HP 83572A RF Plug-in 26.5-40 GHz: only a fuse away from the highest frequencies

2016/10/25 Simon Leave a comment

Not one of the most preferred things to repair – a rather rare 26.5-40 GHz sweeper plug-in, not producing any output. Despite its rather simple function as a signal source, typically, not easy to fix if any of the microwave parts are faulty. New, close to 18 kUSD, so it is nothing you can easily replace from a hobby budget, and 26.5-40 GHz sources are not easy to come by.

Inside, all full of heatsinks, and a few waveguides.

The modulator.

The YTO. Not many companies around that can manufacture such devices.

First thing to test, with no output present – the YIG oscillator. This has two main items: the bias supply, which is more or less just a variable voltage power supply which is tuned along with the frequency sweep. Secondly, the main tuning coil current, providing the magnetic field for the YTO. Checked both – and found the bias supply at 0 Volts. No wonder there is no output.

Upon inspection of the schematic, I noticed the fuse, which is rather hidden down on the motherboard. And, it was blown. No idea why – maybe just because of its age? Sure enough, HP did not use just any ordinary fuse, but a BUSS GMW model.

USD 9 per piece – that’s a steep price for a fuse.

Cut the fuse open, and connected a 5×20 mm European style fuse. All protected by a piece of shrink tubing.

Well, an about 1 hour later, the YTO is oscillating again, and you can see a nice and strong signal, well over 90 dB useful range, to test attenuators, or whatever 26.5-40 GHz device you want to put to test.

Various

Ultra-cheap LED Spot Lights: Failure mode analysis, and some reverse engineering, and some concerns

2016/10/24 Simon 1 Comment

Something amazing about the advent of LED technology for general lighting is not only the brightness, efficiency, and so on, but also the amazingly low price. Here, 20 light fixtures, including 3 LED elements each, 34 EUR total. That’s a bargain a friend of mine could not resist. But think twice, after about 1 year of occasional usage of these lights – several failed. Brightness is gone, some lightly flashing lights remains.

Still the price is amazing – considering the price of a singe 1 W LED element, with about 1 EUR retail. Plus the case, heat sink, aluminum circuit board, heat conduction paste, external case, 3 lenses!! No idea how this is made in China, for about 1.5 a piece delivered.

The first suspect – the drivers: each lamp has their own little driver box. Type S3W-0103.

The parts, and a good quality aluminum board, named CQ-LV8072. This is a universal board, found in many kinds of Chinese LED light fixtures.

Tested the LEDs – turns out, one of the LED elements is dead, and this ruins the whole thing, as all three LEDs are arranged in a series circuit. We can fix this easily by replacing the LED elements, all three, with some good quality elements. Albeit, at almost non-economic cost. Hint – the case and be unscrewed with the heatsink turning vs. the outer case. No need to apply brute force like I did, to open it up.

Some reverse engineering reveals a rather simple, but practical circuit. Using S8050 and MJE13003 TO-92 transistors, and a little transformer.

As you can see, no protection elements, what if the input capacitor shorts out, or if some overvoltage blows the transistor. Could it set your flat on fire? Well, my guess is, yes.

Precision Timing

Digital Delay Line: sawtooth corrections of an ultra-precise GPS-reference 1 pps signal, and thermal effects

2016/10/24 Simon 2 Comments

In an earlier post, I have already introduced the Motorola M12+ timing receiver, which is really a nice and affordable gadget for everyone who needs a precise and accurate time signal. Taking about nanoseconds here. All these timing receiver have something called a sawtooth error, linked to their internal clock. See earlier post: M12 perfect time.

Various methods exist to account for this sawtooth error, first and foremost, correction by software. However, I felt the need for a hardware solution here, to simplify the usage of the 1 pps trigger as a reference signal for phase measurements, and other purposes where the recording of sawtooth correction values would be rather troublesome.
With any such attempt at nanosecond scale, considerable thought needs to be put into the system to avoid introducing any errors larger than those we want to correct. In particular, thermal effects can lead to great long-term jitter, aka, randomly wandering phase.

How can we achieve compensation of the sawtooth error? Well, rather easily, by introducing a variable delay element in the signal chain, and adjusting its delay second by second, to the expected sawtooth error, in ns. Fortunately, the M12+ can be programmed to send out a message, called @@Hn TRAIM Status Msg, which provides, every second, the expected sawtooth error, of the next second. One single command is need to make the M12+ send out this message, very second, from now and forever, until other instructions received, or until the M12+ backup battery is taken out…

See below diagram, a AVR processor is tapping the TxD line, from the GPS receiver, to any host controller or PC (if connected), and whatever messages are send out are checked for the @@Hn message (and @@Ha message, just to display the current time, UTC, and date, on a LCD display connected to the AVR). Note that this works perfectly fine, even when another host, or PC is used to control/read/monitor the M12+. The M12+ uses 3 V logic, but an AVR input can easily handle this as a valid signal, even with the AVR running at 5 V.

Glad a processor is doing the decoding work… the GPS messages, a bit too cryptic for me:

Rather than implementing a discrete solution with various delay lines as coax cables, switches, etc, Maxim Integrated provides a marvelous chip, a silicon delay line, DS1023 series, at not marginal, but still acceptable cost, USD 8 per piece.

This chip comes in various versions, varying by the delay-per-set, and an 8 bit register, to set the actual delay. Sure, the minimum delay is not “0 ns”, but some odd number, corresponding to the delay of the signal before and after the actual delay line.

According to information found in the datasheet, this chip is trimmed for best accuracy, and high thermal stability. Further documents also say that the thermal drift is non-linear, and that no coefficient can be provided. Rather, the delay is specified as an absolute number, over the full temperature range. Well, fair enough, but what does this mean for our present case and actual device under test? With no information available anywhere, it seems, the only way to find out is to measure it. The datasheet maximum error would be a bit more than we want.

The schematic is nothing to write home about, a 74F04 is used to buffer the input signal, and a the same F04 is used as an output buffer, providing a nice and fast-rise (or, respectively, fast-fall) 1 pps signal.
The only specialty, a thermistor, and two resistors epoxy-glued to the DS1023-50 top surface! This can be used to heat up the device rather quickly, to 60 degC or more, by providing power from a regulated DC power supply.

Note the heating element and the thermistor (a rather small, fast response, 100 kOhm NTC) – red frame.

The test setup – to measure the temperature effects, is running without the GPS, but with a ~1 kHz fast rise-time pulse, from a HP 8012B pulse generator. Both input and output are connected to a HP 5370B Timer Interval Counter. The latter is a great device, single-shot accuracy of 20~30 ps, if you are into any precision timing tasks, very much worthwhile to get one of these, or a Stanford Research Systems SR620. Time intervals are then recorded as averages of 1k measurement, giving very stable readings with high resolution, certainly to 0.01 ns. For the test purposes, the AVR monitoring the RS232 signal can also be programmed via USB, to set any delay value from 0..255, corresponding to a 0..128 ns delay, plus any baseline delay of the gates and the DS1023-50.

All connected to a PC via GPIB, and recording the delay values at various settings.

Rather than many words, please inspect these diagrams, which will give you a feeling of the delay and drift to be expected with temperature cycling of the device at various rates (slow cooling, fast heating, slow heating, etc.). These were all recorded at the maximum delay, register set to 0xff, 255. Diagrams show delay, in ns, vs. time, as MJD.

In absolute numbers, 152.1~152.7 ns variation. Not much. About 1 step. So maybe good enough, and no need to apply any temperature compensation, or to put everything into a thermostated box.

HP 8566B Spectrum Analyzer

HP 8566B Spectrum Analyzer: A19 board, YTO unlock, bad precision trimmer(s)

2016/10/11 Simon Leave a comment

Not the first HP 8566B on the bench, and not the first at all showing the famous “YTO unlock” message. Most of these YTO unlock message issues can be traced to defective capacitors, but not this time.

With the 8566B, take my advise, don’t touch any of the assemblies if you aren’t really sure which one is at fault, it is a fairly complex machine. To troubleshoot, a microwave counter is handy, to check the LO frequency.

Next, the PLL was disengaged by disconnecting the cable from the sampler/LO pll. Still, no good LO frequency output. This leaves two main assemblies to be checked, the LO pretune DAC, and the YTO driver assembly, A19 and A20, respectively.

Quickly traced the issue to the A19 DAC assembly, and luckily enough, had a spare one around, albeit, an older version. After swapping the boards, it was confirmed that the A19 assembly is really the faulty part.

Next – desoldered all the capacitors at one end, but, to my surprise, all useless work, all caps in best working order, even after 25+ years!
Checked various components, and finally, found some issue with the precision trimmers – seems a cold or aged solder joint. To be sure the the fix is as permanent as possible, all the trimmers were removed, the solder connections cleaned, and all installed back in. Easy fix, all working again.

With this unit, there was no intention to do a full calibration, but as an extra service, I checked the power at the reference signal outlet – see below. Quite amazing how accurate, and pretty sure that this unit hadn’t been at a cal lab for at least 15 years….. this is really a superb level of lasting precision and quality, and ingenious engineering.

YIG

Avantek S081-0321 YIG oscillator: not oscillating at all

2016/10/01 Simon Leave a comment

One of the best sources of microwave signals still are YIG oscillators/YTO. These do require a good amount of power, magnetic coils, etc, but provide stable and rather low noise output, and good modulation capability. Core element is a small YIG sphere, placed in a magnetic field.

However, for the current unit under investigation (from a 18-26 GHz frontend), type S081-0321, 8.0-13.4 GHz, all the magnetic field and effort is wasted – no output detectable at all, not even a faint signal (checked with various equipment). Knocking it with a (small!) hammer, no effect. Varying the coil current – no effect.
Current consumption on the 15 V rail is normal.

Well, with all the basics checked, what to do with such hermetically sealed unit, other than using it to satisfy my curiosity about its internals. Hope to trace the defect to some specific part.

But before we consider more destructive measures, let’s try to re-tune the YIG by slightly adjusting the YIG sphere. This is possibly throught the side opening, which is usually welded shut, but can be drilled up rather easily.

Still no luck, no signal, even after turning the YIG quite a bit.

To look inside, carefully removed the top weld seam on a lathe, and the you can pry the case open.

What you can see is pretty straightforward, despite all the gold wires. There is an input voltage regulator, from +15 V rail, down to 8 volts (measured about 8.15 V), this is then distributed to the 4 active parts via resistors (the bluish elements). Voltage at the resistors is about 4.3 V, so all stages seem to be adequately powered and current flowing as usual. Still no signal. Also probed other parts of the circuit, with a thin wire, under the microscope. No obvious defect. The gold wires and contact point reveal a good amount of adjustment done by placing/removing bond wires as need to adjust bias currents, probably also frequency response, etc.

The coil – rather, the coils. The thick wire is the main tuning coil, which accepts 0.4~0.6 Amps, the small coil around the magnetic center pole is the FM modulation coil. This is for much lower currently but high bandwidth modulation. All is sealed and soaked with epoxy resin. Note the hand made labels which may explain the cost of these units if purchased new… looks like US style handwriting to me.

Well, seems that fixing this is beyond what I can do here with the tools at hand. So will need to look for a spare/used 8-13.4 GHz YIG/YTO somewhere.

Various

EIP 545A Microwave Frequency Counter: another dead tantalum repair

2016/10/01 Simon Leave a comment

It’s not the first fix of an EIP counter here, and I have to say, these units are still very useful tools around any RF and microwave workshop, despite their age. The unit under repair, judging from the date code of the parts, is about 25 years old.

The symptoms – the EIP is just not counting. Showing 000000 zeros in the display, but no count. To confirm, you can use the build-in test function 01, by pushing test-0-1, and normally the counter will show “200 000 000”. But no indication of any counting activity for the current unit.

First thing you do, checked all the rails, and turns out the 12 V rail is dead. Starting from the right, removed all boards inside. And soon the defect was traced to the A107 board, gate generator assy.

Checking with the schematic, there are several ceramic and one tantalum cap.

Desoldered the tantalum, and it is no cap any more, just a short.
With all the various tantalum shorts repaired in the past years, there seem to be to cases, some tantalum go fully short and activate the power supply protection circuit, with little powder dissipated over the tantalum. This is the good case. Some other tantalum seem to develop a short with some resistance, leading to considerable power dissipated in the cap, causing stench and risk of fire. The latter more seems to be more common with SMD tantalums, for reasons unknown. Also, the tantalum story is one of the many reasons why you should design current limit circuits, and power supply protection circuits, even in low power equipment, especially, if there are any valuable components that you might want to protect from a power supply failure.

The 10 uF replaced, with a new orange cap, but same “drop” style.

After carefully installing the A107 assy back into the counter (take care not the damage any of the wires), all is good. Counting at 200 MHz with the test function 01 activated.

Precision Timing

A precision pendulum clock, and an even more precise time interval counter

2016/09/14 Simon 1 Comment

Several years ago, I managed to get hold of a rather special piece, an electromechanical master clock, including an Invar temperature compensated precision pendulum. Such clocks were use to control various remote clocks at a train station, in large factories, or huge governmental offices, etc.

These clocks have long been superseeded by crystal oscillators, nevertheless, they are marvelous pieces of precision engineering, and it has been a long held thought of mine to measure how accurately this time-piece is performing, not only long-term (which can be easily timed by daily checks), but also short time, for each individual tic.

The pendulum has a 1/2 period of 0.75 seconds, which is quite common for such kinds of electromechanical clocks. Only the most wealthy businesses opted for a 1 m (full) pendulum, and the instrument is large enough anyway even with 3/4 lenght.

The dial is quite beautiful, and every piece appears to be hand-made. There is no date on the clock, but the age of the capacitor suggests 1920~1930 time-frame. I fully rebuild the mechanics about 3 years ago, all degreased, checked, and freshly lubricated with clock oil. There was no need for any other repair, all parts and bearings are still in good shape.

The clock has a rewind mechanism that is activated once per minute, and also turns a polarity reversal switch, used to steer the remote clocks.

To get the signal out of the clock, a small light gate has been setup inside, and a 1 mm wire connected to the lower end of the pendulum (in a way to ensure virtually no movement of the wire). The wire interrupts the light gate approximate at the lowest point of the pendulum, i.e., when it has its highest speed – this is to ensure sharp edges of the signal.

The setup, currently it is just a set of boards running the TIC4 and LOGGER5 software discussed earlier. A Dell OptiPlex FX160 is used to collect the data, but you can use any kind of computer that can handle RS232 input.

Here, some first results – more to come. Phase is given in seconds, and horizontal axis shows the tick counts – about 115k ticks per day. The software uses narrow time gating to sort out any incorrect ticks, caused by electrical interference, or other random disturbances. There are no more than 1-2 of such events every day. The phase reconstruction algorithm also handles any missing ticks, and the measurement accuracy is not compromised if one or more tick events are not registered for any random reason.

Removing the linear part – not a lot of residual phase error left. Plus minus a fraction of a second a day. Now I have slightly slowed down the clock by removing a tuning weight from the pendulum.

As described earlier, the LOGGER5 setup also records real-time (not to a high degree of precision, just to keep track of time and day), and temperature/pressure. See earler post, LOGGER5, and TIC4.

Below, and interesting feature of the data – with the re-loading of the clock every minute, there is some slight variation of the frequency. It is really not much considerable the notable “CLICK” with every re-wind, done by a large magnetic coil, actuating a lever mechanism.

The hourly variation, most likely, related to the travel of the minute hand – will check this later, simply by removing the hands!

Some first correlation of frequency vs. pressure, but will need to collect much more data, and then correlate with pressure and temperature.

Finally, the Allan variation of the clock, determined from a few days worth of data. Short term stability is compromised by the bi-directional pick-up of the pendulum (detection is at the rising edge of the pulse, which corresponds to two different positions of the pendulum relative to the light gate – because of the discrete thickness of the wire).

Precision Timing

Perfect time: upgrade to a Motorola M12+ receiver, and new GPS antenna

2016/09/13 Simon 1 Comment

For years, a Motorola UT+ GPS timing receiver has served me well as a frequency reference and source of accurate time (and location). While I primarily use a DCF77 locked 10 MHz OCXO, the GPS time is useful for various purposes, be it, to confirm that DCF77 is actually delivering the proper time.

One drawback of the Motorola UT+ is the rather large “sawtooth” error, which is caused by the quantization of the 1 pps signal derived from a 9.54 MHz clock. This results in a +-52 phase inaccuracy – which can be corrected, but only with further effort.

The later model, which is dated by now and available at low cost, the Motorola M12+, is much better in this respect, featuring a +-13 ns sawtooth, which is not a lot, and good enough for most purposes without any further corrections.

Below, some tests on an OCXO vs. GPS 1 pps pulses, for a OCXO under test (10 MHz, divided down to 100 kHz, and phase displayed in microseconds).

This is the small board, not a thing of beauty, but working. The only parts needed are +3.0 V and +5 V (actually using +4.4 V) voltage regulators: 3.0 V for the M12+, 4.4 V for the GPS antenna.
The 3.0 V also powders a MAX3232 TTL to RS232 converter.

Also procured a second-hand GPS timing antenna – this one has a nice radome, a quadrifilar helix element, and a 26 dB amplifier to compensate any cable losses. The cable, LMR-195, features N to SMA connectors, and a considerable of PVC tape was used to protect the N-connector from the elements. Still it would be better to use some special outdoor N connectors, but, sorry, don’t have.

A handy program to control the GPS – TAC32. Usual procedure is to carry out a location survey, which will take about 2-3 hours, and then continue in position hold/timing mode.

One drawback of my location – there is no way to get full 360 deg view, so reception is limited to the more southern satellites. But usually 6-8 satellites are in sight.

Still contemplating if it is worthwhile to put this in a larger box, together with a 10 MHz OCXO, and possibly a DS1023-50 delay line to implement a hardware sawtooth correction. Maybe a good project for winter time.

Various

A Thermal Fuse, and HP 10811-60111 Repair

2016/09/04 Simon Leave a comment

Usually, I don’t care much about high precision oscillator options being fitted to frequency counters, etc., because in the lab, any critical equipment is anyway connected to an external well-controlled 10 MHz reference, locked to DCF77. However, this time I need to install a OCXO (HP 10811) in a HP 5335A counter for service outside of the lab.

The only thing that needs to be done is to remove a jumper on the board of the 5335A (see red box in picture below), and mount the 10811 in the slot already prepared for the OCXO inside.

While such installation is fairly straightforward, it turned out to take more time than expected – simply because of the OCXO not showing any stable output signal.

After a few quick tests, the cuprit was found, a defective (open) thermal fuse. This is apparently a quite common issue for the 10811 oscillators, and you might get away with just putting in a wire jumper. However, I didn’t want to take any risk of overheating in case of a failure of the 25+ years old OCXO circuits. An exact match for the thermal fuse could not be found, so just soldered in (very carefully, cooling the case and leads!) a 10 Amp 109 degC fuse.

This is the OCXO with the new fuse installed.

This style of fuse as a non-insulated outer shell, so a shrink tubing sleve serves as insulation.

Finally, a note found in a datasheet of a common thermal fuse – it clearly states that lifetime will be limited when operating the fuse to close to the cut-off temperature. So clearly, thermal fuses are not the best protective mechanism for the OCXO case. Maybe better would be a bimetallic switch (self-resetting, but at least no subject to any significant aging), or some other device like a PTC.

Sure, we can slightly blame the HP engineers, because it is stated on most thermal fuse datasheets, like the one below, that the operation temp limit should be about 30 degC less than the cut-off, which is not quite the case for the 10811 OCXO. 80 to 84 deg C operation, 109 degC fuse cut-off.

DCF77

DCF77 vs. GPS time comparison: not a lot of uncertainty…

2016/08/07 Simon 2 Comments

Some folks were asking about the accuracy of the DCF77 10 MHz standard described earlier, DCF77 10 MHz – which has an Piezo brand OCXO, steered by a long-time-constant PLL locked to the DCF77 77.5 kHz carrier.

But, how to assess the short and long term stability of such a ‘standard’ in practical terms? Well, short term accuracy – it will simply be that of the Piezo OCXO, and some noise injected by the power supply. Mid- and long term, the drift will be determined by the DCF77 master clock (which is dead accurate), and the propagation conditions of the long wave signal (which is by far worse).

With my location at Ludwigshafen, Germany, I’m reasonably close to the DCF77 transmitter – maybe 70 miles? So there is hope that the transmission induced effects are not all that bad.

To measure the mid and long term stability, see below two plots of the DCF77-locked phase of the Piezo OCXO, vs. the instantaneous phase of GPS, stable to 40 ns or better, and obtained from a Motorola M12+ timing receiver. Measurements were done by measuring the time interval from the GPS 1 PPS signal, to the rising edge of a 10 kHz signal – derived from the 10 MHz OCXO by a good divider (using a ADF41020 REF input – R divider routed to MUX output) by HP 5335A counter.

In short – DCF77 is tracking GPS extremely well, and the OCXO phase is stable to within a few 10 to 100 ns. In practical terms, 1 second of observation time would be well enough to calibrate any frequency standard to 1 ppm or better, by comparison with the DCF77 locked OCXO. In other words, the DCF77 locked OCXO instability appears to be dominated by the propagation of the DCF77 signal more then anything else.

SimonsDialogs

HP 83572A RF Plug-in 26.5-40 GHz: only a fuse away from the highest frequencies

Ultra-cheap LED Spot Lights: Failure mode analysis, and some reverse engineering, and some concerns

Digital Delay Line: sawtooth corrections of an ultra-precise GPS-reference 1 pps signal, and thermal effects

HP 8566B Spectrum Analyzer: A19 board, YTO unlock, bad precision trimmer(s)

Avantek S081-0321 YIG oscillator: not oscillating at all

EIP 545A Microwave Frequency Counter: another dead tantalum repair

A precision pendulum clock, and an even more precise time interval counter

Perfect time: upgrade to a Motorola M12+ receiver, and new GPS antenna

A Thermal Fuse, and HP 10811-60111 Repair

DCF77 vs. GPS time comparison: not a lot of uncertainty…

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