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Old 17th Apr 2021, 1:51 am   #21
G0HZU_JMR
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Default Re: RF signal generator - Marconi 2022?

Yes, I would always recommend to cover the screen wall artwork with tape before going anywhere near a specially plated PCB like this with an iron. I've seen the same thing happen at work and it is really easy for a blob of solder to jump off the iron and then the damage is done. It really is worth the minute or so it takes to protect the PCB with tape before attempting any kind of rework on a PCB with critical surface finishes like this.

Luckily, the old solder was really soft and it came off the gold finish like peeling pieces of soft paint off a metal surface. It just took ages to do it under the microscope at high magnification.
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Old 18th Apr 2021, 2:10 pm   #22
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Default Re: RF signal generator - Marconi 2022?

I've been playing a bit more with this 2022C today and discovered a few things.

The good news is that the service manual has a procedure in it that allows the fine tune (10Hz and 100Hz) spurious terms to be nulled. In other words, the close to carrier spurs that seem to affect every 2022 that I've used at work can actually be nulled away. I assume that when these generators get sent for calibration every year this procedure isn't carried out. So the generator gets worse and worse year on year for these spurious terms.

A typical calibration at a calibration centre will probably just check everything is within a fairly sloppy tolerance for amplitude and frequency and modulation etc. So fine details like close to carrier spurious may get overlooked.

I managed to null out the close in spurious terms so they are lost in the noise. Before they were at -30dBc and looked awful when tuning in 10Hz or 100Hz steps. Now this is not an issue at all and the sig gen is clean even on fine tuning steps.

The calibration procedure in the service manual seems to contradict itself when it comes to the RF level procedure. I think it has to be done a certain way or the calibration will be corrupted on one of the ranges.

I also discovered another repair in this sig gen where someone has fitted an incorrect ALC detection diode. They probably thought they did a good job but the correct diode is needed or the fine attenuation step sizes will not be correct over the entire ALC range.

I think this highlights the perils of buying RF signal generators that are popular in hobby or ham radio circles. Sooner or later someone is going to do a dodgy repair and leave the generator with performance that is well below what the instrument is capable of. The Marconi 2022 is sadly not a reliable generator so this makes it more of a lottery if you buy one from ebay that may have been repaired numerous times over the years. I've only just started checking this one out properly and so far mine has had the following issues:

Solder splash under the VCO lid caused leaky screening leading to significant sideband spurious on the VCOs.

External modulation input connector lead wasn't fitted to one of the internal PCBs (so the external modulation didn't work)

Incorrect ALC diode fitted during previous repair (caused poor attenuation accuracy)

Poor/intermittent connection on internal multiway connector (caused intermittent RF level)

Several OM345 modules replaced with ebay alternatives (impact on performance unknown so far)
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Old 18th Apr 2021, 2:27 pm   #23
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Default Re: RF signal generator - Marconi 2022?

I should point out that I'm still very pleased with this generator as I was extremely lucky (and very happy) to get it for such a low price. I think I can restore the performance such that it operates close to factory specs but this might take a while. Ideally, I'd like to swap out all the ebay OM345 modules for real ones but these may take a while to find.

I do think the service manual is poorly written in places and may lead to incorrect adjustments during the calibration procedure.
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Old 18th Apr 2021, 4:51 pm   #24
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Default Re: RF signal generator - Marconi 2022?

I've been through the RF level calibration process now and the sig gen is transformed now it has the correct type of ALC diode on the high band range.

The AM performance at high % levels is also much better now on the high band range. With the wrong diode the AM envelope was a real mess and it wasn't possible to get through the AM calibration procedure.

The 1dB step accuracy is now very impressive on both low and high band ranges and the RF levelling seems to be well within spec across the full frequency range. This sig gen is now performing very well and now that I've managed to null out the 10Hz and 100Hz spurs when tuning in fine step sizes I think I'll be using this sig more often. The AM modulation performance is very good especially when used with external modulation. There is very low distortion in the AM modulator for example.


NOTE:
I'm still uncertain what changes were made for the 2022E. I've seen it described as a special version for military with lower output power and it might not have an internal 10MHz OCXO. This might explain why the 2022E at Stewart of Reading hasn't sold.

It should be possible to find/fit a small 10MHz OCXO or feed it an external 10MHz reference if it doesn't have one but I think I'd want to pay a bit less than £400 for a 2022E if I had to do this. It probably has a standard 10MHz crystal oscillator rather than an OCXO. Can anyone confirm this?
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Old 18th Apr 2021, 5:05 pm   #25
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Default Re: RF signal generator - Marconi 2022?

This data sheet confirms the reference oscillator in a 2022E is not very good:
http://www.testequipmenthq.com/datas...-Datasheet.pdf
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Old 18th Apr 2021, 6:32 pm   #26
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Default Re: RF signal generator - Marconi 2022?

Thanks! That confirms that the 2022E only has a standard 10MHz oscillator and not an OCXO.

I noticed the odd typo in the service manual for my 2022C. I'm not sure if this is an issue with the PDF conversion process but I think it should be corrected somehow. Otherwise there are going to be some confused service operators when their 2022C performs worse (or not at all in places) after they follow the service guide to the letter. Other versions of the service manual seem better in this respect so I think it may be a PDF conversion issue for the 2022C manual that is commonly available online for free.
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Old 18th Apr 2021, 7:04 pm   #27
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Default Re: RF signal generator - Marconi 2022?

I've seen recent discussions about replacement OCXO modules being created for a few Marconi instruments in this group that has some ex-Marconi service staff: https://groups.io/g/Marconi-Test-Instruments/topics

Although I never realised that the 2022x ones had OCXO's available, as they seemed a rather a budget model compared to the main 203x/204x series ones
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Old 18th Apr 2021, 7:13 pm   #28
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Default Re: RF signal generator - Marconi 2022?

Yes, it appears the 2022C version has an internal 10MHz OCXO. The datasheet is also on the website that Dickie linked to earlier.

http://www.testequipmenthq.com/datas...-Datasheet.pdf

It looks like it has a fast warmup OCXO (5 minutes) and the specs for temperature and ageing aren't that great. There's a trimmer hole on the rear panel so it can be trimmed very easily with a small screwdriver. It probably needs this because the drift performance over time and temperature is not that impressive.
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Old 18th Apr 2021, 8:14 pm   #29
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Default Re: RF signal generator - Marconi 2022?

Ought to be possible to engineer a good replacement for the OM345 thick film module on 1.6mm PCB. RO3010 would even put you close to the Er of alumina. The active parts can't be anything not available in SOT23 or SC70.

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Old 18th Apr 2021, 8:25 pm   #30
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Default Re: RF signal generator - Marconi 2022?

Jeremy, the write-up on your repair efforts has been interesting. I didn’t realise that a small gap due to a splash of solder could cause such a problem due to a compromised seal. Interesting notes as well regarding the lack of OCXO on the ‚E’ variant, ALC diode and the tweak. Glad you managed to get your unit sorted. As you say, there is always the risk when buying used goods, particularly on eBay that some poor quality repair was done previously or some incorrect component fitted and the instrument then performs well below par. I also hadn’t realised that it was difficult to correctly engineer a substitute for the OM345 modules.

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Old 18th Apr 2021, 8:28 pm   #31
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Default Re: RF signal generator - Marconi 2022?

I did have a go at designing a BJT based alternative to the OM345 a few years ago but never got around to milling a prototype.

The only niggle with this 2022C is the low level FM that creeps into the AM modulation when on the 10kHz to 62.5MHz range. At first, this was dominated by the poor fit of the VCO lid but it may now be affected by the reverse isolation of one of the OM345 clones. I think the OM345 has about 22dB reverse isolation at about 160MHz and a classic +12dB gain MMIC probably only has about 18dB reverse isolation. That's not much difference but I think it would be possible to make a 160MHz version of the OM345 that has vastly improved reverse isolation.
Using a pair of DC coupled BJTs it looks to be possible to get about 50dB reverse isolation. I simulated a design that managed to deliver just over 5dB noise figure and 12dB gain and 50dB reverse isolation and it had similar P1dB as the OM345. It ran at 12V, 13mA and was also unconditionally stable.

The only thing between the BJT based diff amplifier (used as the AM modulator) and the 160MHz VCO is an OM345 module. So some of the FM could be caused by the limited reverse isolation here. However, the VCO is really lively and I'm not sure how effective the VCO screening can is. An improved OM345 might not actually make any difference because of this.
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Old 18th Apr 2021, 8:30 pm   #32
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Default Re: RF signal generator - Marconi 2022?

Quote:
Originally Posted by WaveyDipole View Post
Jeremy, the write-up on your repair efforts has been interesting. I didn’t realise that a small gap due to a splash of solder could cause such a problem due to a compromised seal. Interesting notes as well regarding the lack of OCXO on the ‚E’ variant, ALC diode and the tweak. Glad you managed to get your unit sorted. As you say, that is always the risk when buying used goods, particularly on eBay. Its random chance as to what may have been done previously on the inside.
Thanks! I probably should start a thread on this stuff. I've learned quite a bit about this sig gen in the last couple of days.
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Old 18th Apr 2021, 9:27 pm   #33
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Default Re: RF signal generator - Marconi 2022?

AM to PM mechanism, perhaps?

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Old 18th Apr 2021, 10:05 pm   #34
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Default Re: RF signal generator - Marconi 2022?

Quote:
Originally Posted by G0HZU_JMR View Post
Yes, it appears the 2022C version has an internal 10MHz OCXO. The datasheet is also on the website that Dickie linked to earlier.

http://www.testequipmenthq.com/datas...-Datasheet.pdf

It looks like it has a fast warmup OCXO (5 minutes) and the specs for temperature and ageing aren't that great. There's a trimmer hole on the rear panel so it can be trimmed very easily with a small screwdriver. It probably needs this because the drift performance over time and temperature is not that impressive.
Maybe they were smaller and lower-power versions to fit into the 2022 series. And the 203x/204x may have used a larger double-oven for better stability (I do recall it taking quite a while (upto 15mins) for frequency to be accurate - But not sure if it told you when it was still heating upto +70C etc).

On the 203x/204x all the adjustments could be made over the IEEE bus etc. without removing the lids, and I presume that also covered the OCXO as there were stored & displayed-dates in the menus of when it was last adjusted.
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Old 18th Apr 2021, 10:13 pm   #35
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Default Re: RF signal generator - Marconi 2022?

Quote:
Originally Posted by Radio Wrangler View Post
Ought to be possible to engineer a good replacement for the OM345 thick film module on 1.6mm PCB. RO3010 would even put you close to the Er of alumina. The active parts can't be anything not available in SOT23 or SC70.

David
I presume they didn't use any exotic components that were only produced in naked-die (or ang HP-Avago MMIC's, now it seem Broadcom have killed-off many of these since acquiring them) or any laser-trimmed parts (Although can get quite precision thin-film resistors and 0.05pF tolerance 0.1pF increments capacitors in low values).

A quick Google, finds this interesting webpage (that refers back to threads here + reference to G0HZU's work on these), where it seems they did use printed resistors and that one of these fails more than the transistor, but could be repaired:
https://ea4eoz.blogspot.com/2017/09/...id-module.html
And results of a rebuild: https://www.eevblog.com/forum/repair...-repair-om345/

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Old 18th Apr 2021, 11:08 pm   #36
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Default Re: RF signal generator - Marconi 2022?

Looking at the eevblog thread it's interesting that someone tried to make my original guess of what was inside an OM345. At the time that circuit was a blind attempt to design a circuit with similar gain at VHF. I had no idea what was really inside an OM345 at the time. Even on the simulator that first attempt lost gain up at UHF so wasn't really worth making.

Later in the thread I had access to the insides of a real OM345 and I did design an improved version based on the real OM345 circuit as this has added inductance and an extra resistor and these help with bandwidth and stability. This would hopefully work just as well as a genuine OM345 up to 1GHz but I never built one.

The MMIC based solutions for the OM345 are a risky alternative because they won't have the same large signal performance as an OM345. They are unlikely to have the same reverse isolation either. At VHF the MSA0735 has 18.5dB reverse isolation compared to about 22dB for the OM345.

At 12mA bias current the P1dB of the MSA0735 is about -2dBm and I recall the OM345 manages over +3dBm for P1dB. In some circuit locations in the 2022 this won't matter though.

I am tempted to have a go at making a VHF only alternative that has high reverse isolation. This might actually perform better than the OM345 in the locations in the 2022C where it is used as a buffer after an oscillator. In these locations it won't matter if it only has flat gain to about 300MHz. It might help to reduce the annoying FM that appears on the AM modulation on the 10kHz to 62.5MHz range.
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Old 19th Apr 2021, 2:19 am   #37
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Default Re: RF signal generator - Marconi 2022?

Sadly I have to clear my workbench for the coming week as I am still working from home. Therefore the 2022C has to go back on the shelf for a few days. However, I did manage to measure a detailed IV curve for one of the HP 5082-2826 RF detector diodes used in the ALC circuit in the 2022C.

Hopefully the image below will have enough detail for it to be interesting or useful. The diode has to have this IV curve or it won't be possible to set up a linear response for the 1dB attenuator steps.

The curve below is for a diode taken from the 2022C and it is one of a matched pair. To get this level of detail I have a test jig that connects to a couple of Keithley lab multimeters and a programmable power supply. It is sensitive enough to measure down to tiny fractions of 1uA so the curve below has more detail than a typical datasheet. It was taken at 22degC.

If I enter 1nA for Is and a diode ideality factor of n=1.05 and Vt = 0.0255V into the classic Shockley Diode Equation then this agrees very closely with the diode graph below up until it starts to droop above about 0.32V across the diode junction.

https://en.wikipedia.org/wiki/Shockley_diode_equation

The diode that had been fitted during a previous repair had a different IV curve and it wasn't possible to achieve the correct response for the 1dB steps and also the AM modulation was very distorted when the wrong diode had been fitted.
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Old 19th Apr 2021, 2:37 am   #38
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Default Re: RF signal generator - Marconi 2022?

To give some idea how well the 1dB attenuator works when the RF levelling is correctly calibrated using the 2nd functions in the service manual I measured the accuracy of the 1dB steps from +7dBm down to -20dBm on a lab power meter at 28MHz, 70MHz and 200MHz. The worst error across this 27dB range was about 0.05dB on some steps.

I'm lucky to have a very nice spectrum analyser here with a very accurate log display due to the digital IF it uses. I used this to measure the accuracy of the attenuator system at 70MHz from 0dBm down to -127dBm.

Over most of the range the error was less than 0.15dB. By the time I get down to -127dBm I'm stretching the test system a bit but I think the error was less than 0.3dB across all the 1dB steps down to -127dBm.

That is quite impressive and is much better than the datasheet spec. However, I didn't try and do this test at 1GHz. I suspect the errors would creep up at 1GHz as the 10dB step attenuator isn't going to be as accurate at 1GHz as it is at just 70MHz.
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Old 19th Apr 2021, 2:56 pm   #39
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Default Re: RF signal generator - Marconi 2022?

There's a Marconi 2030 on eBay currently at £385, but once again, way too big for me to accommodate, the 62.5cm depth being the big problem.
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Old 19th Apr 2021, 3:09 pm   #40
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Default Re: RF signal generator - Marconi 2022?

Quote:
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now it seem Broadcom have killed-off many of these since acquiring them
Avago bought-out Broadcom. It looked like a minnow attacking a shark, but they were fronting for Silver Lake financial group. They then decided that the Broadcom name sounded less silly then Avago (have-a-go) and changed the company name.

Then they set about stopping all their small RF parts, all the MMICs the microwave diodes and so on. I don't think there's anything left. They did the obsoletions in two rounds, with no prediction of the second round. So a lot of us expended blood sweat and tears redoing products onto using the parts that survived the first wave, unly to have to do it all over again some months later.

A lot of firms have just got a ban on their designers using anything with the Broadcom or Avago name on it ever again.

Still narked!

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