Electronics > RF, Microwave, Ham Radio

Interesting & unusual RF/microwave boards

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D Straney:
Cubic Defense Systems mystery board (large)
Here's one of a couple modules I got from here that were being sold for parts << $20.  "94987", which pops up everywhere, is the CAGE code for "Cubic Defense Systems", for what it's worth.

Because signal paths are nice and easy to follow on single-layer RF boards, we can see that a signal comes in at one end, passing through one of the bottom-side-metal-can amplifiers(?) and then a circulator...

...passes through a couple discrete-transistor amplification stages, with tombstone'd SMT power filter caps...

...and then goes to a directional coupler, where some of the signal gets picked off:

A second signal input goes through an amplifier of its own, with some big blobs of foam to hold inductors in place...

...and then goes to a coupler of its own:

By the way, those small metal boxes on each input & output are confirmed to be filters, with very "ship-in-a-bottle" construction.  This one looks like maybe a bandpass:

Both output signals sampled from those directional couplers then travel to a middle section, where they're mixed together in the "CF049" module, and then boosted by the 2nd bottom-side-metal-can amplifier before being sent out over yet another coax.  Opening it the "CF049" module got messy, so no good photos there, but it had 3 tiny magnetics and a tiny diode bridge inside: a double-balanced diode mixer.

Overall, the block diagram looks like this:

The signal flow vaguely suggests that one of the signal paths is a transmitted RF signal, and the other is an LO signal, and the "monitor" circuitry in the middle is downconverting the RF-Tx signal to an IF to make sure that the output amplitudes of both RF & IF are in a normal range etc. etc.
I'm not sure why the top signal path has the most-heavily-heatsinked amplifier (the metal-can one) coming first in the signal chain, and followed by a circulator.  It might be performing some non-linear function, like a frequency multiplier?

Let's look inside both those metal cans to see what's happening there, because I couldn't find anything useful with the part numbers, and also because finding miniature accidental-hybrid-artworks inside mystery boxes is fun.  As a quick side note, you can see some creative power routing on the bottom of the heatsink here, with grooves milled into it where discrete wires are run for distributing the supply voltage.

Here's the metal can that boosts the mixed output:

It's a pretty standard-looking RF amplifier in most ways, a common-emitter (Q1) with a large spiral coil that forms a tapped auto-transformer for stepping down the voltage from the transistor's collector.  Q3 and associated components form a biasing circuit that attempts to regulate the DC collector current of Q1: R5 works as a current sense resistor, and so when Q1's collector current is too low, the voltage on Q3's emitter increases.  This increases Q3's Vbe (because its base voltage is fixed by the R6/R7 voltage divider) and increases Q3's collector current, which increases the base(/gate?) bias on Q1.

The strange part though is Q2.  I have no idea what this is doing here.  It has a larger die than Q1, and the patterning I can see on the surface makes it look more like a high-frequency transistor, rather than a very "normal" BJT like Q3.  The connections don't make sense, unless it breaks the standard pinout of "bottom of die = collector/drain".  If it really does have the base(/gate?) connection where I showed it, then it seems like it would operate off of a kind of "current sense" for Q1.  Any insights here would be appreciated.

Let's take a look at these nice spiral inductors, and resistive traces in the biasing circuitry:

Now, here's the metal can the precedes the circulator:

The biasing is much simpler here because it appears to be a common-base amplifier, with Q1's DC setpoint determined only by the R2/R3 voltage divider, and R1 (ignoring Q1's Vbe, that is).  The DC-blocking capacitors here are little vertical single-layer flat plates, rather than the big SMT multi-layer ceramic caps in the previous metal can, which suggests that the operating frequencies here are higher.  The step-down auto-transformer/tapped-inductor L1 here makes sense, as a common-base amplifier has a very low input impedance equal to the incremental resistance of Q1's Vbe: so here you get the best performance by stepping down the 50Ω(?) input to match the much lower transistor impedance.

So, I'm still not sure why the whole board's signal chain is laid out the way it is.  This device isn't a mixer or anything else obviously non-linear, and it seems to be used as an amplifier rather than a frequency multiplier: even if Q1 was driven heavily to the point where it saturated each cycle and produced a lot of harmonics, you'd expect to see a lot of filtering following it, to select only the desired harmonic and suppress all the others.  I have to admit that I'm also not sure what the benefits are of a common-base vs. common-emitter amplifier are in an RF context where impedances are fairly constant; the tradeoffs are much more obvious at low frequencies, but I'm not a full-time RF guy.  Might have something to do with noise figure, or specific types of transistors available?

There's nothing else particularly noteworthy in this metal can, except that the inductors are rectangular(!!!) instead of circular:

Fuck, I love microscopes.
You can also see some trimming (or parameter selection?) of one of the resistors, with wirebonds selectively bypassing sections of it to tune the resistance.

Anyways, hope you enjoyed the close-ups.

D Straney:
Cubic Defense Systems mystery board (small)
This is the second of the Cubic Defense mystery RF boards:

You can see there's two identical channels of.....something.

There's a few pieces of semi-rigid coax running over this board, suspended like an elevated railway on some metal posts.  Two of them actually terminate on this board, but the other(s) just seem to be passing through.

There was actually a pair of boards: one marked with "Rev. L", and the other with "Rev. N".  They look just about identical, until you cut open the hybrid modules:

Rev. L (now with EvilMonkeyz) has a more complicated circuit inside the hybrid module:

...while Rev. N has been simplified somewhat:

Next microscope-trip to the local hackerspace, I'm hoping to get a look at the dies inside my Rev. N hybrid and see if I can get a part number from each one.  Meanwhile, though, here's the circuitry on the rest of the board:

It looks like an RF signal comes into each hybrid, when then produces a low-frequency differential output of some kind.  The obvious function would be a power sensor / level detector, but the hybrid internals look a lot more complicated than I'd expect for a simple function like that.
One clue also comes from the lack of local DC feedback on the LM118 amplifier: this means that either...
(1) it's being used as a comparator to produce a digital output: not unheard-of, I guess, but doesn't make sense that it has local AC negative feedback, or
(2) each channel here is being used as part of a larger control loop; for example, automatic gain control (AGC) for the level of the RF signal coming in.

Additionally, there's a couple common connections between the two hybrids, and it looks like they may be supplied only with a negative voltage.  I'll have to see what ID'ing the hybrids' internal dies turns up.

D Straney:
Mystery boards from L3 Communications
Kenton (Evilmonkeyz) and I swapped some duplicate scrap a while back, and so I ended up with these mystery modules.  Based on the construction style and part numbering, they look like they were originally part of some kind of aerospace or military radio system made by L3 Communications (CAGE code 06401, which shows up everywhere on these).

The many wires and coax cables branching off each of these boards, along with their irregular shapes and small size, suggests that each one lived in its own shielded compartment separate from the others.  Would've loved to see what the whole system looked like intact before it got taken apart and the pieces ended up on eBay.

Amp and filter board
Let's get the simplest one out of the way first.

This looks like just a couple amplifiers (one of which is missing), followed by a filter module.  The filter helpfully lists its frequency (12 Ghz) and manufacturer (12855 = Smiths Interconnect).  The "wj" on the amplifier module suggests it's made by Watkins-Johnson, a big radio manufacturer:

There's also a directional coupler on the output for sampling the output signal and bringing it off-board somewhere.  The very non-coaxial-looking wire connection exiting at the top-right (very much not appropriate for a 12 Ghz signal) suggests that the 2-terminal gold device there is some kind of power sensor - this could be just as simple as a fast diode and a capacitor.  It's also interesting to see the "pass-through" ceramic block in the lower-left, where maybe an attenuator would've been placed in a different design variant.

PLL board

The main feature of this board is the Qualcomm Q3036M, which is a mil-spec version of the Q3036 PLL controller.  I couldn't find a datasheet for this chip directly, but there's one for the slightly upgraded version, the Q3236, which mentions being backwards-compatible with the Q3036: https://www.qsl.net/n9zia/omnitracs/q3236.pdf

This contains everything for a frequency synthesizer except the VCO (for flexibility), and the actual analog portion of the control loop, between the frequency/phase-detector output and the VCO input.  This second part seems to be provided by the Philips 5534A op-amp next to it.

You can see a bunch of slightly-unusual passives:

...and a couple metal cans on the bottom:

Wasn't able to find data on either of these, but my guess is that one of them is a stable oscillator that provides the reference for the frequency synthesizer.  I may saw them open soon to have a look inside.
(Edit: the smaller can is an LM126 dual-tracking voltage regulator, probably used to generate bipolar analog supplies for the PLL circuitry.  The larger one looks like the reference oscillator - I sawed off the lid but was immediately greeted by a foam filling...no easy decap here)

VCO board #1

The board is extremely simple, with just a closed VCO module, and a pass-through trace for something unrelated.  The module here is listed as a "SAW Oscillator" with an 862.15-862.85 Mhz tuning range at this semi-sketchy parts sourcing website.

Let's open it up and have a look inside:

You can see the rough arrangement from the schematic: there's an amplifier with a frequency-dependent feedback loop, forming the oscillator.  The oscillator's output gets split off and also goes through a separate amplifier to buffer the output signal, followed by a bandpass filter to clean up any harmonics.

The main source of phase shift (and therefore what mostly determines the oscillator frequency) in the feedback path is the SAW filter, in the gold can.  SAW stands for "Surface Acoustic Wave" (SAW); it's essentially a mechanical delay line but which can propagate even multi-Ghz signals along the surface of piezoelectric material, transmitting at one end and receiving at the other.  The delay is set by the distance between the transmitter & receiver, and the propagation velocity, so it's known for being stable with temperature, and if I remember correctly, being more resistant to vibration and shock than a quartz crystal oscillator.  I'm not an expert on these, though, and they're an entire rabbit hole of physics on their own.

The same structure with two coupled inductors pops up a lot in this circuit: I haven't gone deep enough into the RF world that I recognize all the common circuits off the top of my head, but it sure looks like a lumped-element transmission line at a specific frequency.  A couple articles (1 and 2) explicitly call out this circuit as a lumped-element version of a coupled-line coupler, which makes sense from looking at it.  This is used as a power splitter on the output of the oscillator, to send half the power back through the feedback network, and half to the output-buffer amplifier.  However, I don't understand the use of this coupler in its two occurrences in the feedback network.  My best guess is that it's doing something like generating 180°-separated signals to feed to the tuning varactors, and then recombining them, to cancel out any asymmetries generated by the non-linearity of the varactors - haven't worked through the math though.

In the output filter, the tiny sections of wire across the capacitors to ground are used as inductors: this makes sense when you look at the values.  For a parallel LC circuit to resonate at 862 Mhz (and therefore allow the fundamental output frequency to pass, while shorting others to ground), 15 pF → 2.3 nH.  (It's convenient that these RF caps have the values written on them)  2.3 nH is a very plausible value for that short loop of wire.

I'm not sure what type of transistors are used here, as strangely there's no DC biasing on their inputs.  They might have some kind of output-to-input biasing resistance added internally, or they could be something like JFETs.

VCO board #2

This is similar to the other board, except the VCO module's output gets fed to a Merrimac FDF-4A-750 frequency doubler, amplified by an (upside-down) transistor, and then sent off-board.

Oops, looks like the VCO module here took a dent:

This VCO module is an HO1313 model made by RF Monolithics Inc., and the patent number referenced on the case (US4760352) refers to an oscillator tuned by, again, a SAW filter.  The HO1300 and HO1301 are 700 Mhz and 750 Mhz SAW oscillators in the same series, which look similar even down to the pinout.  The difference, besides presumably the frequency, is the "hi-rel" packaging here which might explain the lack of available datasheets if they gave it a custom part number for the special application.  I'm going to assume that like the other VCO, though, this oscillator works somewhere in the high-100s-of-Mhz / low-Ghz range.

Let's open up this one too and compare the internals:

This one is much simpler than the other, based around just a single amplifier: the µPC1677.  If you scroll to the 2nd page of that datasheet though, you can see it's not just a single transistor, but a whole 8-transistor amplifier circuit, 4 of which are diode-connected likely for DC biasing.

This VCO circuit is different in that it lacks a separate output buffer circuit, and it also uses a lot of winding PCB traces instead of inductors: I haven't measured lengths and substrate thickness, but this suggests it may be working up in the low-Ghz range as opposed to the other VCO.  It also has a resistive power splitter to divide the oscillator output between feedback network and output pin, which is much lossier (dissipates half the output power) but I'm guessing the lossiness also might make it less susceptible to "load pull" on the oscillator output, making the output buffer amp unnecessary? (I haven't worked through the math on that either though)

The varactor-tuned portion of the feedback network is interesting, with 6 identical back-to-back varactors fed the same bias voltage: this amplifies the capacitance variation, and therefore the tuning range, of a single one of these varactors by 6x.  The back-to-back configuration also I think should help cancel out some of the asymmetric non-linearities (= even harmonics) created by the RF signal moving each varactor back and forth slightly along its tuning curve.  The SAW filter here also gets its own supply voltage, which suggests that there's either some kind of electrostatic thing in there that needs a DC bias, or that it has its own internal amplifier (either to drive the transmitter, or the amplify the signal from the receiver).

Anyways, hope you enjoyed - let me know of any mistakes I made interpreting the VCO circuitry.

D Straney:
Cubic Defense Systems mystery board (small) - continued
(Continued from here)
As part of the same local-hackerspace visits where I got some rough die shots from the aircraft voice-warning-generator's hybrid modules, I managed to get some nice views of the dies inside this mystery RF hybrid module from before:

It looks like there's a lot of wirebonded resistors(?) to ground, scattered around on various signals.

Unfortunately this didn't help much: I have no idea what these dies do, as there's no obvious large-scale structures or markings that correspond to searchable part numbers.  But they sure are pretty!


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