Rohde & Schwarz SWP.339.0010.02

I have a new thing to fix! It's a sweep generator that can produce signals from 400kHz to 2.5GHz and -130dbM to +13dbM. It's very big, heavy, old and broken!

So What's it Do?

This thing is a specialized  sweep generator that can quickly sweep across a frequency range. In addition to the RF output the generator produces a voltage which is proportional to the output frequency that you can use as the X input to your oscilloscope in X-Y mode. This allows you to produce frequency vs amplitude response curves for filters, amplifiers etc using the generator and a scope. It's pretty much the tracking generator part of a modern spectrum analyser. R&S sell other bits of gear that work with this one to provide network analysis and other features.

The thing uses a YIG (Yttrium , Indium, Gallium) tuned oscillator to produce the output frequency since YIG oscillators can be rapidly tuned over a broad frequency band. Unfortunately they are not frequency accurate without additional circuitry. R&S sell a 'synchroniser' option which is pretty much a fancy PLL circuit that allows you to lock the output to a crystal oscillator to get frequency accuracy. My unit has this option fitted.

In addition R&S sold a marker option and while my unit is fitted with this option I've not really touched it. I think it generates pulses so you can create tick-marks on your scope at specific frequencies.

Another option is the mechanical step attenuator that allows the unit to generate a broad range of output levels. Mine is also fitted with one of these which is pretty nice and it makes an amusing array of clunks as you wind the knob to adjust the output level.

Physically the thing is massive - weighs nearly 20kgs and is packed with boards.  It's got a huge linear supply with a shielded torroidal transformer and the back panel (which acts as a heatsink) gets quite hot when operating.

It has a big bright LED user interface and lovely clunky keyboard interface. The adjustment knob feels lovely to use as it is really solid and has a gentle cogging that feels a bit like a turning a very soft stepper motor. The knob will easily whizz if you flick it and it comes to a gentle stop.

I found the user interface a bit confusing at first - still do really. You have three numerical displays which are the start, marker and stop frequencies. The keys have second functions that are activated by a second function button marked with a red circle. To get a constant frequency output you have to use the second function button for example. There is an LED marked RF off and a button. By default the output is on unless you hit RF-off and then the light comes on. I feel is exactly the opposite of what I would expect.

I had no idea what 'synchronizer' meant and assumed it was something to do with interfacing it to a network analyzer. 'Synthesizer' would have made more sense as that is what this option does - makes the unit a synthesizer.

Condition

Overall the unit is in pretty good physical condition for its age (I think it is an early 80s device). There are little bits of corrosion on some boards/connectors which is concerning but probably Ok. It's got minimal amounts of the usual cal sticker residue and the paint on the rear and top/bottom panels is in reasonable state.

The first time I turned it on it was apparent the switch is broken as I had to hold it in to keep the unit on. This might have been transit damage.

In the ebay listing the seller noted it displayed error 10 A when the self-test runs. Before I bought the unit I looked this up and found it indicates the settings battery is dead. Could I be that lucky? When I opened it up I found the battery is in fact a pair of AA alkaline batteries and they had not been replaced in such a long time that they have eaten their battery holder. The battery doesn't matter too much as it is only used to store pre-sets. The unit operates fine without it.

I connected the unit up to my spectrum analyzer to look at the RF output but unfortunately it didn't look like it was working. The output was constantly around 38Mhz regardless of the settings. Doing this test  required some creative operation  as I needed to keep my finger on the power button to keep the unit on!

Power Button

First things first - the power button had to be sorted out as I couldn't do anything sensible with  the sweep generator while holding the power button in. The power button is at the back of the unit near the mains input and is connected via a long shaft to a power button in the front panel.

Knowing the likely outcome I removed and disassembled the power button anyway. The main shaft that is pushed by the rod has a hole for a pin that implements the push-on/push-off mechanism and this hole had been wallored out from many years of use.

I started looking for a replacement but the switch was made by Petrick and they went out of business long ago. The other problem was I found it pretty hard to find the right keywords to specify a push-on/push-off  plunger type switch with a square rod. It also needs to handle quite a few amps at 240V.

Even if I could repair the existing plunger I don't think the one I had was going back together any time soon :)

In the end I found a replacement switch that is a KDC-A04 and they are pretty common in cheap Asian electronics. They have the right number of poles, roughly the right throw and the right size plunger. The one problem was the size of the tabs used to screw it down where different. I found I could get enough purchase under the screw on the opposite side (even though the screw didn't go through the hole) for the switch to be pretty secure. I did have to slightly shorten the plunger but it worked just fine.

No Output

I got a partial copy of the user and service manual before I bought the unit from the R&S yahoo forum. Unfortunately these included the first 20 pages or so and the manuals are a mixture of English and German. I realized quickly that I needed more and began looking around for the rest of the service manual. The most economical option was to buy the two service manuals for 20Euro each from an online vendor. The paper manuals were nearly triple that! That's actually more than I paid for the generator!

Once I got the service manuals and started reading the more I started to figure out the self-test mechanism. The generator has an extensive set of self-test lines and can localise faults to some extent by itself. The first error displayed was the battery failure but you can hit the clear key to get past this and see other errors.

The first error was a failure of the 600MHz IF. This explains the lack of output. The other errors were likely a result of this first error.

How it Works

So the YIG oscillator generates frequencies from 4.2GHz to 6.7GHz. The output is generated by mixing this with an internally generated 4.2GHz signal generated from the 10MHz reference clock. The YIG is then tuned by sweeping the control voltage to vary the output frequency. This is all achieved within one of the large, RF shielded boards called the converter board. The block diagram below illustrates how this works:

The process is broken into:

• A 100MHz VCO that is locked to the 10MHz crystal reference using a PLL
• A frequency multipler to generate the 600MHz
• A helical tuner to filter the 600MHz
• An amplifier for the 600MHz signal (as it is used elsewhere in the unit)
• A multiplier to generate the 4.2GHz signal from 600MHz
• A filter for the 4.2GHz 
• A mixer to combine the 4.2GHz with the YIG output

Debugging  down Rabbit holes

In my case the 600MHz was missing so I disassembled the converter and started tracing the signal through. The converter internally was quite interesting as it has a number of distributed element filters and a lot of hand construction.

Here you can see the 4.2GHz filter composed of the series of diagonal sections followed by a filter and at the top left is the mixer

The section at the top right is the 100MHz VCO and the bottom section to the right of the coils is the multiplier that generates the 600MHz from 100MHz. The  area at the top is the phase detector and frequency dividers etc that form the PLL.

The next trick was how to debug this. The case is quite closely packed so there is no hope of sneaking a scope probe down between the boards when they are in place. I found that R&S kindly supply a set of riser boards inside the unit that you can use to operate modules while they are up above the rest of the unit. The problem then is all the coaxial connectors that go in/out of the module which are all SMC (not SMA or anything common). I found a set of short SMC male to female patch leads that were perfect for the job on ebay. They look like this and allow me to connect the board to the coax cables inside the case

Running the converter board up on the extender boards with the patch cables plumbed in, I used a scope probe attached to my spectrum analyzer to measure what was going on. I found that the frequencies were way off. The 10MHz signal from the reference looked fine but...

The 100MHz was off a bit so instead it was more like 98MHz.

This got worse when it was multiplied so instead of 600MHz it was more like 589.

The output of the PLL PI filter was at the limits of the voltage range so clearly it was trying to force the VCO to 100MHz but couldn't. Because the 600MHz was so far off the filter was knocking it down a lot before it got to the next stage. Could this be the problem?

Apparently Not

I then looked at the service manual again. Apparently you can't operate the converter with the top cover removed as it effects the VCO (face-palm). Re-attaching the top cover and re-testing I found that the 100MHz signal was just fine as was the 600MHz coming into the following amplifier. What came out was pretty much nothing however. So in the schematic below the signal coming out of the 600MHz filter was fine but nothing was coming out of V82. What's more the test voltage at MP-10 and MP11 were off.

I also checked the circuit feeding V83 which is used to disable the 600MHz output (for example when re-tracing during a frequency sweep) but this appeared Ok.

So I ordered some BFR35 transistors and replaced V82 (feeling pretty sure this was it). I tested this again but no luck.

Intermittent

One really annoying thing about this unit is that the faults are very intermittent. I found that by fiddling with the controls it would randomly come good and start producing output. I figured out that the control that seemed to fix it was the one that disabled the output while the sweep re-traced.

I started monitoring the signal that disables HF output and looking at V249. See below.

On one of the occasions that this worked I realized that the 'HF off TTL' signal was effectively active-low. That is when the signal is high the HF is disabled and when it is low it is enabled. So actually when the signal coming in on pin C15 of the motherboard connector was high the output was disabled.

So actually the fault is elsewhere!

HF Enabled

The service manual is not much fun to read through. It's a huge file and so it is slow to scroll. The manual has been scanned as images so you can't search and it alternates the sections between English and German. Maybe this is because I don't have all  of the user manual but I couldn't find information anywhere that showed the location of each card. I had to effectively draw my own map by pulling each one out and comparing it with the drawings in the service manual.

Each schematic has the IDs of the connectors but then the cables have different designations and so it isn't always easy to figure out what signal is what and where it goes. The big block diagrams don't label the signals at all.

Now the HF off signal is referred to in other sections as HFSW1. There is also a HFSW and HFSW2 which are different! It turns out the HFSW1 signal is generated by the Sweep Control and Modulation amplifier.

So I connected up the sweep control and modulation board on the risers so I could start debugging this issue and half-way through I could no longer reproduce the fault. Before this I could re-start for the fault to return but now I can't in fact since then this fault has only come back a couple of times.

So I have to put this fault on the back-burner for now as I can't fix it if I can't reproduce it.

Next

So now it produces an but the frequency is pretty far off. The synchroniser does a good job of locking the frequency to the crystal but between 20.0MHz and 70.0MHz it totally looses it.

More debugging next time!