Tag Archives: Micro-Tel

Micro-Tel MSR-904A Microwave Receiver: phase lock test, YIG driver bandwidth modification

Some final parts added to the MSR-904A digital interface/PLL: the actual PLL circuit (frontend), an Analog Devices ADF4157 fractional-N PLL, together with an ADF5002 8:1 prescaler. The phase detector is set at 1.25 MHz, to allow 10 MHz integer-only steps. Some experimentation with other phase detector frequencies might follow later.

Here – the schematic of the PLL frontend. The circit is wired point-to-point, sure enough, with VERY short wires, soldered using a microscoped – hope you have a steady hand. After a quick test (using the MUX output of the ADF4157), the wires and the very tiny ADF gadgets, all sealed with a few drops of epoxy.

msr pll adf5002 adf4157 schematic

On the main board, the PLL loop filter. Build around the remaining half of the already installed OPA2703 (other half used for the DAC output buffer).

msr pll loop filter

With all these parts now put together, to do some basic tests on the PLL – a Gigatronics 605 Microwave Synthesizer was connected to the MSR-904A input, and the LO sample output of the MSR-904A connected to PLL. A sample of the “LO sample” taken by a broadband -10 dB coupler is used to monitor the frequency, using an EIP 545A. The 10 MHz reference output of the EIP is used as the ADF4157 reference.

msr pll phase lock test setup
msr pll test setup 2

The MSR-904A down-converts the signal to a first 250 MHz IF (by fundamental LO), the 250 MHz IF is then mixed with 410 MHz (this can be locked to a 5 MHz signal – not locked at the moment, but the signal is very clean and stable anyway).

The 160 MHz 2nd IF is available at the rear panel, and connected to a R820T RTL SDR. This is a very handy method to monitor noise, and do some basic adjustments on the PLL. Using headphones – and the human ear as a phase noise meter… more quantitative analysis to follow.

Here, the transition from manually controlled CW mode, to PLL controlled mode.
msr-904a locked at 7250 mhz lo

A close-up:
msr-904a locked at 7250 mhz lo 2

For these tests, the LO was locked at 7.25 GHz, receiving a signal at 7.0 GHz (SDR offset set to about 160 MHz).

Note – same as for the Micro-Tel 1295, and the SG-811 – the YIG driver has a bandwidth limit (by a 100 uF Tantalum capacitor – and a 499 k resistor!) that is controlled by a reed relais on the YIG driver. This doesn’t allow low phase noise operation, even with the best PLL. Well, 100 uF is a bit too much. Therefore, a 100 n capacitor was added – this is enough to suppress most of the noise of the YIG driver stage, and still the circuit remains fast enough for full band sweeps at moderate scan rates. Might modify this later, by adding a bit of logic that adds the 100 n capacitor only when the external frequency control is active, but disconnects it during full band sweep, etc.
msr-904a YIG driver board

Micro-Tel MSR-904A Remote Interface: pretune DAC, precision reference, and some auxilliary circuit

Not a very exciting circuit today, but definitely, a very important one: the pretune DAC for the MSR-904A. This DAC will drive the 0..10 V input of the MSR-904A, to set the frequency for a given band. The frequency needs to be set to about 1 MHz or better, and the DAC needs to be virtually free of noise – any noise will be converted to phase noise, and cause a lot of hazzle for the PLL circuit to be added later.

Rather than a dual supply, the intention is to use a single +18 V supply for the whole remote control circuit. Therefore, we need a few linear regulators, to derive the +5 V for the digital circuits, including the ATmega8-16, and a positive voltage of about 12 V, for the analog circuit. The output driver for the pretune (0..10 V) uses an OPA2703 rail-to-rail opamp. So, I decided on a 11.4 V positive supply, for convenience of resistor values available – 270 Ohm, and 2k2, for a LM317 regulator.

The DAC, a Texas instrument DAC8831. A highly linear device – 1 LSB of INL error. Low noise, low power. The DAC is connected to a +5 V precision reference, a MAX6350. This is a pretty stable and low noise reference, very much recommended for 16 bit converters.

Well, not much more to say, here is the schematic:

msr-904a interface pretune circuit and power supply

And, a quick glance at the board:

msr-904a interface pretune and digital control

There is some space left on the board – for an ADC (to monitor signal strength), and for the PLL power supply (needs 3x 3 V, UA723 – for low noise), and a 10 MHz/5 MHz distribution circuit.

Micro-Tel MSR-904A Microwave Receiver: remote control (digital interface)

The MSR-904A has a remote control interface, to control most of the front panel settings by TTL level signals – operation mode, band, filters, IF attenuator, detector. All in all, 22 signal lines are needed.
The circuit will also need provisions for latter addition of the PLL filter and PLL control – just a few digital lines. All will be controlled by a single USB interface.

First, we need a cable – the MSR-904A uses a DSUB-37 connector, but not all pins are used – so an adapter cable was fabricated to convert this to a much more common (and available) DSUB-25:
msr-904a remote control cable
– quite a few wires!

msr-904a remote interface wiring and register layout

The digital control is implemented by a set of three 74LS164 shift registers, serial in, 8 bit out. These registers are very fast, can be set in a few microseconds. The three registers are named 1-Q0 (LSB of register 1) to 3-Q7 (MSB of register 3).
The micro is an ATmega8-16PU, running at 16 MHz – this has plenty of power to handle the USB interface, the digital control, and later, the PLL loop. There is also a standard 10 pin ISP header, not shown in the schematic.

msr-904a digital interface schematic

The circuit – build on a perfboard. No plans to fabricate a PCB, I don’t anticipate a big demand for MSR-904A remote control units, but still it should last many years. This is why a proper FR4 perfboard with plated-through holes is used.

msr-904a digital interface

Micro-Tel MSR-904A Microwave Receiver: a broken trace, a replaced pot, and an escape proof guarantee

First of all, mystery solved – the remote enable input for the IF attenuator. After quite painful tracing of wires and disassembly of the logic boards, a broken trace! Just a little crack, but big enough to block the electrons’ flow. A little bit of solder, and then, suddenly, the MSR-904A’s IF attenuator can be remotely controlled.

Today, a also the dual pot for the F2 adjustment arrived – fitted, also this, working again.

But most interesting, a little box, with a label not seen before:

msr-904a mains cable label

The part has a NSN number, original unit price was no less then 71.38 USD!

Enclosed – the most special mains power cable I have ever seen:

msr-904a mains cable
-it was well packaged in a multilayer heat-sealed bag.

And, a label, which will be kept – Federal Prison Industries: Escape Proof Guarantee
unicor federal prison industries escape proof guarantee

Micro-Tel MSR-904A Microwave Receiver: crosshair, imagesetting film printing, remote control input

Some more progress related to the MSR-904A:

(1) The crosshair. The old one is badly damaged.
msr-904a crosshair

Sure, I could just use a laser or inkjet printer, but with a laser, the resolution is not good-doesn’t look sharp enough. With inkjet, I doubt it would be permanent enough, and also there, the printout is never sharp enough. So I decided to go for the solution that also Micro-Tel used, so-called imagesetting film. Back home in Germany, not problem, but here – first I need to find a source. Turns out, not too difficult, about 20 USD for 8 pcs of crosshair (minimum order fee), a company located in New Jersey, not around the corner, but not far. 2400×2400 dpi, acetate/emulsion film.

With a film printing service identified, we need to get the digital data, of the crosshair. With the human eye being a pretty precise tool to determine even small differences, and aiming for perfection with the replica – first, determined the line width and distances with a measurement microscope. Such a little microscope is extremely handy, I use it all the time for inspecting circuit boards, etc.

msr-904a crosshair measurement microscope

msr-904a crosshair measurement
one of the big divisions: 0.275 mm (26 pt at 2400 dpi) wide. Line is about 0.125 (12 pt at 2400 dpi) wide.

Here – a reconstruction drawing:
msr_crosshair

The new prints should be in the mail tomorrow!

(2) The remote control input: a 37 pin connector! Fortunately, not all wires are connected (x in the draft).

msr-904a remote control input

Controlling the function, bands and IF bandwidth, and detector characteristics (log-lin), all fairly straightforward. All of these inputs seem to have pull-up resistors, so grounding them works fine to switch. This is quite hand for control via optocoupler – no external voltage required.

The IF attenuator – traced the lines to the control board, it is a BCD control input, 2×4 bit. Unfortunately, I can’t get it to switch… the ‘enable’ signal doesn’t seem to reach the control board – more effort will be required to trace this last line! – Solved: a broken trace on one of the logic boards!

Some of the pins, despite having wires attached to them, remain unidentified – or might be reserved for options not implemented. Except for the IF attenuator control enable signal, all functions needed have been identified anyway.

msr-904a remote control input pinout

(3) The remote frequency control input – analog voltage: quick check with a frequency counter connected to the LO sample, and a DC voltage supplied – it is a 0 to 10 V input.
Scaling of the input voltage can be adjusted on the A6B2 board, R56 is for the mid-range adjustement (offset – supply 5 V and adjust for mid-band frequency), R68 is the gain adjustment (set at 0 V, and adjust for lower band limit; check setting at 10 V – should be at the high end of the band – and it really is).

Micro-Tel MSR-904A Microwave Receiver: some progress

With the basics done (power supply, potentiometer), a few hours were spent to get everything tuned up.
And, quite amazingly, it is receiving:

msr-904a first sign of activity
– notice the dirt, and the sticker residues. Also the crosshair (which is printed on a piece of plastic foil), will be replaced.

Finally, the exterior. The front panel, easy enough, all brushed and cleaned with diluted isopropyl alcohol.

The top, bottom and side panels with the sticky green stuff – all the old junk (“paint”) has been removed, my soakin the panels in methylated sprits, sanding, solvent cleaning, sanding. Then, a layer of aluminum primer (self-etching automotive primer). Followed by a light sanding, and a layer of ‘Hunters Green” alkyd paint. After 10 hours of air drying, final curing at about 150-170 deg C, for a bit over an hour.

All in all, quite an effort. The result –
msr-904a panels - newly painted

Missing items – one fastener like this – no idea where to find, seems like a part from the aerospace industry.
micro-tel quick release fasteners
Should you have any of these around, even of somewhat different length, or if you know a source, please let me know!

msr-904a receiving at 8.1 ghz
Sweeping test around 8.1 GHz – with the refurbished panels installed.

Two more handles are still needed – either need to get spare handles from a parts units (which may be impossible to find), or ship the MSR-904A with 2 handles only, and provide the remaining two later, once I had a chance to fabricate them back home in Germany. At least, I have the exact dimensions measured, just a matter of some CNC milling.

msr-904a receiving AM modulated signal at 8.1 ghz
-this is a test using a 8.1 GHz AM modulated signal, with about 1 kHz modulation frequency. Carefully checked the IF chain (different chains are used, depending on filter setting) – the MSR-904A uses 250 MHz, 160 MHz (by mixing the 250 MHz IF with 410 MHz, from a low noise LO), and 21.4 MHz (for the 100 kHz BW setting).
All seems to be functional.

With the receiver now basically functioning – some weekness of the AFC circuit alignment, and the frequency control was noticed. Therefore, some more effort was spent on the frequency control and AFC circuits, and the tuning indicator circuits. Really tough without any instructions or schematic.
There are some nice indicators on the front panel, LED bar graph displays – one for signal strength, and one for tuning.

These displays, now, in working condition and properly adjusted, are great fun to use. They are extremely responsive – nothing to compare with the time lag and sluggish nature of a modern SDR.

After several hours – here, receiving at 6.1 GHz, with 1 MHz bandwidth, and the AFC keeping the frequency, counteracting artificial drift:
msr-904a receiving at 6.15 GHz with AFC active

Monitoring of the AFC control is by looking at the IF frequency, 250 MHz (on the EIP 545A counter), derived from the (non-phaselocked) MSR-904A LO frequency, minus the RF input frequency (from the Gigatronics 605 Microwave Synthesizer; the EIP 545A is locked to the 10 MHz signal from the 605).
This setup allows me to check for any drift of the MSR-904A IF chain (and AFC, if activated), to 1 Hz resolution.
Had it running now for several hours, no issue, signal stays perfectly tuned.

The only remaining item, internally, is the alignment of the cross-band assembly – still lacking one CD4051 multiplexer circuit – which is on its way. A quick check with a CD4051 taken from another board showed that there is no defect, the board just needs some alignment of the band-to-band transition points. The crossband assembly allows a full 0.5-18 GHz sweep, with automatic band selection.

Micro-Tel MSR-904A Microwave Receiver: the monitor output

The MSR-904A has a few outputs (and inputs), most of them, easy to identify, but two, are pretty much a mystery to me, with no schematic – their function is clear, but with all these wires TTL logic boards -difficult to guess the pinout.

msr-904a monitor output

The more easy thing first, the monitor output. Arguably, this was intended to be connected to storage displays, digitizer, chart recorders, or the like – to more permanently record the activity over the bands.

So, what do we have. Using a scope, and a multimeter, and activating more or less all the functions of the apparatus, that’s what I found out.

msr-904a monitor output - pinout

Sorry for the rough draft, but any questions, please ask – for the given purpose, good enough.

The only pin that doesn’t do much, is pin 9 – always stays on logic 5 V – maybe a +5 V supply line? Doesn’t seem to be an essential function, anyway.

Micro-Tel MSR-904A: RF tuner block diagram and component specs

Just to learn a bit about the way Micro-Tel was doing their engineering, let’s have a quick look at the way in which it functions, at least for the RF front end. After some study and tracing, here is the block diagram:

msr-904a rf tuner block diagram
Some parts don’t have model numbers – because these are hidden, and I don’t want to take the thing apart.

Luckily quite a few of the parts are still available, and datasheets are available for most. The YIGs are Avantek, have a customer part number, but I assume, essentially, slightly modified line item parts, with some specific specications. The only really uncommon part, is a “TREK MICROWAVE” 0.48-2.01 GHz 3-stage YTF. Didn’t know that such low frequency YIG filters existed, this has serial #00003.

YIG filter 4021-104

Even more, TREK doesn’t ring a bell for me, and the filter really looks very much like Systron Donner technology, with the characteristic color, and square-cube shape.

As it turns out, TREK acquired the YIG division of Systron, some time around 1984… so, this mystery solved.

The only part where no data seems available is the 2-18 GHz mixer, Avantek SX83 series, but can’t find a -1612 model anywhere.

Some datasheets:
5B120-2330_25-O_OP – 5915011438953 _ chebyschev 2330 MHz 25 MHz bw

k&l 5L120-300-0 low pass 300 mhz

8L120-2050-0 low pass – 5915012428744 _K&L filter

narda 4244-6 082-Couplers

a34 datasheet

qbh 101 amplifier datasheet

narda 4203-10 coupler

Micro-Tel MSR-904A Microwave Receiver: the big box arrived!

Look what I found on the doorstep yesterday:
msr-904a big box

Wrapped in 20+ feet of bubble wrap, nothing less than a Micro-Tel MSR-904A Microwave Receiver, needing some TLC, later, to be added to a special equipment collection elsewhere.

The MSR-904A. Arguably, it is the last member of a series of 18 GHz+ receivers, build by Micro-Tel at Baltimore, MA, and intended for surveillance work, by governmental agencies. If you ask the right people, these receivers are pretty famous, and have been considered a strategic item for a long time.

They are build using all discrete parts, and hardwired CMOS and TTL logic. After all, many parts, but if you have seen other Micro-Tel instruments, not too unfamiliar. Some say, 80s technology, but actually, is is build in time-less style – from the best components available (not only at the time – these components, YIGs and filters haven’t really improved since).

Some performance data:
Frequency range: 0.5-18 GHz – fundamental mixing; fully YIG pre-selected over the full range (using 18 dB drop-off filter, i.e., three YIG spheres; one preselector for 0.5-2 GHz, the other, 2 to 18 GHz).
1st Image rejection, 70 dB, and 65 dB at above 12 GHz.
IF rejection: >70 dB
IF filters: 100 kHz, 1 MHz, 5 MHz, 30 MHz – quite handy.
IM3: about 5 dBm
LOG and LIN detectors
AM and FM demodulator
Spurious: 90 dBm at input equivalent over full range.
Noise figure is about 20 dB

Note: All in all, 3+3+1+1 = 9 YIG spheres are used, and an uncounted number of filter crystalls. The 100 kHz 21.4 IF filter, it’s quite impressive.

In the 2 to 18 GHz range, a 250 MHz-21.4 MHz IF chain is used, with LO 250 MHz above the signal.
For 0.5 to 2 GHz the signal is mixed with an additional 2.08 GHz from an auxilliary LO. I.e., LO frequency is 2330 MHz (2080+250 MHz) above signal.

The other great things about it:

(1) Fully fundamental mixing, using YIGs – lowest phase noise possible. Fully preselected.

(2) Unit has a 250 MHz IF output, with about 40 MHz bandwidth – this makes this unit ideally suitable as down-converter, if you want or need to receive at medium to high GHz frequencies. Can be directly fed to any SDR for demodulation. The MSR-904A has very small group delay, seems pretty suitable for handling of digital modulation schemes.

(3) It is fully remotely controllable, and has a phase lock input – will hook it up to a ADF41020, and/or a fractional-N PLL, same PLLs as already developed and tested for the Micro-Tel 1295 receiver. Such PLL unit will go along with the MSR-904A, once the repair and proper adjustment and testing is finished. Micro-Tel used to offer a frequency stabilizer (PLL) for the MSR-904A, but I have never seen one offered. If you have one, please let me know!

Two downsides – NO serice manual, no manual or documentation at all. If you have one, even if only for another MSR unit (MSR-901, MSR-902, MSR 903), please, let me know.

Second downside – the condition. Well, there don’t seem to be many of the MSR-904A around for sale any more. One unit I know off, but it doesn’t have the panoramic (scope) display. Other might be available, at outrageous cost. This unit was sold even blow the market value of a fraction of the components.

msr-904a as received - front

Note the tuning know – different from the typical Micro-Tel style. But nevertheless, seems to be the original, unmodified part.

msr-904a as received - top

msr-904a as received - bottom

msr-904a as received - back

The full repair, it will be a major job, because currently, it is a bit beat up – I wish, the earlier owners would have treated it a bit more carefully, and Micro-Tel should have never touched the green paint that just isn’t lasting and a sticky mess on a good number of their instruments – fortunately, only the panels are affected, and these are easy to remove – re-painting already in process!
Also, it doesn’t seem to work well, powers up, but seems to have a leaky supply – keeps tiggering the RCD. It needs a through inspection.

Cosmetically at least, the inner working are in much better shape than the exterior would suggest.

Center – edge-connector boards, mainly YIG driver and analog control. The metal box on the right – the RF box with the microwave stuff. The other items – IF converters, detectors and so on. Everything: very well shielded.
msr-904a top view

2.08 auxilliary LO. Mixers.
msr-904a aux LO filters mixer

A Narda 2-18 GHz broadband -10 dB coupler. Still available from Narda today!
msr-904a narda 4203-10 LO sample coupler

The preselectors: S082-1630 (2-18 GHz, might work up to 20-22 GHz), and a custom Systron Donner 0.48-2.05 GHz YTF.
msr-904a preselectors