E4433B RF sig gen repair

[microbug]

I picked up this sig gen from work before they threw it away ('beyond economical repair'...). I was hoping for the same MOSFET that was blown in many other people's to be the problem, and was very disappointed when I opened the output board only to find a sea of custom Agilent parts. The layout was different too so they aren't just rebadged. These parts all have the prefix 1GG (e.g., 1GG3/1GG7) and appear to be internally 50 Ohm matched.

The instrument fault is an 'unlevel' displayed error, and the output of the unit is fixed at 2.628GHz regardless of the frequency set on the front panel. The output attenuators seem to work (the signal is reduced in 5dB steps) but fine-grained attenuation doesn't work. The output drops slightly to 2.622GHz when the set frequency is 0-250MHz. These sig gens have a different signal path in this low frequency range -- the synthesiser board generates a 1GHz - f_set carrier, and then it's mixed down with a 1GHz reference on the output board.

I poked around the board by soldering coax to different points. The output power amp seems to be doing fine. There is all sorts of crap coming out of the IQ mixer (SOIC-14 part labelled 1GG7 4225): there is always a peak at 2.72GHz. The correct carrier peak is also there when the set frequency is above around 2.95GHz -- below that it rolls off quickly. I probed at the input to the 1GG3 amp after the mixer (i.e., after the two small transformers).

The IQ mixer input looks correct -- single peak at the carrier frequency, like the carrier output on the back of the unit.

This leads me to suspect the IQ mixer (ignore the missing DC block cap in the photos -- I damaged it but will replace). I'm still a bit confused about where this 2.72GHz signal is coming from though -- I don't see why the mixer would need a fixed reference that could leak through (and besides, the fixed reference available is 1GHz).

I'd appreciate any ideas or suggestions. I've attached some pictures. The block diagram of the output board is at page 35, http://www.doe.carleton.ca/~nagui/labequip/synth/manuals/e4400335.pdf.

I might try to get a copy of the CLIP (including the full schematic) from this guy. I'd have to pay for it, but it would easily be worth it to get the sig gen running.

[R-1125F]

Have you seen this video of an E4433B repair?

[microbug]

If you mean the Signal Path videos, yes, I've watched both of them.

[R-1125F]

These generators have trouble with the frac-N chip in the synthesizer board.  Have you checked the synthsizer and reference board?  A bad frac-N can cause all kinds of problems. 

I have bought the CLIP.   I think it for an A model but it is still useful for troubleshooting.

[microbug]

These generators have trouble with the frac-N chip in the synthesizer board.  Have you checked the synthsizer and reference board?  A bad frac-N can cause all kinds of problems. 

I have bought the CLIP.   I think it for an A model but it is still useful for troubleshooting.

I haven't checked the synth board, but its output looks good (as measured at the coherent carrier output and on the output board before the IQ mixer). Carrier between -5 and 0dBm, with some small harmonic peaks.

Edit: I've emailed Milan to buy a copy of the CLIP.

[microbug]

Ok I now have the full schematic. Screenshot of the IQ mixer section where the RF path starts going wrong is attached. Any help would be appreciated to try and identify where this 2.7GHz signal could be coming from.

I'll try to have a poke around some more testpoints and check the synth board this weekend. I'll also check the I and Q inputs to the mixer.

Edit: sadly the CLIP doesn't have much info about the 1GG7-4225 mixer IC. There's no info except what you can see on the schematic.

Edit: any ideas as to what Vdi/Vdq, Vci/Vcq, and Vblo do on the mixer IC would also be useful. My RF hardware knowledge is a bit thin.

Edit: so far I have found that:
- QUAD LOOP is muxed onto ABUS, which is an internal analogue bus used for instrument calibration.
- I/Q DRIVE and I/Q BAR DRIVE come from either the instrument I/Q inputs or another off-board source. An AD7568 DAC and some op-amps provide gain and offset.
- VBLO DAC and QUADRATURE DAC come from the same AD7568 through some LF412CD op-amps.
- PRE LEVEL DET appears to be compared to PRE LEVEL REF DAC, which also comes from the same AD7568 to generate a signal called PRELEVEL DRIVE. That goes off board.
- Based on a public Agilent/Keysight datasheet for a similar part, Vdi/Vdq are probably the detector diode bias current, and Vci/Vcq are probably phase control voltages. Not 100% sure about this though.

[R-1125F]

Just to be clear; you are getting the full range 250kHz to 4GHz from the coherent carrier output?

[microbug]

Yes. The coherent carrier output is actually only 250MHz-4GHz. When the instrument output frequency is set below 250MHz, the coherent carrier output is 1GHz - f_set (i.e., 750MHz-1GHz as you decrease from 250MHz towards 0).

I have confirmed that this is the expected behaviour using the CLIP -- the coherent carrier output is a buffered copy of the input from the synth board, which behaves as described above. The block diagram also confirms this.

Edit: added screenshot of block diagram (see first post).

[vaualbus]

If you have an old x86 computer, you can download the service SW and used to access the ABUS reading maybe that help. You need a GPIB adapter (as I think the service SW does not work over the serial port unfortunately).
Also, if you need that part you can search for some spare vector board from the E5515C wireless test set as they use the same output board. (Early models use exactly the same board, newer use a modified board where they remove the final mixer for the frequency below 250Mhz and change a lot of the gold chips to standard SOT something chips, but the IQ modulator is the same! and the spare boards seems to be cheap).
PS early on I suspect I had a failed output board (was not the case ) and I ended up buying all the RF board of the E5515C for like 50$ (indded I spend more on shipping than the boards ) so I have a lot of custom ICs ahah 

[microbug]

That's a good idea. I'll see if I can borrow one from work.

Edit: also hoping I can get away with a VM rather than a real PC.

[vaualbus]

I think I get it to work via serial on a VM, no idea If you can share the USB though.

[vaualbus]

LIke this one is for E5515C, probably there are cheaper boards on ebay but I left this for "reference"
https://www.ebay.com/itm/351978940844?itmmeta=01HTPP8C54WVKYDSHBZ7JT78JM&hash=item51f39469ac:g:ldAAAOSwIgNXwKJ0&itmprp=enc%3AAQAJAAAA8O%2BU5csbRSVAQXQxB%2FArsZ16JBwdUHxXmWYRqoI%2BHYaM0C04oeYIYNFx3ClTyG7zLPVz8ZaI2DpnORLI6caCHqO7cwaiVSt6ksVGZtXJqggOz7sBRG6%2FlY13DW2IiKsfbpGwx2cLughO1YyJAF7RxCiWV7eiO1rjcA2gTTAtledSB2wC5S%2BhOALahS%2FZeBT7k4CJ1bAVeATjajLIraIsQAvIomqzSKiAnRftFhRhYMjxI9JN%2FNWbNscP3i2U52f3VnXtLV2kPBtKKMGBCwffxTJJGYtJeWAMHz%2F%2BMXTUA81PqvbGiAGkeIvou2N7U%2FbcBQ%3D%3D%7Ctkp%3ABk9SR9TCodbVYw

[microbug]

Thanks, good to know there is a source for the part. I did find a few on grey market sites, but who knows if they're genuine (and the price is similar to the whole board). I'll borrow a GPIB adapter and a spectrum analyser this weekend and try to dig a bit deeper. This 2.7GHz signal doesn't make much sense, I think I must be missing something...

[vaualbus]

This one for 50 bucks is like good anyway as you get way more money just from the parts in it (excluding the custom IC, I guess  )
https://www.ebay.com/itm/264970900545?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=a_NoMhZNTx6&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

[microbug]

Thanks, I've asked about shipping to the UK.

[microbug]

I must apologise to R-1125F. On checking again, the coherent carrier output is looking pretty suspicious.

After boot, the sig gen is set to 4GHz. Notice the peak at 2.72GHz, which is the same as what I'm seeing at the main RF output.


Sweeping the output through the 250kHz-4GHz range, it got worse. The 2.72GHz peak is always present, and is particularly bad around f_out = 2.434GHz.



The correct carrier does always seem to be present, and is always the loudest signal (amplitude of 1-3dBm).

I have forgotten to bring an N-type to SMA adapter home, so can't check the actual RF output today. I'll have a quick look at the synth board...

If anyone knows what the coherent carrier output should look like (in terms of images present and carrier amplitude), do let me know. I will have a dig through the service manual if not.

[vaualbus]

I think you can look at the coherent output in one of te TSP repair video. it had a failed generator with ALC error and it foe sure measure the coherent output.
The video should be around 4 years old(never mind it's already 9years, wow time is passing so fast!)  let me post here the link:
https://youtu.be/RSCGAY5z_44?si=z0MPqTE5SF31ma4G

[microbug]

Found the coherent output discussion, at 18:30 in the video. Looks like my levels are ok (his were -6 to -3dBm, mine are 1-3dBm).

There are spectral images in the video, but otherwise his spectrum looks fairly clean. My spectrum is generally bad with the 2.7GHz signal, and at 2.434GHz is particularly bad, with multiple peaks that shouldn't be there. There's nothing like that in the video. That does lead me to suspect the synthesiser board.

The fact that this 2.7GHz signal is always present narrows the search down to areas of the synth board that are common to all frequencies. There are some switched bandpass filters on the synth board, in tight bands between 250MHz and 1GHz. They are quite aggressive (11th order). I would expect these to cut the 2.7GHz signal out. Either they aren't working, or the signal is appearing after the filters. I need to sweep the frequency slowly through the full range and monitor the 2.7GHz signal level. This should reveal whether different filters are switching in and out.

Another clue: the PLL out-of-lock LED (DS1) is on in some frequency bands. There are at least four different bands where the PLL is out of lock. I will look closely at U201 (UPB584G -- NEC 2.5GHz div2 prescaler, marked Agilent 1GC1-4209), because its datasheet warns of unwanted self-oscillation if the input is too small.

I also have been trying to install Windows XP to get the Agilent Falcon service software working (using this post). Have had some difficulties with GPIB-USB-HS drivers and the Agilent software crashing.

[microbug]

Here are some pictures of the synth board. The right side is the phase locked loop and splitter. The middle generates FM baseband to mix with the VCO tuning voltage (I think). The left includes a /2 divider and two doublers, and a lot of switched filters. The gold part near the connector is the output buffer.

Edit: the label that says 'microstrip to output section' should be 'microstrip to filters/dividers/doublers'. The output section comes after these parts.

[vaualbus]

Well more interestingly is that, if the PLL get unlocked, why the instrument never report such error?
Maybe check the reference 10Mhz signal and the different supplies on the board.

[microbug]

It now reports error 508, frac-N loop unlocked.

This definitely wasn't there before -- the only error was oven cold. UNLEVEL was displayed but no error was present. I have seen someone online mention that the frac-N error can be intermittent, but it's also possible I've caused it. It goes away if cleared when the out-of-lock LED is off, so I think the error reporting is working.

I checked the supply rails on the synth board (had to solder wires to testpoints and re-attach the board), and the 10MHz reference output on the back of the unit. All were fine.

Edit: the frac-N error has now disappeared again. It is quite likely that it appeared and disappeared before and I didn't notice -- after I first checked the error queue and found only oven cold, I didn't check it very often again.

[vaualbus]

Oh I see! Anyway the interesting thing is that the Frac-N locks the signal before the doubling and filtering stage, I rather would check, as you said, that part.
So If something is wrong on that path it would never unlock the PLL. Maybe also having the board case removed cause more unlocking due to  the board not been shielded?

[microbug]

Yes, it could be that the board being unshielded is causing the PLL to unlock. I will put the shield back on and do a frequency sweep. If I don't get the unlock error then I think I could ignore it for now.

[vaualbus]

Any new regarding the repair? I am curious  if you found something bad on the filtering  and doubling section!

[microbug]

I’ve been quite busy this week. Don’t worry, I’ll have a look at the weekend!

[microbug]

I put the synth board back in with the lid loosely resting on top, and the PLL was still losing lock. However, when I screwed the lid down tightly, the PLL stopped losing lock -- so I will ignore this error for now.

I had a look at the VCO output, which is well marked on the PCB with a socket for an RF probe. It's all over the place -- I expected to see 0.5-1GHz, but it was a mess of peaks that were almost never in the right place. This testpoint was on the output of RF amp U16, just after a 4dB pad. I also checked directly on the output of U16 and saw the same.

Next I checked the input to U16. I swept the sig gen from 2GHz-4GHz, and lo and behold, a clean peak moving across the spectrum! Power level is consistently -16dBm, which is about right if we assume the coax I soldered on forms a splitter that drops another 6dB. The coax is quite thin and lossy too.

Then I googled 'Agilent "A06" repair'. What do I find but another thread pointing the finger at the same part number. Very suspicious...

I will probe around a bit more and check the other Avago MMIC (A03/A06/A07) parts on the board.

Edit: will take a better picture of the problem area later, but U16 is visible on the zoomed picture of the synth board a few posts ago.

[R-1125F]

It looks like you found the problem. This should not be too dificult to fix.  MSA-0686 is available on ebay but who knows if you get real of fake.  The TSP repair video has a similar repair with a part that needed a different bias.   

[vaualbus]

Cool!
Seems you found at least something "wrong"! I would first check the BIAS T of that amplifier maybe some the resistor/inductor got open? 
Also I would check for soldering cracks, I had a failing 8643A generator failing self test, I track it down to the output amplifier than I try resoldering the main output RF transistor amplifier and be hold afterwards it works and passed self test, I guess there was a cracked join in the soldering! The HP 864x series were the first RF generator made by SMD components that HP designed, very beautiful machine!

[microbug]

The Minicircuits MAR-6SM+ looks like a promising replacement that is available from Mouser.

Thanks vaualbus, I'll check that too. I don't know if a broken U16 explains all the symptoms -- why would the PLL still lock? Maybe it is less broken than it looks when it's loaded correctly.

[vaualbus]

I think is because the lock is before the divider!
As you see the schematic block F of that page, we have the VCO output that is 3db splitted into two paths: one is than feed to U15 and go to the prescaler and than eventually yo the phase detector locking the loop, the other side is feeding U16 and than the dividers/filters and eventually generate the Frac-N module output signal.

As it can be seen, U16 that seem suspicious is on the other side of the splitter so the instrument is "deaf" of the problem. I guess they can only figure out it by having the ALC detect an invalid level of signal?   

Never mind I confused U15 with U16  U16 is feeding the lock, U15 the dividers and output.
Still dunno what happen if one that amplifiers are bad and could create a problem on the others.

[microbug]

Good news: I had another U16 (MSA-0686) on an old PCB from an HP 8922M GSM test set.

Bad news: I swapped it in and the symptoms are exactly the same. The input and output of U16 look identical to before.

The bias tee DC voltage is around 3.5V at U16, which looks about right (datasheet). The 9.1V rail measures 8.6V. I checked each resistor and both capacitors in the bias tee, and they all look ok. Inductor is a short so I am assuming it's fine.

Edit: I checked U15's output and it looks a lot like U16, i.e., seems like it might be broken. I will now have a look at U12, which is MSA-0386 that drives both U15 and U16.

[microbug]

I have found that by pressing the synth board down into its RF gasket (as it would be when assembled fully), the PLL unlock error goes away AND the U12 output switches between a single, stable peak at the correct frequency, and a mess of peaks all over the place.

Sadly, pressing the synth board down doesn't change the output of the whole instrument, which remains a fixed 2.72GHz at -3dBm. So it seems I've just solved a problem I created myself  -- and a problem I identified earlier (the PLL losing lock)

I put the synth board back in its shielding block and tightened the screws. Now I get no frac-N unlocked error and the coherent carrier output looks perfect -- one peak at the desired frequency, and some harmonics visible (which should be filtered out later on the output board). Main peak between 1-3dBm. The problem must have started when I opened the synth board RF shields.

So I'm back to suspecting the I/Q modulator chip. I'll have to poke around the output board again, and see if I can find the source of the 2.72GHz signal -- it's not coming from the synth board. Perhaps the modulator chip is self-oscillating.

Edit: screenshot of the coherent carrier output attached.

[vaualbus]

It is still strange to me that the only fault could be the I/Q modulator....
I guess looking, at the schematics, you could bypass the IQ modulator chip?
The output is AC coupled to next amplifier I am expect the chip to have some loss so if you have a 3db attenuator maybe you could insert that there to avoid demage the next stage chip;
Also I see that the IQ modulator output is differential and is converted back to a single ended signa by two transformer (T50, T51) maybe one is open?
I still do not get why the output is that magic 2.72Ghz signal at all.

[R-1125F]

You didn't mention what options you have.   Is it possible that the digital generator boards are somehow interferring with the IQ modulator?

[vaualbus]

Doesn't really make sense thought anyway?
The IQ is at IF frequency, how they ended up at a so high frequency?
If the IQ modulator is a multiplier of some sort, it would produce some spurious armonics at multiple of the carrier frequency, so why you are stuck at 2.7Ghz?
So, let's said it failed how the hell the output is stuck at 2.7Ghz, if the input signal is at 4Ghz as it was shown in the first pictures of this thread?
It is still puzzling me, either the IQ is oscillating, and we see the result of it or who knows.
What I guess is possible to do is to bypass it temporary with a coax cable, the output is DC coupled into the next stage MIMC amplifier so I guess it should "work"?
Or maybe better if you have another sig generator you can try feeding a signal after the IQ modulator and see if you get any output at all!

[microbug]

There aren't any options installed except the high-accuracy timebase (OCXO).

I took some more measurements today. I started with the coherent carrier, and noticed the 2.7GHz signal there as well as the correct signal. This made me suspicious, and I tried the same measurement with the vector output board shield properly installed. The 2.7GHz signal disappeared. If it's coupling into this part of the circuit, it could be coupling into other parts too.

I measured before the I/Q modulator, immediately after, and after the next IC (U80, a 1GG3-4205 RF amp). Screenshots attached.

The following stands out to me:

- Across the I/Q modulator, the signal at the set output frequency loses 30-35dB!
- The gain block U80 appears to be effective only at the set output frequency. The 2.7GHz signal is relatively constant on either side of the gain block.

These measurements were all taken with a piece of coax connected straight to an RF trace, so they won't be very accurate in absolute levels. The very large I/Q modulator losses do seem like a smoking gun suggesting this part needs replacing.

Unfortunately I've damaged a DC block cap again, so need to leave it here for today until I can replace it. I will try bypassing the modulator IC with some coax and a 10dB pad when I've replaced the cap.