Category Archives: HP 8561E Spectrum Analyzer

HP 5086-7803 YIG tuned Filter and Switch: “SYTF” details and adjustment

With the repair of many spectrum analyzers, it turns out, the preselectors are usually not easily damaged, because of their self limiting characteristics, and because of the absence of active parts. The SYTF is diffient in that it has an active part, a switch.

It is already some years old, but no reason why such parts should have much aging at all.

The symptom of this unit, it is working in the low frequency region setting, LOW band out, but the high band is dead, loss in excess of 35 dB, at all frequencies, and independent of the tuning current supplied.

So my first assessment was, this unit needs replacing, and I found a replacement part online, from a US seller, not cheap, but OK, the 8561E analyzer is worth it, if it is working again after the repair. Unfortunately, several week of waiting were all for nothing – the seller shipped the wrong part and it took a while to get the money back, but finally all settled, except, we still have the defective filter.

Let’s try to investigate the nature of the defect, and open it up. Fortunately, these filters are not hermetically welded like some other YIG parts.

You can clearly see the coil, the inlet and outlets (low and high band) by rigid SMA cable (1 mm size!).

First, let’s study the switch. It is not actually switching the high band, as I originally assumed, but it is switching on and off the low band.

It is a series-shunt-series type FET switch, controlled by about -10 V negative voltage (1 kOHM vs. -15 V connection is the usual control method, floating or ground to switch off).

I could not find the exact die and model for this switch, but there are many similar models that clearly show the structure. The shunt and double series construction will provide very high isolation.

After removing carefully the gold mesh (it is only lightly glued on, I will use some tiny traces of epoxy to stick it back on), some study under the microscope.

Clearly, the spheres are misaligned! The spheres must be placed in the center of the coupling loop, to allow for RF to couple. Generally speaking, during alignment, the sphere is only turned, and then the position fixed by some epoxy – which all seems to be intact, and solidly fixed. So what has happended? I think it has to do with the mounting blocks, which are of different material compared to the based (which needs to be magnetic Fe-Nickel alloy). With frequent temperature cycling, I believe there is some migration of the mounting blocks, fractions of a micrometer every time (keep in mind, the YIG spheres are heated during operation). While we can speculate about the reason of the migration, the result is clear, and the action as well: we need to realign the spheres.

I decided not to undo the screws because the coupling loops can be easily damaged, and used a screwdriver to carefully push the mounting blocks away from the coupling section, bit by bit, under control with a microscope.

Finally managed to get all the sphere properly aligned. If you don’t know how it works, never turn the YIG spheres! These need to be aligned for thermal stability effects, not only amplitude – something which you may have trouble doing at home.

After all the alignment, a quick test setup, with a current source for the main coil, and another supply for the heater and switch connection. Note that the current source is set in parallel with a capacitor (22 uF) to allow for stable regulation with the strongly inductive load.

The insertion loss test – done by checking at several frequencies, using the lines of a good comb generator.

The insertion loss, in my simple setup, it is about 4-6 dB for a 3 stage filter, not bad. I don’t know the original performance spec, but it is definitely in the typical range of such filters, and good enough for a spectrum analyzer in any case (in the worst case, we will loose 1-2 dB of sensitivity). Maybe I will eventually find a new filter at a reasonable price, to check it – it could also have moved spheres.

The tuning current is very linear, I don’t expect any issues with using this part in the analyzer (the tuning characteristics can be programmed and stored in the EEPROM of the analyzer, to control its DAC appropriately, also, we will need to recalibrate the flatness).

HP 8561E Spectrum Analyzer: 100/300/600 MHz system fixed!

Progress with the HP 8561E – the 100 MHz to 600 MHz system, driving the 2nd converter, and other sub-system including the 300 MHz output.

Initially, it looked like a failure of the 600 MHz doubler, so I decided to open up the RF case of the A15 assembly, and to go to component level troubleshooting.

According to the block diagram, the 8561E first uses a trippler to convert the 100 MHz VCO output to 300 MHz, and then a doubler to convert the 300 MHz to 600 MHz. Note that there are various revisions of this board, not all use the same frequency multiplication scheme.

Very soon it became clear the the double is not getting enough 300 MHz power to work. So, to check it, I injected a 300 MHz signal after opening the signal chain after the tripler (there is an attenuator, just desoldered the middle resistor of the PI configuration attenuator, and checked all the components around this area)

Only about 2 dBm are needed to drive the tripler, there is an amp stage in front of the doubler. A quick test – the doubler is working just fine!

So, probably a fault in the trippler? At least, there is 300 MHz present. What is going on? Let’s go one step further back – removed the tripler transistor, marked “Hb” which is a NE85635 transistor.

Let’s drive the 300 MHz circuit from an external generator. This is running at quite high power – about 18 dBm!

With the injected signal, all is working fine! So, the 300 MHz transistor probably failed? By luck and coincidence, I found a spare 2sc3603 transistor, marked “Oq”, and soldered it in.
Surprisingly, the old transistor, once desoldered, tested just fine. And, to confirm this, the 8561E still not working!

The 300 MHz system can be conveniently monitor by checking the 300 MHz cal output with another (working) spectrum analyzer.

Well, we need to go back one more step – to the 100 MHz amp.
The tricky fact – the 100 MHz system is working, but after some careful measurements and calculations (I don’t have a precise active probe here), the power at the output of the 100 MHz amp is clearly low. This needs to deliver well over 10 dBm of power, otherwise, all the following systems won’t work properly.

Fortunately, the 100 MHz amp is a fairly common part, a MSA-0505 gain block.

These MSA-0505 are used in many HP circuits, just took one from an old board:

The A15 board, still with the 100 MHz gain block, and the tripler transistor replaced…

A last step – replaced the Oq transistor, with the old/original HP part (300 MHz circuit).

Finally, some tests – the 8561E (at least the low band up to 3 GHz is working again, and the CAL output is in spec (0.02 dB difference to another calibrated 300 MHz source!).

HP 8561E Spectrum Analyzer: 600 MHz troubles

Some investigations into the 600 MHz generation of the 8561E. The target is clear, we need to get a clean 0 dBm 600 MHz to the 2nd converter to make it work (the converter itself is running at 3600 MHz). The source, it is on the A15 board, which is one of the more complex circuits that have been designed by some clever engineers at HP. And, there are multiple versions of this board to make things a bit more complicated.

The 600 MHz system is located in the corner close to the J701 connector (which goes to the 2nd converter). It works by first locking a 100 MHz source to the 10 MHz reference, and then multiplying it to 300 MHz (which also drives other circuits, and the calibration output – which is working just fine), and then doubling it to 600 MHz followed by some amplification. Only output J701 is used, the other output, no use in the 8561E (and also this has low output).

The suspicious section, what could be wrong. Most suspicious, the output amp, because there is 600 MHz present, just not enough signal power. So this amp could be blown, and absorbing energy rather than adding RF energy.

It is a quite common amplifier, MSA-0386 which is a 12 dB silicon bipolar amp (labelled A-03 on the package). Ordered a few pieces, less than 1 EUR each, and these are useful parts anyway.

Other faults could relate to the input amplifier, but would we then get 600 MHz at all? The diodes – these are not likely to fail. The power supply decoupling – which has several capacitors and tantalums – may be faulty, but in general, the A15 boards don’t suffer from bad tantalums.

Well, first we need to get some small Torx bit, to get into this assembly…

HP 8561E Spectrum Analyzer, 30 Hz to 6.5 GHz: some consequences of a past disaster

Even today, 6.5 GHz bandwidth spectrum analyzers don’t come easy, you are looking at 6+ kEUR for even the most basic Rigol or similar low cost brand unit (which won’t be very useful to design VCOs or other precision gear), and all the high quality low SSB noise instrumentation, it is well outside the budget of the common hobbyist or even small company in this frequency range. This may be one of the reasons why the earlier HP and similar name brand equipment still is in high demand, and good working units still have their market, even being 30 years old. Some claim that with age, the performance might degrade, but as per my own tests and experience, this is not the case. These YTOs and YTFs and mixer still work like at the first day. Well, only if they work at all…

Here, we are dealing with a unit described as non-working, in good general condition, but not showing any signal, except, noise. The price was right, there doesn’t seem to be a big demand for defective spectrum analyzers in Japan – I was the only bidder.

Soon after the auction, a large box arrived, and the 8561E powered on just fine, with a good display (well, after some cleaning of the CRT and filter). Checked with the 300 MHz calibration signal – no response.
Further to check, it is quite handy to use a comb generator. This one has good output well into the high GHz range, at 100 MHz spacing.

The result – no response of the 8561E in the low band, and some minor response in the upper band (the 8561E uses 1st harmonic mixing, but two bands, one below 2.9 GHz utilizing a second converter, and one above 2.9 GHz, directly mixing down to 310.7 MHz first IF).

That’s how the comb gen output should look like on the analyzer (below, tests with a different – working – analyzer): all equi-desitant peaks, 100 MHz spaced, and somewhere between -10 and 0 dBm.

Next, I checked the LO, and it is osciallating and it is phase locked, and the power level is right, at least at the LO output. That’s a relieve – at least the YTO is working and locked, and no complicated PLL troubleshooting.

Well, what can we do further, we need to take the thing apart (at least, I have the manuals and even the component level information package, CLIP), starting to probe the signal chain. The most easily accessible part is the mixer (all bias/switch voltages checked OK), let’s take it out and see if it works – feeding a test signal directly to the mixer, and taking the output to a working analyzer.

Low band – looks fine.

High band – working, but we are missing 25 dB signal strength! Something is not right.

Let’s have a look inside – nothing obvious like a broken bond wire, or burn mark found – but also hard to find without a microscope.

Reparing the mixer itself, no option here, this a marvel of engineering, and you need a larger workshop than mine to fix all these tiny pieces of silicon, mounted with gold on axis-oriented saphire substrate…

This may become a more serious repair, over several weeks to get spare parts in, so best to keep all parts and screws well arranged, I keep them in plastic bags, so I can check later if all made it back to the instrument.

Now, with the mixer situation clear – it has a defect in the high band, lets turn to the incomming path, from the main connector, to the mixer input.
Low band is fine, but high band again – weak signal. What can we do? We can at least isolate the fault. Turns out, the attenuator is working just fine, but trouble with the SYTF. HP introduced this combination of a solid state switch and a filter with 8561E, the 8561B still had a separate switch (which would be easier to fix, compared to a fancy switch-YTF combination.

Next, let’s check the IF processing path – just feeding a 310.7 MHz signal to the analyzer, past the mixer and 2nd converter – all working fine. Great!

Next, let’s check the 2nd converter that is needed for the low band using a 3.9107 GHz 1st IF. It is not converting, and no 3.6 GHz signal at the test port (the LO of the 2nd converter is generated by multiplication of a 600 MHz signal). Further check shows that the 600 MHz signal is low, about -20 dBm (should be about 0 dBm). Still the 600 MHz is phase locked, and the reference output (300 MHz, but derived from the same circuit), is working, so probably something with the 600 MHz output stage.

Let’s review. (1) a dead mixer, (2) a dead SYTF, (3) a probably working 2nd converter (at least as much as I can tell now, with feeding a good 600 MHz signal), (4) working LO system, (5) working IF and other processing system, (6) good CRT and mechanical structure. And, looking at the noise specifications, and the total package of properties – is it worth the repair? Well, used units in mixed state go for 2 kEUR, good calibrated working units for 4-5 kEUR, so if we can find the spare parts (a mixer and a SYTF) for a few hundert EUR, it will still be a very economically reasonable repair. Fortunately, I was able to locate a “working and guaranteed” mixer, and a spare 2nd converter (just in case) for a good price. Only the SYTF, a bit more expensive, but it is guaranteed working as well. Now, we need to wait for these parts to ship from the US and China, to Japan.

I wonder what has happend in the past, what kind of disaster? Usually, the mixer is not easily damaged, because of the limiting characteristics of the YTF. And the 600 MHz fail looks completely unrelated. But who knows the sequence of failure, maybe the 600 MHz failed long ago, and the analyzer was only used in the high bad, well, until someone connected it to a high voltage generator.

A few other things to consider – the lithium battery. It is still good. Usually I only change them once defective, or in case I sell an instrument.

The fan, a Papst Multifan 8312 (working, but a bit noisy) is still available, but I will only replace it after fixing all the RF chain.

Another bonus – the 8561E also came with a working mass memory module. Mass refers to a massive 250 kilobytes in this context!