Tag Archives: Micro-Tel 1295

HP 11708A 30 dB Reference Attenuator: less than 0.0005 dB drift per year?

One of the products that have been in the HP/Agilent/Keysight catalog for 3 or 4 decades, or more, the 11708A reference attenuator. Specified at +-0.05 dB, it is a remarkably simple device – it just provides 1:1000 attenuation, chiefly, 30 dB. It’s main application is the calibration of 8484A power sensors, from a 1 mW source – the 8484A needs a 1 µW reference level.

Unfortunately, it doesn’t come cheap, when ordered from Keysight today, at least for a hobbyist’s budget. So I got mine used, aged (30 years?), and at a minor fraction of the cost.

11708a keysight

Before using it for a considerable number of power measurements, it is a good idea to confirm it’s performance. Measuring attenuation to +-0.05 or better is no easy tasks, but fortunately enough, a tractable one, with a 8642A signal generator, and a Micro-Tel 1295 precision attenuation measurement receiver. The Micro-Tel is specified to +-0.02 dB, plus +-0.02 dB for each 10 dB, say, +-0.08 dB. Actual performance, of a well-calibrated and well-heated-up unit is considerably better, but only in combination of other high quality components, like, a stable source (the 8642A has virtually no measurable drift), and, good test cables (using Suhner Sucoflex).

The Micro-Tel 1295 employs IF substitution to determine attenuation, and the IF attenuator works in 10 dB steps. Therefore, for best accuracy, the tests should be done at various power levels, to use various combinations of x0 dB segments, of the IF attenuator.

The results, quite remarkable!

11708a low level

11708a low level2

11708a high level2

One thing to consider for the test – the input and output matching losses. Neiter the source nor the cable/receiver are perfect 50 Ohm terminations – but the 6 dB pads will ensure only very minor losses. Obviously, you need to use high quality pads here, specified to small return loss, 18 GHz parts preferred.

First step – reference measurement is taken without the attenuator-under-test:

11708a test atten 1

Second step – actual measurement is taken with the attenuator-under-test installed between the two 6 dB pads:

11708a test atten 2

Before the start – best to check reproducibility and repeatability. With good cables and hardware, +-0.005 to +0.01 is achievable with the current setup.

Well, let’s say, chances are that the 11708A is +-0.02 off its nominal value, most likely, it didn’t drift at all over the last 30 years.

Micro-Tel Precision Attenuation Measurement Receiver: an all-electronic manual for an almost all solid state device

Look at what I have here, finally, an (well, almost, 1 page missing) PDF manual for a Micro-Tel 1295 Precision Attenuation Measurement Receiver. Including all schematics…

1295 manual

…a pleasure for the eye and joy forever!

1295 a4 if module

1295 a3b6

If you have any rare manuals, let me know, much appreciated!

The 0 to 40 GHz SDR: Micro-Tel 1295+R820T USB RTL SDR

Having repaired two Micro-Tel 1295 microwave receivers recently, I noticed a IF (intermediate frequency) test port – this as a sample of the 30 MHz IF signal, from fundamental mixing of the input with the LO, for 0-18 GHz. Using 2nd and 3rd harmonics, and external mixers, the full range up to 40 GHz can be covered.

micro-tel 1295 if port

The 1295, despite its sensitivity, is actually not build for reception of real-world signal – it is an IF subsititution attentuation measurement receiver. However, this doesn’t mean it can’t be use to receive GHz signals… Recently I have been working on a 2-20 GHz digitally controlled preselector, and adding this to the 1295 will already help to get pretty much excellent selectivity.

0-40 GHz rtl-sdr using a Micro-Tel 1295

Now, a quick test: the IF test port, which is normally terminated in 50 Ohms, needs to be connected to the RTL USB SDR. To avoid overload of the RTL SDR by mirror signals, a little Micro-Circuit PBP-30+ filter was added, the silvery can, on the ESD foam, on top of the receiver.

pbp-30+ elliptical bandpass filter

This filter has a 6 MHz passband, 10 MHz 3 dB bandwidth – plenty for the USB SDR. Using a test signal at 11.02 GHz, with neither the receiver nor the source phase-locked, this is the result:

sdr at 11.02 GHz

Divisions are 100 kHz, so there is a bit of drift. But keep in mind: 0.1 MHz for 10000 MHz, that’s just about 10 ppm! – and a PLL will be added to the 1295 anyway.

After all, maybe a good idea to build a little 2-20 GHz downconverter, using a YIG pre-selector (currenty being developed anyway), a mixer and a LO (possibly using harmonic mixing). Stay tuned!

Micro-Tel 1295 Precision Attenuation Measurement Receiver: cleaned, painted (!), fixed, modified, and fully adjusted/calibrated

The 1295 receiver – before working on the internals, the external parts – the panels – needed a makeover.

(1) Sticky paint removed from side panels, top and bottom panels, using methylated sprits. Imaging scraping off dark green chewing gum, several square feet covered with it. Hope the company that sold this paint is now out of business, that’s what they deserve.

(2) Some more cleaning and sanding, with 400 grit paper.

(3) Primed with self-etching automotive primer. For coating aluminum metal, always use a suitable primer – don’t trust any suggestions on paint cans that it will work without a primer. It won’t.
After some drying, a quick sanding. Not aiming for perfection.

(4) Top coat with a modified alkyd resin. “Hunters green” appears close to the original color shade.

(5) After several hours air-drying, burn-in at about 165 °C, for 60 minutes. This improved adhesion, at least based on my past experience, and no need to wait for days before the instrument can be re-assembled.

(6) Clean the newly painted surfaces with isopropylic alcohol, this gives an even shine, and to confirm that the new paint is fully resistant vs such solvents.

(7) Re-assemble all the small hardware and screws, feets, etc, of the panels!

The other items:

(1) Added filter caps to the YIG driver, when under remote control (more or less a bug in the Micro-Tel circuit).

(2) Added a parallel ot serial converter to the display – the readout values are now transmitted via 2400 baud, via the external control connector. See post in the attenuation measurement section. The circuit involves an ATmega32L which monitors the display for an update, and with every update occurring, it reads out the value, and does the transmission – no handshake.

(3) All frequency related and AFC adjustments, YIG driver adjustments etc. have been performed. Calibration of attenuation levels checked – seems OK – precise calibration, I can only do back in Germany. But seems to be in-spec, and will compare more throughly vs the “master” 1295 – the first unit.

(4) The light of the mains switch, using a T1-1/4 28 V 0.04 V incandescent bulb, with broken filament – replaced by a LED, with an added 1 k resistor in the supply line.

(5) Fitted a spare 2″ display bezel, with red filter – the original one was missing.

That’s the gem, receiving at about 16.260 GHz.

1295 cleaned and painted

Micro-Tel 1295 Precision Attenuation Measurement Receiver: 2nd unit!!

Recently, I haven’t been acquiring a lot of test equpiment, for my own workshop aka museum, because space here in the US is limited, and carrying all these things over to Germany again in 1 or 2 years will be a hazzle. But this time, I could not resist – a Micro-Tel 1295 receiver, for less than 1 ct. per USD 1980s list price! The parts alone, a 2.33 GHz low noise LO, 2-8 GHz Avantek YIG, 8-18 GHz Avantek YIG, a 2-18 GHz broadband coupler, various microwave mixers and attenuators, all of the best mil-spec quality, well worth it.
Also, it will be a great addition to the precision attenuation test set-up: a dedicated receiver each, for the through and reflected power! And, we can safe one coaxial relais (to switch either through power, or reflected power, to a single receiver), and everything will be faster, by almost a factor of 2. The only downside – another PLL will be required, but well, this is just a matter of a rainy weekend.

It arrived well packaged, no damage, except a missing frequency display bezel (which was easy to source, exact fit), but one thing I did not expect: the paintwork on the upper and lower cover, and side panels, has converted into a mixture of honey and chewing gum, a sticky mess, and dark green! So, first task was to strip off this “paint”, which was pretty easy using some methylated spirits, and engage a bit in spray painting. Hunter’s Green.
micro-tel 1295 sticky paint
See the dark green side panel – covered with sticky paint! Now, it is finally clear to me, why the 1st unit, the 1295 acquired earlier, had been re-painted by his former owner….

Note that the lid and side panels have numerous screws and nuts (more than 100 single pieces!) – quite impressive, how little Micro-Tel had to consider manufacturing cost!

One issue found so far: the IF distribution relais had some intermittent noise – most likely a bad contact somewhere. So took out all the board, cleaned up the edge connectors (all gold plated), and moved the connectors around a few times – and, the issue is gone.

A note on edge connector cleaning: this is best done, from my experience, we some special type of eraser. Don’t use anything harsh, abrasive, or natural rubber. It will either scratch it gold coating, or leave traces of residue behind that isn’t going to improve contact resistance and reliability.
Best suited at vinyl erasers made especially for PET film or tracing paper. Prismacolor Magic Rub.
prismacolor magic rub box

These crumble a bit, but the vinyl materials absorbs all the dirt, and can be brushed of easily, with an ESD brush.

The unit is overall very clean, just the frequency calibration/display seems to be quite a bit off. This will be the next step, after a thorough warm up.

And, as for the first unit, I will add a parallel to serial converter for the display, same as for my main Micro-Tel 1295, because I don’t want to use the IEEE-488 bus for this device. It will have the same 2400 baud (TTL-level) serial output. Also, a little capacitor will be added, to limit the bandwidth of the Freq Control/YIG driver amp when in external mode – this seems kind of a bug of this device, because the larger bandwidth only increases noise, and the receiver is not build for fast sweeping anyway.

Attenuator calibration – first real dataset!

Some items of the mighty precision attenuator calibrator setup are still missing, like the automatic auto-zero/through calibration, and the adaption of the reflection bridge (see earlier post), but nevertheless, all parts are now in place to do some first real measurements (and generate, thanks to computer control, more data than anyone could have ever recorded manually, without getting close to nervous breakdown).

The device unter test (DUT). It is a HPAK 33321 SG step attenuator, 35 dB, in 5 dB steps – it is more or less a transmission line, with 3 resistor pads that can be individually switched in and out.
33321 sg step attenuator DUT

Also, note the SMA to SMA connector needed to get things connected vs. a through line. No allowance was made for this connector, it is a 18 GHz qualified precision part and will have very little loss.
hp 33321 sg data
As you can see, it is specified for 4 GHz operation – there are multiple models of these attenuators, both from Weinschel/Aeroflex and HP/Agilent/Keysight, up to 26.5 GHz or more. The 4 GHz models are relatively easily to come by, and others have claimed that these are actually quite usable up to higher frequencies. Let’s see.

While I don’t have exact specs for the 33321 SG model, there are similar models around, and typically, the numbers given by HP are +-0.2 dB at 10 dB, +-0.4 dB at 20, and +-0.5-0.7 in the 30-40 dB range. Repeatability about +-0.01 dB, which is quite impressive.

To be exact, we will be dealing with insertion loss here – not quite attenuation, but close, because no corrections have been made for any return losses (due to the SWR of the receiver and of the DUT, which might also change with attenuation setting).

Now, the test:

Step (1) – the system was calibrated without the DUT, just with the cables (from generator and to receiver) directly coupled (“through” calibration)
Step (2) – the attenuator was inserted, and tested at all the steps from 0 to 35 dB, 0 dB was measured twice. For all steps, 10 readings were taken, 1 per second, and averaged. Standard deviations are very small, showing the excellent short-term stability of the setup:
sdev vs frq at various attenuations

Step (3) – again, a through calibration. The measurements took about 3 hours – the drift was small, and distributed linearly with time over the measurements. Drift is pretty much independent of frequency. Later, there will be a drift correction anyway, by the yet-to-be-implemented auto-calibration feature.

Drift – 3 hours; about 0.1 dB absolute per hour.
drift in dB, 3 hours period

Insertion loss – at all the various steps, relative to “through” connection
insertion loss vs. through connection

Insertion loss – relative to “0 dB” setting. This is relevant for most of the practical cases, where the 0 dB values are stored in a calibration ROM, and the levels corrected accordingly. Repeatability of the 0 dB setting was also checked – standard deviation is about 0.04 dB, but might be much better short-term (more like the 0.01 dB claimed in the datasheet). However, keep in mind, 0.04 dB at 10-18 GHz is not more than a few mm of cable movement, or a little bit more or less torque on a connector.

insertion loss (corrected for 0 dB loss) vs frequency

Deviation of 0 dB corrected loss from the nominal values (5-10-15-20-25-30-35 dB steps)
total insertion loss - 35 db to 0 db, 5 db steps - deviation actual-nominal

As you can see, the attenuator works great well above the 4 GHz, easily to 12 GHz. Still usable up to 18, with some error. This error seems to mainly come from the 20 dB pad. Rather than relying on just the 20 dB pad measurement, some maths were done on the data to determine the insertion loss difference (attenuator switched in, vs. switched out), for each off the pads, e.g., for the 20 dB pad, by subtractions of these measurements:

(1) 20 dB in, 5 and 10 out; vs 0 dB
(2) 5 and 20 in, vs 5
(3) 10 and 20 in, vs 10

So there are actually 3 combinations for each pad that allow determiation of the actual insertion loss, for each individual pad. Furthermore, this utilizes the 1295 received at different ranges of the (bolometer) log amplifier, and with different IF attenuators inserted – and will average out any slight errors of the log amp, and calibration errors of the IF step attenuators of the 1295. For even more cancelation, the source power could be varied, but fair enough.

Results of a lot of (computerized) number crunching:

Insertion loss difference in vs. out, for each pad
attenuation of each pad vs frequency
The 5 and 10 dB pads are performing great, the 20 dB pad – a bit less. Well, there must be a way to tune this a bit – but don’t have a cleanroom here, and the fixtures, to scratch a bit of resistive mass from the pad, at certain places, etc. Wonder how they do this at the factory, and if in fact there is some manual tuning, at least for the higher frequency units.

Deviation from nominal, for each pad
calculated attenuation for each of the pads - actual minus nominal

This is really a quite striking level of accuracy – much better than specification, and also indicates the level of precision already achievable with the still temporary attenuation calibration setup. Up to 12 GHz, no issues at all.
33321 sg pad in vs out values

The 0 dB loss – some might be in the connectors, some in the transmission lines, some in the “through” switches of the 3 attenuators. Simply assuming that there aren’t any losses in the connectors and transmission lines, this is the loss per attenuator switch, when in “through”=”pad switched out” position.
0 dB insertion loss (calculated), per contact

All in all, the best way to use these attenuators obviously is to very accurately measure the 0 dB insertion loss, on a pretty narrowly spaced frequency scale. For the attenuator pads, these are best measured by recording values at various attenuations, and polynomial fits give very good approximation, without the need for a lot of density on the frequency scale, and seem to be merely additive, with little cross-correlation errors.
Sure, such things can all be analyzed with much more maths involved, but I doubt it will impact much the application-relevant aspects, and would be rather just a numerical exercise.

Micro-Tel 1295 receiver: parallel to serial converter – digital readout

The Micro-Tel 1295 has a GPIB (IEEE-488) interface, and in principle, can be fully controlled through this. In principle, but, not with ease; and, as it turns out, the build-in processor is running on 70s hardware, and doesn’t respond well to my National Instrument GPIB card. The only thing I need are the attenuation readings, in dB, same as shown on the front LED display.

Also, these GPIB cards are expensive, and I would rather like to control the whole attenuator calibration rig through one single USB port – also to be able to run in with various computers, not just with a dedicated machine.

In brief, after trying hard, I gave up – there need to be a more practical way to read the 1295 data.

First, how to get the data out, if not through the IEEE-488 interface? The case if fully closed, and drilling a hole, mounting a connector – NO. The modification should be reversible.
But there is a solution – the band selection connector, which is already used to remotely control the band switching, has a few spare pins!

This connector is a sight by itself:
micro-tel 1295 band control (and now serial output) plug amphenol wpi mini hex series 126

AMPHENOL/Wire-Pro “WPI” 9-pin “126 series” miniature hexagonal connector, 126-220; these connectors have been introduced in the 1940s, or latest, in the 50s. Still, available today… but the first piece of test equipment that I have ever seen that uses such kind of connectors. 500 Volts, 7.5 Amp – seems like a lot for such a small connector, at 14.99 USD each (plug only).

So, how to run the full display info over one or two wires? Update rate is 1 reading per second, or 1 reading every 4 seconds, not a lot of data – still it needs to be reliable, easy to use.
After some consideration, I decided to use a RS232 interface, with TTL level logic (rather than RS232 voltage levels-only using a short cable), and running it at 2400 bps, transmitting the data from the 1295 receiver to the main micro. This main controller, an ATmega32L can easily handle one more incoming signal, via its USART, and buffer any data coming from the 1295 before it is requested over the USB bus, by the PC software.

There are 5 full digits, plus a leading 1, a optional “+”, a leading-0 removing signal, and a blanking signal, which is set to low when the display is updating, or when the receiver is not giving a valid reading (over/underrange). Each digit needs 4 bits, binary coded decimal (BCD), so in total: 5×4+4=24 bits. Perfect match for the A, B, and C ports of a ATMega32L. This micro will monitor the blanking signal, and after sensing an updated display, read out the BCD information, convert it into a readable string, and send it out at the 2400 bps, via a single wire, no handshake, or anything. Will just keep on sending.

The easiest way to get the signal was determined to be directly at the display unit (A7) itself.
1295 a6 assembly schematic

There is also some space to fit the micro board, a commercial (Chinese, called “JY-MCU”, Version 1.4) ATMega32L minium board, with USB connection. These are really great, running at 16 MHz, with some little LEDs (which are on port B – disabled for this application), and a bootloader. It just saves a lot of time, and these boards are really cheap, below $10.

A 34 pin ribbon cable, with double row connector, salvaged from an old PC – so the controller/parallel-to-serial converter can be removed from the 1295 if no longer needed, and even the cable de-soldered.

The modified assembly
1295 a6 assembly top
MC14511 CMOS latches-BCD decoders-LED drivers – very common for 70s/early 80s vintage, and still working great!

1295 a6 assembly

Data is being transmitted, no doubt:
2400 bps signal

Now it is just a matter of some lines of code, and soon, some real insertion loss tests can start! Stay tuned.

PLL characterization – final results for the Micro-Tel SG-811 and Micro-Tel 1295 circuits

After some experimentation, measurements, etc. – as described before, time to wrap it up.

The PLL loop filter output is now connected to the phase lock input (the additional 1 k/100 n low pass in the earlier schematic has been omitted), with a 330 Ohm resistor in series. This will remain in the circuit, because it’s handy to characterize the loop, and to provide a bit of protection for the opamp output, in case something goes wrong, to give it a chance to survive.

With the charge pump current adjustments now implemented in the software, that’s the result, all pretty stable and constant over the full range.

The SG-811 signal source
micro-tel sg-811 pll bandwith vs frequency

The 1295 receiver
micro-tel 1295 pll bandwidth vs frequency

Micro-Tel SG-811 PLL: frequency response
sg-811 final gain

sg-811 final phase

Micro-Tel 1295: frequency response
1295 pll final gain

1295 pll final phase

Fractional-N PLL for the Micro-Tel 1295 receiver: some progress, more bandwidth, two extra capacitors, and a cut trace for the SG-811

Step 1 – Programming of the ADF4157, no big issue – fortunately, all well documented in the datasheet. The 1.25 MHz phase detector frequency selected will allow tuning in integer-only (no fractional divider) 10 MHz steps (considering the :8 ADF5002 prescaler).

One sigificant difference to the ADF41020 – the ADF4157 uses 16 steps for the charge current control (0=0.31 mA to 15=5.0 mA).

Step 2 – Checking for lock at various frequencies – in particular, at the low frequencies – the thing is running really at the low edge, 250 MHz input for the ADF4157. However, despite all concerns, no issues, prescaler and PLL are working well even at the low frequency. Quite a bit of noise! Not out of focus…
1295 noisy signal

The PLL is locking fine, but still, significant noise in the loop, and also visible in the 1295 scope display, with a very clean signal supplied to the receiver… bit of a mystery. When the PLL is disengaged, and the 1295 manually tuned – no noise, just some slow drift.

Step 3 – Increased loop bandwidth to about 8 kHz, even more noise – seems to PLL is working against a noisy FM-modulated source…. a mystery. Checked all cables, nothing is changing when I move them around.

Step 4 – Some probing inside of the 1295, and review of the signal path for the PLL tune and coarse tune voltages. And, big surprise – there is a relais (K1) on the YIG diver board, and this disengages a low-pass in the coarse tune voltage line – it is a 499k/22 µF RC, several seconds time constant.

See the red-framed area:
micro-tel 1295 A3B9 YIG driver loop damping

Tackling this through a lowpass in the coarse tune feed line (from the coarse tune DAC) didn’t change a thing – the noise is getting into the YIG driver from instrument-internal sources, or partly from the opamp (U5, LM308) itself, when it is left running at full bandwidth. As a side comment, note the power amplifier – it is a LH0021CK 1 Amp opamp, in a very uncommon 8 lead TO-3 package. Hope this will never fail.

Usually, I don’t want to modify test equipment of this nature, because there is nothing worse than badly tampered high grade test equipment. All conviction aside, 2 X7R capacitors, 100 n each, were soldered in parallel to the R38 resistor, so there will be some bandwidth limitation of the YIG driver, even with the K1 relais open.
micro-tel A3B9 YIG driver board - modified

With these in place – the noise issue is gone.
1295 clean signal

Now, triggered by this discovery – the SG-811 uses a very similar YIG driver board, which also has a low pass engaged, in the CW mode – however, not in the remotely controlled CW mode, with externally settable frequency… easy enough, just one of the logic traces cut, and now the filter stays in – don’t plan on sweeping it with a fast acting PLL anyway.

Back to the fractional-N loop: after some tweaking, the current loop response seems quite satisfactory. Set at 3 kHz for now, with plenty of adjustment margin, by using the 16-step charge pump current setting of the ADF4157. Getting 45 degrees phase margin (closed loop) at 3 kHz – therefore, should also work at higher bandwidth. Will see if this is necessary.

PLL gain
1295 fractional-n loop mag

PLL phase
1295 fractional-n loop phase

PLL measurements continued… ADF41020 locking the Micro-Tel 1295

With the work on the Micro-Tel SG-811 generator PLL mostly completed, some trials with the Micro-Tel 1295 receiver – this instrument has similar YIGs fitted, just needs to be tuned 30 MHz above the actual frequency tuned, because the 1295 is running on a 30 MHz IF (all diagrams have tuned frequencies, not LO frequencies).

After some crude analysis of the schematics, the 1295 seems to be able to handle a bit more PLL bandwidth – so the target set more in the 500 Hz to 1 kHz region, and some calculations were carried out with the ADIsimPLL program, to determine the rough capacitor and resistor values – otherwise, the loop filter is the same as for the SG-811 PLL, also using an OPA284 opamp.

Otherwise, pretty much comparable results (earlier post related to the SG-811), for example (17.8141 GHz tuned/17.8171 GHz LO frequency, Icp setting 6):

Gain (disregard 1 to 10 Hz)
micro-tel 1295 17814100 kHz cpc6 gain

micro-tel 1295 17814100 kHz cpc6 phase

After quite a few of these measurement (doesn’t actually take too long), the results.
adf41020 pll bw phase margin 1295

Phase margin vs. bandwidth
pm vs bw adf41020 micro-tel 1295

Bandwidth vs. charge pump current Icp setting, at various frequencies
bw vs icp at various frq adf41020 micr-tel 1295 pll

Again, a bandwidth frequency^0.7 product could be used to get the numbers down to two parameters – slope and intercept of the bandwidth*frequency^0.7 vs. Icp setting curve.
Finally, suitable Icp settings for a 600 Hz target BW:
bw vs frq adf41020 micro-tel 1295 with Icp adjustment

The result seems quite satisfactory, pretty much constant 600 Hz BW can be achieved over the full 2 to 18 Ghz range, at about 47 degrees phase margin. This should allow for stable operation. No locking issues were observed at any of the frequencies, even with full Icp current.