harmonics of the input frequency.  For frequency multiplication, you can then apply a bandpass filter to pick out the specific harmonic that you want, while suppressing the others.  In this case, you can see the non-uniform parallel-coupled lines filter directly following the SRD.
I don't know the dielectric constant of the ceramic substrate, but the length of the filter sections (~3 mm for 1/2 wavelength) suggests somewhere in the 20-40 Ghz range.

Oscillator #2 & amplification
Let's back up a bit now before shit gets weird, and look at the cavity at the top-center: this holds an oscillator tuned by a dielectric resonator, or a DRO (Dielectric Resonator Oscillator).  This type of oscillator is common in satellite dish up/downconverters, and uses a high-permittivity ceramic material as an E-field resonant cavity due to the cylindrical resonant modes it can support.  The resonant frequencies depend only on the geometry of the resonator & its permittivity: go here and click on "Read More" on the first article (https://doi.org/10.1016/j.pnmrs.2014.09.003 by Andrew Webb) for an explanation of resonant cavities.  There's also some pretty good diagrams from that article here and here which help visualize what's going on.

In our case, that yellow disc is the dielectric resonator, the trace running next to it loosely couples the signal to and from the resonator, and the gold part to the left of it is the transistor that adds amplification to turn it into an oscillator.  I assume that the transistor's reverse-transfer(/Miller) capacitance is what adds the feedback needed to create the oscillation.

Also coupled to the resonator (better visible in photo below) is a separate trace with a white varactor, which is used to slightly detune the resonator, and therefore adjust the tuning of the oscillator:

You can see that the anode and the cathode of the varactor both go to semicircular RF-blocks, and then to feedthroughs: the op-amp(s) in the DC half seem to drive the voltage across this varactor, to set the fine-tuning control of oscillator #2.

Splitting & mixing
The oscillator #2 output signal goes through a couple stages of amplification, and then enters a branchline coupler which splits the signal along two paths.


One path goes to the output coax connector, through a chain of amplifiers (along the left side in photo):


The other path goes through a circulator to a mystery module.  This mystery module gets its other input from the filtered SRD-multiplied reference frequency from oscillator #1:

Unscrewing and flipping over the mystery module reveals it's made by Watkins-Johnson:

It has a 3rd connection, which goes straight through a feedthrough to the DC side above, where it connects to the board with the op-amps.

What's actually happening here?
The only thing that makes sense to me, with this set of connections, is this:
Oscillator #1 is a stable reference frequency source (generating 20-40 Ghz via the SRD), while oscillator #2 is an inaccurate but tunable oscillator, which is also used as the output of this whole box.  The mystery Watkins-Johnson module is a mixer, which is used to compare the two oscillator outputs.  The op-amps in the control section here adjust the DC tuning voltage on oscillator #2's varactor until the two oscillators match frequency - this keeps oscillator #2 (and therefore the final RF output) frequency-stable in the long-term.  However, the varactor-driver op-amp also sums the feedback control with an external modulation signal from one of the DE-15 connectors, which allows short-term changes to the tuning to produce frequency and/or phase modulation on the final RF output.  Something like this would normally be used as the RF signal source for a transmitter.

There's other ways to produce a stable-but-modulated frequency for communications: you can...

• split your frequency reference into I and Q signals (in-phase, and 90-degrees shifted), amplitude-modulate each one separately, and then re-combine to produce phase and amplitude modulation
• mix directly with your frequency-modulation signal, if you have a double-balanced mixer to suppress the LO in your output
• do multi-stage upconversion like the "exciter" from a SINCGARS military radio I looked at before: this is similar to the previous item
• etc.

However, the I/Q scheme doesn't allow for frequency modulation, and working in the 10s-of-Ghz range severely limits your options for active devices; you'll have a very hard time implementing a good balanced mixer (especially considering the 1994 date code on one of the op-amps), so I can see why you'd go for this dual-locked-oscillators scheme.

Hope this was interesting, let me know if you've got any info or insights I missed.

[D Straney]

Meant to mention, the external "T.P." coax connector is connected to the filtered, amplified, pre-SRD output of oscillator #1 (the metal-can frequency reference).

Also here's a better photo of the SRD: you can see the package standing on end, with one terminal attached to the case (ground) and the other terminal with "pass-through" foil bonds to the circuit sections before and after it.  There's a small gap just to the left of the SRD, on the output side.  This acts as a very small-value capacitor, which serves as a high-pass filter to block all the unwanted low frequencies (such as the input frequency) that come out of the frequency multiplication.