8663A Synthesized Signal Generator: a 1 year long repair story

2015/01/28 Simon 2 Comments

Over a year ago, I got hold of a defective 8663A. It seemed beyond repair, but hardly any equipment is, provided, you put in sufficient effort. The 8663A is certainly worth any resonable effort – it is a marvelous piece of test gear, providing full evidence of human ingenuity. It is said that a team of no less then 20 of the best HP engineers took about 5 years, with full support of the mighty HP organization at the time, to develop the 8662A and later the 8663A generators. Ever since then these were the gold standard for any low close-in noise source, for phase noise measurements, and so on. Keysight is offering a replacement now, the E8663D, about USD 50k, nothing compared to the 8662A or 8663A, for its historic value, and comparing the sheer mass of metal, the amount of gold on the assemblies, and the variety of strange little parts.

After some analysis, these were the main defects of the unit discussed here:

(1) A missing attenuator. The 8663A uses a pair of mechanical attenuators; these come in a set, together with a calibration ROM; with one of the attenuators missing, we might need to check flatness and level accuracy once replaced.

(2) Some intermittent failures of the A6A4 output sum loop. Seems to depend on frequency but not limited to any particular range. Supplied some test signals to the A6A4 assy and the fault really seems to reside with this assembly.

(3) A defective output amp. P/N 08663-67002. This is a real disaster. The output amp is a microwave microcircuit, with HP GaAs FET transistors. Needless to say, such assemblies were very expensive already at their time, and spare assemblies, despite long search, are fully unobtainium.

First things first – the attenuator. Found a spare attenuator that should be resonably close to the orgininal one. Did a quick check – it has very flat frequency characteristics anyway, so the ROM calibration coefficients might not be too significant (the 8663A has +-1 dB level spec, but typical accuracy appears to be more like +-0.2 dB; relative levels about +-0.1 dB). Someone tool the attenuator from the ‘dead’ unit, including the bracket – well, I didn’t spent too much time (as you can see) to fabricate a replacement. Good enough.

The intermittent fault of the A6A4 assembly – these assembly seem to be the weak point of the 8662/8663 series – I have already fixed a few of these. With schematics around, no big issue to fix – still it took a few hours this time. Turns out, the pre-tune DAC, which is a discrete circuit using 4 FET switches (1 per bit), showed erratic behavior. This was traced to one of the FETs, of the common 1855-0020 type – I took one from an early 80s HP parts units (8569A).

This is the bad guy:

The most critical item, the 08662-67002 amp. This has a low frequency (<120 MHz), and a high frequency input (>120 MHz), which are routed to a common output, providing about 20 dBm of power, at low distortion (about 35 dBc), over the full band from DC to 2.5 GHz.

After opening up the microcircuit, it is pretty clear that the last of the FET transistors is blown, and shorted to ground. This is all sapphire substrate, wire bonding, high frequency art. Beyond my capabilities (do you have a wire bonder at home, and a steady hand, and the skill and knowledge? Please teach me!). However, this world is not all bad, and rescue came along, back in good old Europe – in its South-West corner.

Turns out that a HMF-1200 is a suitable replacement for the proprietary-unknown original HP part.

Here the work of the kind friend, who certainly has tremendous skill and is a master in his field:

This is the associated board, with a PIN switch, and some bias regulators.

After all these repairs, and some adjustments (which took another few hours; including amp bias, lock detectors, ALC, FM VCO, …), the machine is working again – uptime so far, 48 hours at full power – consuming 500 Watts, and 100 mW at the output. Like powering a 100 W light bulb, from a 500 kW supply…. not quite efficient but a good heat source for the house, during these cold winter days.

To come: some flatness and level checks of the attenuator, but don’t expect any bad surprises.

An interesting document, found during the search for spare assemblies – the US Air Force also seems to be looking for repair, for 4 pcs of the amplifier assemblies, for their F15 Tactical Electronic Warfare System Test Set, P/N 001-006730-003.

May be a good business opportunity, but not for me!

Micro-Tel MSR-904A Microwave Receiver

Micro-Tel MSR-904A Microwave Receiver: AM detector/AGC circuit fix, A3B5 assy

2015/01/17 Simon Leave a comment

After fully refurbishing and fixing the MSR-904A receiver, and some months of service, another look at the AM detector. It is working fine, but at times, is seems to have some microphonic resonance and random shifts of level. Nothing that prevents use of the machine, but a small flaw worth correcting.

Locating the faulty assembly – pretty obvious when knocking at the A3B5 assembly. This takes in a sample of the 250 MHz IF signal, and performs the AM demodulation, and also generates the AGC signal.

These are the inner workings, the signal is first attenuated, about 20 dB, then amplified, about 25 dB, using an Avantek GPD-202.
GPDGPM gpd-202
The signal is then futher amplified via a tuned transistor amplifier.

Demodulation is performed by a zero-bias HP Schottky diode, a HSCH-3486. This was state of the art at the time, and still, there aren’t many diodes around that are any better.

hsch-3486 hsch-3206 hsch-3207

Turns out that this diode must have suffered damage earlier, and it was fixed with a drop of epoxy!

The HSCH-3486 are not so common any more (with the SMD package HSMS-2850 being widely used), here a comparison:

hsms-2850 hsms-2860

The HSMS-2850 and HSCH-3486 use the same technology – the HSMS-2850 is more or less a SMD version of the earlier HSCH-2486 (glass package, similar to DO-35).

Now, should I replace the flaky diode with a HSMS-2850? Well, after some thought, rather keep the parts as they are, for such a rare instrument. With some effort, this is what I found:

Glad I didn’t have to buy at USD 20 each, that’s what some vendors are asking for.

A bit of soldering, without taking the whole assembly apart:

Finally, a quick sensitivity and microphonic test – no issues at all.

Quite interestingly, someone must have fixed this assembly before – note the ECG opamp, 1991 datecode (it is a LM1458 replacement; for the AGC circuit).

Various

ADCMP580 Ultrafast Comparator: it’s really fast!!

2015/01/12 Simon 1 Comment

Some experiments with a tiny part that has remarkable qualities: the Analog Devices ADCMP580. It is a comparator, and a very fast one indeed, based on a SiGe semiconductor.

Its features are true extraordinary – everything specified in GHz and picoseconds.
Well, can this be made work without a lot of money invested in a special HF or microwave board? Yes, it can. You just have to keep the wires short:

This test board was constructed by first soldering thin wires to the chip, and then mounting it on a small piece of perf board, with some epoxy glue. The output is wires, within less than 2 mm, to a (50 Ohm) SMA connector.

Measuring the performance characteristics is not an easy task. I used a 54750a sampling scope, with a 54751a 20 GHz bandwidth plug-in.

The comparator is fed by a 8642b signal generator, which is also used to trigger the 54751a plug-in.

At the output, a 13.5 dB attenuator is used, at least for some of the tests, to get best output SWR (to avoid issues caused by reflected signals). However, as it turns out, the 54751a has quite reasonable input SWR, and the connecters are better than it first seemed. So the attenuator can be left out. Still good to have some attenuators at the 54751a input, remember, this can only handle +-2 V, and no ESD!

In these tests, “squaring up” a 500 MHz and 2 GHz (!) signal. Rise and fall time are about 50 ps, not bad at all for the simple construction.

FFT shows bandwith to 10 GHz and up.

Output power is also quite useful, 400 mV Vpp.

Miscellaneous

LinuxCNC EMC2 HAL Files: 3 axis mill, 2 axis lathe with encoders, jog wheel, axis compensation, camera view

2015/01/11 Simon Leave a comment

Due to frequent requests – here are the configuration files for my LinuxCNC (EMC2) controlled mill and lathe.

The mill is a 3-axis machine, with stepper motors and jog wheel (see earlier post).

emc2 linuxcnc fkm 3 axis mill hal with jog 150101

The lathe has 2 axis, stepper motors, and digital readouts. No feedback on the readouts, but they are great for highest precision work. Configuration files also include the setup for spindel-synchronized movement and spindle speed readout. I have run spindle-synchronized toolpaths for cutting regular and tapered threads with no issues at all, up to a few 100 RPM. The GUI (axis) is also configured for use with a little camera that is very handy to set the coordinates of the tools.

emc2 linuxcnc jet lathe with optical scale and spindel index 150101

Any questions, please ask. These files are meant as a source code collection for you to code your own HAL files, etc.; if you need help with a particular configuration, feel free to contact me.

Please consider that some fragments of the code might be copyrighted by others – however, I have modified it so many times that it is virtually impossible to trace back.
My contribution to these HAL files: You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission. The work may not be free of known copyright restrictions in all jurisdictions. Persons may have other rights in or related to the work, such as patent or trademark rights, and others may have rights in how the work is used. I make no warranties about the work, and disclaim liability for all uses of the work, to the fullest extent permitted by applicable law.

Various

Rong Hua (Bianhuan) “50 Watt” Travel Adapter: not enough iron, for this “wattage”

2015/01/10 Simon 2 Comments

In interesting find, Made in China, a 50 Watt travel adapter.

This is used to convert 220 V (despite the US plug!) to 110 V – same device also seems to exist for conversion from 110 to 220 V.

The build quality is exactly what you expect for less than USD 7. A crude plastic case, two screws, no protection circuits or fuses, and a rather small transformer.

Total weight of the transformer – about 140 g.

Looking at some transformer tables, this is about 6-10 VA (=”Watt”, if you wish) nominal size. For autotransformers like this, the nominal size needs to be converted to the actual power rating, by using the conversion ratio (voltage ratio). P_nominal=P_actual*(1-voltageratio). I.e., for a 220 to 110 Volts transformer, the ratio is 0.5, and a 10 VA nominal transformer can handle 20 VA if configured as an autotransformer.

Quite obviously, 20 VA is not 50 VA – please use these these transformers with great caution, and only for really small appliances. Never leave it plugged in unattended, it might catch fire any time if overloaded, or if it fails!

HP 8642B Synthesized Signal Generator

HPAK 8642B Synthesized Signal Generator: backlight replacement

2015/01/06 Simon 2 Comments

The 8642B is an excellent generator, very clean, at least at offsets >1 kHz, hard to beat. It is also very heavy, thanks to a special modular concept that HP was pushing at the time. Their intention was to make the unit more serviceable, with the result that the generator is super heavy, and so expensive that it never was a real commercial success for HP. Frequency range is from below 100 kHz, to 2115 MHz. Pretty useful, with amplitudes from -140 dBm to 20 dBm.

The generator has a rather large (for the time) LCD display – fully story can be found in the HP Journal, December 1985.

The backlight has a very thoroughly designed light diffusor, which directs light from two 5V axial bulbs evenly to the LCD. Sure enough, these bulbs can burn out.

Some webpages claim that such bulbs would only last a few 100, maybe 1000s of hours, but such statements are incorrect. The bulbs used, 5 V, 115 mA, will typically last about 40000 hours, much longer than common household light bulbs.

These little bulbs have 1.9 Lumen each, not bad. To replace with a LED, 14000 mcd, at 25 deg angle, are about 2.2 Lumen. Close enough.

Found some 3 mm superbright white LEDs:

0.04 USD each!! Amazing!

Also these won’t least forever, white LEDs do lose intensity over time, like, 50% remaining intensity after 20000 hours.
I decided to run them below the rated current, at about 16 mA (120 Ohm series resistor with 5.2 V supply). Maybe this will make them last a bit longer.

Note that the backlight is software-controllable (special functions 134 and 234). This is how it is implemented (let me know if you need to full schematics of the 8642A or 8642B):

The resistors (1.8 Ohm) make sure that the lamps operate at 5.0 Volts, not 5.2 Volts, and don’t interfere with the operation of the LEDs. Quite amazingly, running at 5.2 V vs. 5.0 V would reduce the life span from 40000 hours, to 25000 hours!

Agilent sold these bulbs for about USD 18 each!!

No mechanical modification of the 8642B at all, the LED and the resistor fit well into the cavity (don’t worry about the lens of the LED – the light will find its way.

The result:

– not to my full satisfaction (non-uniform brightness, looks a bit dark on the left hand side, albeit, very easy to read).

After a bit of head scratching – turns out I soldered in the left LED with incorrect polarity – so it can’t work.

With this little mistake corrected, all is good:

Now, let’s hope that these LEDs will last. Never mind, I have a bag of spares!

Note: the display assembly is said to be rather ESD sensitive. Make sure not to damage it!

Miscellaneous

Homemade Hard Candy: sugar free extra-strong eucalypt-menthol

2014/12/24 Simon Leave a comment

In winter time, nothing better than some low-calorie, strongly flavored candy. It works against germs, improves general health and well-being, and preparing such candy yourself is fun, and you can make them ‘extra-strong’.

That’s how it works. First, you need to get some isomalt. Isomalt is a sugar alcohol – much less (about half) of the calories than regular sugar, it doesn’t attack your teeth, and is slowly metabolized by the human body (doesn’t lead to spikes of you blood sugar level). It is sweet, but not quite as sweet as sugar. No metallic after-taste, it is purely sweet. And, it is non-hygroscopic (it will not absorb water when stored), resulting in less sticky mess, and eliminating the need to wrap each and every candy separately.
This makes isomalt a nearly ideal ‘carrier substance’ for any flavor.

Take about 150-200 g of isomalt, and melt in a small cooking pot. Ideally, use an electric stove – don’t overheat! Stirr!

Once everything is molten, pour onto a silicon baking pad, or siliconized/non-stick (lightly oiled – vegetable oil) paper.

Watch out! This is dangerously hot stuff – it can cause severe burns! Keep children away! I always wear a pair of cotton gloves for isolation, and a pair of rubber gloves on top.

Now, the tricky part. Using a metal blade, knife, or similar tool, move the molten isomalt around, outside in, until it is cooling down a bit, and getting more viscous. This requires some practice.

Next, most important step – addition of the flavor.

The flavor – a saturated solution of (-)-Menthol in Eucalyptus oil (this is about a 1:1 ratio). This is best done directly in a pipet bottle, very handy for easy dosing. The active ingredient of Eucalyptus oil, 1,8-Cineol is a really great compound, it kills bacteria of all sorts, and constitutes about 80-85% of the oil.

Menthol –

1,8-Cineol –

The right time to add the flavoring is reached when the viscosity is just about high enough to handle the molten isomalt with your hands (make sure to thoroughly lubricate the gloves with neutral-taste vegetable oil).

The flavoring is put directly on the semi-liquid isomalt, and folded in from the perimeter, to the inside of the melt.

Then, take the candy in your hands, and pull it, fold it, pull it, until it is cool enough to be portioned into candy pieces. Can be a bit hot, but never mind. Once the right temperature/viscosity is reached, timing is everything! Now things have to happen quickly!

Using a strong pair of scissors, with some vegetable oil applied, cut the candy into pieces of adequate size. Rush!

Best use some non-stick paper, and let the candies cool down.

The final step – put them into some nice metal containers for storage. I never store too many in a single container, because they will lose favor more quickly, with a large container being opened all the time to take out a few candies only.

The result of a single batch – you can handle up to 250 g of isomalt in one batch with some practice.

Various

Resistive Power Splitter: trying out a low-cost construction

2014/12/22 Simon Leave a comment

For leveling of signals, or test that require two tracking channels, like tracking insertion loss measurements, a resistive two-element divider is very handy. These are broad-band, and rather robust devices.

One input, two resistors (50 Ohms each), in series with two outputs.

Such devices are available from various suppliers, and cost anywhere from 25 to 300 USD, depending on level of precision and frequency range.

Why not try to build one yourself, with some small 0603 resistors; I used China-made SMA connectors, and 4 pcs of 100 Ohm resistors.

How does it perform? Well, let’s connect to it a network analyzer and try:

Port A through measurement (port B terminated):

Port B through measurement (port A terminated):

Tracking is pretty good, 0.05 dB @2 GHz, 0.15dB @2 GHz.

1.2 input SWR – well, pretty acceptable; might still be able to improve by adding some solder or by changing the length of the pin. Good enough.

Here, some specs of a HP resistive splitter:

DCF77

DCF77 Frequency Reference: a resonably accurate 10 MHz source

2014/12/22 Simon 5 Comments

For those out there that need a good 10 MHz source to calibrate their counters – there is an easy method, at least in Europe – the DCF77 transmitter, near Frankfurt. It uses a 77.5 kHz carrier, which is kept very close to 77.5 kHz, all the time, and puts out about 30 kW of power. The carrier is controlled to within 2*10^-13, way better than I need.

To make use of these waves, I build a little receiver, using a tuned circuit, a FET pre-amp (which is located in a plastic case, several meters away from the bench, to avoid interference.
The 77.5 kHz signal is then converted to a square wave by a limiter circuit, and phase-compared to a 77.5 kHz derived from a 10 MHz OCXO. For the OCXO, I used a Piezo brand Model 2920136, but any reasonably good 10 MHz OCXO will do.

No need to go to a rubidium oscillator, which will only consume a lot of power and wear out over the years.

The amplified signal is also available at a rear BNC connector, for troubleshooting, and to find the best spot for the antenna (just connect a scope and align antenna orientation/place for best amplitude).

The tricky part – deriving a 77.5 KHz signal from a 10 MHz source. This requires a fractional divider. First, the 10 MHz signal is divided down to 310 kHz (4x 77.5 kHz), followed by two :2 dividers (74F74 flip-flops). This will give fast transitions, and exact 50:50 duty cycle.

The 10 MHz to 310 kHz divider is implemented using an ATMega8515 (you can use any other microcontroller that can handle a 10 MHz clock). The program does a simple trick – it generates 31 transitions for any 1000 clocks; and it does this with reasonably well distributed jitter.
7 blocks, with 33-32-32-32 cycles; and 1 block with 33-32-32 cycles; in total: 23 sequences with 32 cycles, and 8 sequences with 33 cycles – a total of 1000 cycles over 31 sequences. I am so glad that microcontrollers exist, this would have taken quite a few TTL circuits to realize this hard-wired.

dcf77r_p.c AVR GCC file

The PLL, build around a 4046 has a long time constant, several minutes, however, you could improve the frequency stability by using a constant of several hours – which is not quite practical, and also not necessary, for the given purpose (to provide a reference that is accurate and stable to better than 1 ppm, and that has a phase stability of better than a few microseconds).

Some auxilliary circuits, for the lock detector, and the outputs. Outputs are TTL, but you can also add some transformers, resonant circuits, etc., in case you need other signals. I found these TTL signal very suitable to lock all kinds of test equipment, and never had any issues with ground loops so far. If you do phase noise measurements, I would recommend to use a local Rb reference anyway, or a free-running precision/low noise OCXO, not the output of this device.

Note the shielding of the input circuit, using some copper clad board. A bit curde but works.

The thing, put into a nice box:

After some days of monitoring the output phase vs. a GPS-adjusted Rb oscillator – the device is working just fine. There are some phase fluctuations, most likely, due to the propagation of the 77.5 kHz waves, and these cause phase shifts of about 1 µs. Well, just temporary shifts, and by all means good enough to calibrate any OCXO to full resolution.

Why not use a GPS disciplined oscillator, or a Rb oscillator? Well, the GPS signal, who knows when they will shut it down; and it needs a rather facy antenna, and, you can’t build it from scratch (well, you can, but would be a major effort!). Why not a Rb oscillator, well, I actually have a good Rb, but rarely use it, because it needs so much power, and way too accurate for the general tasks at hand – rather have the DCF77 running, which only needs very little power and generates no heat; and, the OCXO won’t wear out so soon!

Various

Oscillator Driver/PLL: tuning fork oscillator

2014/12/21 Simon Leave a comment

Recently, a “very special” circuit had to be designed – a driver for a mechanical oscillator. The objective – to find the natural frequency of such oscillators, to a very high degree of precision, and at very small amplitudes, in the µm range.
Measurement of the frequency is easily done by a frequency counter – what is needed is a circuit that keeps the oscillator going at a constant amplitude.

The oscillator (a mechanical tuning fork, metal tube) carries a small magnet that can be used, together with a stationary coil, to make is oscillate and sustain the oscillation.
The movement of the tuning fork is sensed by a light gate – an IR emitter diode, and a photodiode.

The oscillator is running at a few 100 Hz, in a very well thermostated environment.

First part, the photodiode amplifier, and signal conditioning circuits.

The second part, the PLL (a classic 4046), and some auxiliary circuitry to provide monitor outputs.

For operation at other frequencies – adjust the VCO timing capacitor, or use an external VCO.

The coil driver – and monitor driver, this is a very low power systems, a few milliamps are plenty for the coil.

SimonsDialogs

8663A Synthesized Signal Generator: a 1 year long repair story

Micro-Tel MSR-904A Microwave Receiver: AM detector/AGC circuit fix, A3B5 assy

ADCMP580 Ultrafast Comparator: it’s really fast!!

LinuxCNC EMC2 HAL Files: 3 axis mill, 2 axis lathe with encoders, jog wheel, axis compensation, camera view

Rong Hua (Bianhuan) “50 Watt” Travel Adapter: not enough iron, for this “wattage”

HPAK 8642B Synthesized Signal Generator: backlight replacement

Homemade Hard Candy: sugar free extra-strong eucalypt-menthol

Resistive Power Splitter: trying out a low-cost construction

DCF77 Frequency Reference: a resonably accurate 10 MHz source

Oscillator Driver/PLL: tuning fork oscillator

SimonsDialogs – A wild collection of random thoughts, observations and learnings. Presented by Simon.