Electronics > RF, Microwave, Ham Radio

Interesting & unusual RF/microwave boards

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D Straney:
I normally get my "shopping therapy" impulses out by buying cheap interesting-looking electronics sold as scrap on eBay, and so besides the avionics in my previous posts, have also ended up with some pretty unusual boards with various bits of high-frequency circuitry.

Here's one to start it off that came through a slightly different route:
Raytheon mystery board
I got this from American Milspec with the intention of removing the RF modules to reuse in my own projects and maybe open one of the duplicates, but eventually realized I wasn't going to have the time to do that and was also moving soon, so sent it back.  Took a bunch of photos first though:

The connectors don't have any function labels, but the card itself is labeled with "1650062-103", which turns out to be a "Receiver-Transmitter Subassembly" (NSN 5895-01-234-0520) made by Raytheon, as part of the AN/URQ-33 radio set.  I couldn't find any references to the AN/URQ-33, but there was also a mention on parts-sourcing websites about the EPLRS, which turns out to be a military location-sharing system that happens digitally over UHF radio.

Let's look at the middle section, which is mostly symmetrical:

The center SMA connector feeds a power splitter, which on either side passes either through a mystery "KWE" module (haven't been able to find any info on these), or into the large "728000-112" modules (no info on these either).  This split signal likely(?) feeds the RF port of the mixers on either side, while the two LO signals come in separately through additional SMA connectors.  The IF signals then appear to go through the 728000-47 RF amp modules, and then the "Hughes"-marked bandpass IF filters @ 315 Mhz.  The 728000-48 limiting-amp modules probably follow these in the signal chain to process the filtered IF signals.

After this, things get much less clear.  The Merrimac modules on either side are listed as electronic phase shifters, but with a 1-6 Hz operating frequency; have to assume this is a typo.  The metal cans nearby are UA733 high-speed (140 Mhz) op-amps, which isn't fast enough to deal with the IF directly (assuming that the 315 Mhz BPFs are in fact the IF filters), but they would be fast enough for data encoded on a 315 Mhz IF.  There may be some kind of phase-decoding loop going on with each channel's combination of a phase shifter module, 2x UA733s and 2x SE521 high-speed comparators (<20 ns), to output a digital data stream.

So far, it looks like this middle section splits up an RF signal and feeds it to two separate receivers.  The LO signals are probably supplied by the identical side sections, which seem to be frequency synthesizers:

The large 728000-174 module is a VCO; no info on frequency range.  It seems to feed two separate outputs, one of which goes through the gold-plated bandpass filter (684-921 Mhz) mounted to the wall.

Programmable Frequency Divider
The column of ICs behind the VCO contains first an 11C90DMQB (equiv. SP8680) ECL frequency prescaler, then a mystery IC (JM38510/06006B), and finally 3x 4-bit ECL counters (10016DM).  This would be the programmable frequency divider portion of the frequency synthesizer; the ECL logic (rather than the "74LS" family used in the slower control sections) is needed to deal with the high 100s-of-Mhz LO frequencies.

There's two 8-bit shift registers (JM38510/00903B = 74LS164) near the rear D-sub connector, one of which has traces running to the ECL counters (through pairs of resistors, which do the TTL-to-ECL level translation; these ECL counters work off of negative voltages).  I think this is a serial control interface which uses 12 of the 16 shift register bits to drive the counter "preset" inputs and therefore set the output frequency, if the counters are chained in count-down mode and use the "underflow" signal both to preset the values again, and as the output of this programmable frequency divider.

Phase Comparator & Control Loop
Two more SE521 dual high-speed comparators sit near the D-sub connector; at least one or two of these must be used as the phase comparator, to produce an error signal between the frequency divider's output and a reference frequency coming in from outside (likely 10 Mhz on the D-sub connector).  The group of discrete components near the VCO is probably what generates the control voltage for the VCO and adjusts it based on the measured phase error.  The two JM38510/31401 one-shots (74LS123 equivalents) next to this section suggests to me that this is some kind of charge pump topology which increases or decreases the voltage on a "memory" capacitor in fixed steps, when an error signal is present.

The remaining 4 bits from the shift registers seem to feed the SNJ5445 BCD-to-decimal decoder, which has 10 different resistors connected to its output.  My best guess is that these resistors set the pulse time for the one-shots, and therefore let the control loop's slew rate of the frequency synthesizer be programmed (setting how fast an error signal can ramp up or down the VCO's control voltage).  The EPLRS Wikipedia page mentions that it uses frequency-hopping for security; because of this, fast frequency re-tuning and therefore fast LO re-programming is likely very important, and so adjusting the control loop gain on the frequency synthesizer would be important for minimizing the time that it takes to lock onto a new frequency.  The two separate receiver channels from the same RF input may even trade off back and forth between each other for fast frequency-hopping, to ensure no gaps in reception: one receiver could be listening while the other is already tuning to the next frequency.

Anyways, there's more photos in the full album, hope you enjoyed.

Neomys Sapiens:
JM38510/06006B should be a OR/NOR GATE, DUAL 4-5 INPUT. At least that is what DLA says.

D Straney:
Oh good to know, thanks.  If it's ECL too might be a way of getting a non-power-of-2 division factor from the prescaler chip, or just be for general control (clock enable, etc.) otherwise.

Rohde & Schwarz CMW 270 RF front-end module
This was from a piece of test equipment to test WiMax transceivers, a wireless standard that never fully caught on and isn't in widespread use anymore,  which is why no one else wanted this module.

It's got 3 type-N ports on the front which correspond directly to the 3 ports on the front panel of the instrument, heatsink fins on one side, and a control connector along with 4 coax connectors which must continue on to the other RF sections deeper inside the CMW 270.

Inside, you can see a control section with a small FPGA (Lattice ispMach) and some linear regulators (the Linear Technology DFNs in the 2nd photo), then a whole lot of RF switches.

This module makes very heavy use of the Peregrine PE42551, an SPDT switch spec'ed up to 6 Ghz and high-ish power for this type of thing (1W or so before it hits the 1 dB compression point).

There's no upconversion or downconversion happening in this module: its purpose seems to be routing signals and setting levels.  From looking at the front panel...

...there's two external RF channels per module, with the middle connector fixed as an output and the other two connectors switchable between outputs and inputs.  A big part of the circuitry here seems to be just switching between Tx and Rx functions for each of these connectors and routing the signals appropriately to those smaller dedicated Tx and Rx connectors for each channel, that feed the mixer & IF stages.

The other big function here is applying gain and attenuation as necessary.  Most of the RF switches involve selectively bypassing attenuators or amplifier stages.  For example:

There's also two sections, one for each RF channel, that look like the biggest power boost, with 2-stage amps right next to large contact areas between the board and the enclosure - I'd bet money that this is what the heatsinking from those fins on the module cover is mostly intended for.

The non-RF section in the middle (partly visible in the next photo) has what I'm guessing is biasing & control for these power amp sections.

I have to admit I'm confused by the circuitry right at the external connectors though:

There's a resistive pad directly on the connector, so it's burning power even when used for Tx (fair I guess, if you can make the thermals work, to improve the impedance match from both ends? add a tiny bit more ESD robustness?).  But there's what look like they'd be final output amp & pre-amp transistors for Tx and Rx respectively: the "O3" device is used in two places, and the "O5" device in the other 3.  I can't see any DC biasing on the inputs or outputs though, and you can see on the middle dedicated-Tx output how the "O3" device's pin connects directly to the type-N's pin, without any DC blocking in between.  The pinouts also wouldn't make any sense as transistors: look at the right-hand connector in that photo, and notice how the same pin (on the same "O5" device) goes to the connector for both the Tx and Rx paths.

Only possibility that makes sense to me is if these are low-capacitance anti-parallel diode pairs to ground, specifically made for RF signal protection.  I've never seen something like that in this package, but then again I'm definitely not a full-time microwave guy so it could be a thing?  I couldn't find any semiconductors listed in the normal places with "O5" or "O3" markings that had the correct package, so this is just my best guess.

Oh also, let's just take a second to admire nicely milled RF enclosures and all the little "tunnels" that connect the different sections:

D Straney:
Remec Transmitter, ED-0421-0 15 Ghz
This is a cute little transmitter module that fits in the palm of your hand.

I'd say it looks like someone removed the connectors, but from lack of mounting points it looks like there were never connectors to begin with.  I'm guessing this could've been part of some sort of rooftop-microwave comms system.

Once we look inside, the layout is pretty clear and the ports are nicely labeled.

Here's a nicer shot, catching the light:

The actual signal path is fairly straightforward:

The LO and IF signals come in separately and get mixed together, pass through a serious bandpass filter to remove all the mixing products except the desired output frequency, and the result then gets 3 stages of amplification with progressively larger amplifiers, until we reach a waveguide transition so that the output can be fed to an antenna (likely some kind of horn shape).  Various DC bias paths can be seen too, feeding the different amplifier stages.

LO path
The LO signal gets special treatment, as it's particularly important to the whole system: a strong LO signal is needed to forward-bias the diodes correctly in a passive mixer and make it work correctly (notice how there's no DC power to the mixer die), and harmonics or other "dirtiness" on the LO signal will create phase noise and other undesirable things for a communications system.  Therefore, the LO gets, after a resistive pad presumably to improve impedance matching, a bandpass filter and its own amplifier to boost the signal right before entering the mixer.  Adding an extra amplification stage here also has the desirable result of keeping the LO signal clean for the whole system - because amplifiers work (mostly) only in one direction, this adds a lot of isolation between the incoming LO signal, which probably is fed to and used by multiple other boards, and the passive mixer whose own imperfections could otherwise inject some of the IF and upconverted RF signals, as well as harmonics of all these, back into the LO for the whole system.

There's also an interesting section between the 1st and 2nd power amp stages, which I'm not sure about but have some guesses.  You can see in the labeled "Limiter?" section from above how the signal is split along two paths (with a standard Wilkinson divider), passes through two small mystery sections, and then is re-combined afterwards.

Inside the cavities are what look like most likely individual diodes.  A DC continuity check shows that the signal is continuous from one side of this section to the other (the diodes aren't in series with the signal), and a diode test mode on the multimeter shows a 0.6V junction from the signal path to ground, but not the other way around.  This all suggests that each cavity contains two diodes in parallel, with their anodes connected to the signal and their cathodes connected to ground.  The bond wires from the PCB trace probably jump down to the first diode's anode, connect across to the second diode's anode, and then back up to the PCB trace on the opposite side.

I'm guessing this would be to limit the power output of the whole device by limiting the signal level going into the final 2 power amp stages, however, I'm not sure why it doesn't use bidirectional clipping with anti-parallel diodes to ground.  I briefly considered the possibility that these could be varactors for frequency multiplication, but it seems very unlikely given that the geometry of the power splitter & combiner are the same (this type uses a quarter-wavelength section) and a filter would be needed afterwards to select the appropriate harmonic and avoid a "dirty" output full of the original signal and all other harmonics.

Splitting up the signal before clamping it with diode(s) serves two purposes, though:
1. The signal gets clipped at a higher power level, because clipping starts when 1/2 the power (1/4 the amplitude) becomes comparable with the diode's forward voltages, instead of when the full power & amplitude becomes comparable with the diode's forward voltages.  Selecting different diodes can also vary the clamping level (/max. allowed power) somewhat, but only over a limited range.
2. Placing identical elements at an odd multiple of a quarter-wavelength apart cancels their effects on the transmission line impedance: see https://www.microwaves101.com/encyclopedias/quarter-wave-tricks#pindiodes for an example.  The two cavities look horizontally separated by roughly 1/4 wavelength (judging by the geometry of the power splitters), and so this could be a way of removing the effects of the diode capacitances on the transmission line impedance.  I don't have the time or network-theory background to know for sure if this works through a power splitter, though, but just putting it out there as a possibility.

Output power sensing
After the final power amp stage, there's a directional coupler (the jagged section) which picks off some small fraction of the output signal to the antenna.  The port at the bottom left has a series diode to ground (if you look closely you can see the diode symbol in copper), which acts as a peak detector and rectifies the output, to turn the sensed output signal into a rough DC indication of RF power.  This section is stuffed full of wideband quarter-wavelength stubs (the paper-fan-looking shapes) which serve as RF shorts to ground, to keep as much as of the RF signal as possible out of the power-sense output so it can remain "clean" DC; there's also some more filtering with discrete resistors downstream.

The external system can then monitor either the output transmit power, or the power reflected back to the final stage amp (which could be used to shut down the transmitter to prevent damage if the reflected power was unusually large, for example).  I believe the way it's connected (with the white RF termination resistor to an RF ground at the top-left port) makes this a reflected power monitor rather than an output power monitor, but I'm not a microwave expert, so take that with a grain of salt.

Anyways, I can dig up the eBay info if anyone's interested in re-purposing this kind of thing, the seller had a bunch of similar microwave transceiver blocks and were open to offers so I got it for something like $20.

D Straney:
On second thought, I think the "limiter" is actually a variable attenuator.  You can see a DC control feeding into this section just before the power splitter, from a little bit of circuitry and a port labeled "VVA" (maybe "voltage for variable attenuator").  This most likely is used to put a variable forward bias current into the diodes to ground, which creates an variable small-signal resistance to ground depending where the diode is in its I-V curve, and produces a variable attenuation of the RF signal.  The same scheme is used all the time in variable RF attenuators such as the Mini-Circuits one I took apart here: https://www.eevblog.com/forum/rf-microwave/a-look-inside-some-mini-circuits-modules/msg4779338/#msg4779338  This would explain why the diode connection to ground is only in one direction, and the variable impedance to ground would provide another reason to place the two diode pairs a quarter wavelength apart and maintain a 50 ohm system.


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