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Old 1st Aug 2018, 12:33 pm   #21
dave walsh
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Yes I'm fairly sure I can find them David. I've got insulated storage rooms rooms in the roof now so it's time to start opening boxes in general before it ends up with someone else having to do it in my absence

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Old 1st Aug 2018, 12:57 pm   #22
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Quote:
Both Marconi 2018 and 2022 give visibly similar results.
Yes, you probably won't see much difference for your application and across the same 2.5-3.5MHz frequency range as the NRD digital VFO I'd expect the Marconi 2022 to have a similar phase noise profile as rhe NRD digital VFO. This is based on a crude/ideal model of your digital VFO.

Down at 2-3MHz the Marconi 2018 will have close in phase noise maybe 20dB lower than the above so it is probably the best choice if you wanted to use it as a clean LO to downconvert the Digital VFO without any risk of degrading the result. The 2022 phase noise will be a bit marginal I think. But if your NRD digital VFO is noisier than it should/could be then maybe it doesn't matter if you use the 2018 or the 2022.

I have a library here of phase noise plots of various lab sig gens we have used at work over the years. The phase noise of the 2022 is quite compromised across the HF bands because of the frequency plan it uses. I have an old library plot of a works 2022 at 10MHz (see below) and the phase noise will be about the same as the noise at 3MHz because of the down conversion system it uses for this band. So I hope this Marconi 2022 plot is of interest.

I'm on my lunch break but I'll post up a phase noise prediction for your NRD digital VFO later. It's going to look quite similar to the profile of that 2022 even though they are quite different systems.
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Old 1st Aug 2018, 11:01 pm   #23
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Here are my initial thoughts having looked over the block diagram and the circuit diagram.

The NRD-515 LO1 system has a large variation in division ratio N across the range (on both loops) which is the first thing I spotted. This usually means the loop filter design will be compromise unless some trickery can be adopted to offset the changes in loop response you will get across the tuning range with such a large variation in N. Sometimes this can be done by tailoring the VCO gain or the phase detector gain across the band. Otherwise I'd expect the loop (and close in phase noise) response to vary quite a bit across the tuning range of each VCO.

None of the VCO circuits look to have been designed with phase noise as a top priority. However, it's hard to tell for sure because not all the component values are known. But if I assume symmetry in the varactor diodes then the loaded Q of each design seems to be very modest.

I noticed that the switch IC after the 4044 PD in the Digital VFO has a (3V?) pedestal on it because of the zener diode. This prevents operation down at low Vt and maybe this improves VCO startup reliability or maybe it's there to prevent operation at low Vt where the VCO gain will get undesirably large.

I can make a few assumptions about the VCO in the Digital VFO and come up with an idea of what to expect in terms of phase noise. However, this is very much a ballpark estimate. I'd expect the resonator power to be a few mW and the loaded Q could be about 10 in the middle of the range. If I put in a flicker corner of 15kHz then I get a free running phase noise plot like the smooth dark blue trace in the plot below. This shows a phase noise response of:

-64dBc/Hz at 100Hz offset
-94dBc/Hz at 1kHz offset
-122dBc/Hz at 10kHz offset

The overall division ratio in this loop is quite large so the divider noise in the loop will be quite high. The 4044 and the dividers used in this design are old devices and are a bit before my time but a crude estimate of the divider noise contribution would be -67dBc/Hz. With a phase detector frequency of 1kHz the loop bandwidth will have to be quite low and there will be a compromise here in terms of tuning speed, reference sideband suppression and noise response. However, the large variation in division ratio (N) makes it more difficult for the designer to come up with a compromise for the loop filter design.

But if the typical loop BW is in the order of 100Hz then the synthesiser noise at offsets above maybe 200Hz will be quite similar to the free running phase noise of the VCO. This gets divided by 10 so there will be about 20dB of noise cleanup once it is divided down to 2.5-3.5MHz.

So my first attempt at predicting the phase noise of the digital VFO is as the image below. I think it could be as good as the green response in the plot below but in reality it might be several dB worse in places especially because of the influence of the wide range in division ratio in the loop. This level of performance isn't exactly state of the art but it is not going to cause any obvious noise issues to an operator unless you are operating in cw mode on a busy band with lots of big signals a few hundred Hz or so away. So something else must be degrading the noise quite a bit. As has already been suggested by others, maybe something has degraded over time that is causing this problem. I doubt it would have been there since new if it is as extreme as you describe.

The 4044 and the 4016 switch will have a dead zone in the phase detector gain when the switch briefly isolates both arms (at phase lock) but I'm not sure this will be the cause of your problems. Maybe someone else who has used the 4044 (or 11C44) can advise here but I don't think it will cause a warble unless the loop BW was designed to be extremely narrow. This would also affect the tuning speed so I doubt the loop BW will be very narrow. It might be 50-100Hz at a guess? Adding a bleed resistor of a few megohms across the loop filter can sometimes help but this can affect the level of the 1kHz reference sidebands. Also, any leaky caps here will affect the level of the sidebands. So component selection can be critical here as you have already demonstrated. If there was an issue with the 1kHz reference sideband levels then you would see/hear an obvious tone on a test signal in cw mode.

The main loop looks to use three VCOs and at first this all looks to promise low phase noise. However, the VCO circuit suggests that the loaded Q of each design is quite modest and so the phase noise at a 10kHz offset of the 100MHz VCO might only be in the order of -120dBc/Hz and this will dictate the phase noise response of the overall LO1 synthesiser as the contribution from the Digital VFO will be insignificant here. However, the Digital VFO will influence the LO1 phase noise at offsets much closer to the carrier and so this is why it is most likely to cause the problems you are seeing. An overall LO phase noise of -120dBc/Hz at 10kHz offset is maybe 10-15dB better than some of the worst (synthesised) ham radios of the early/mid 1980s but it is not as good as a decent design today. The Digital VFO will probably only influence the phase noise at offsets up to maybe 1kHz or so.

The main loop also has a huge variation in division ratio. It would be interesting to see how the close in phase noise of the overall LO1 synthesiser varies across the tuning range of the receiver because of this. But I don't think this is anything to do with your issues because the Digital VFO section appears to be the culprit.

This might not be relevant but the LM723 isn't really a low noise reg unless you add a filter cap to the relevant pin on the IC. This can reduce the output noise from the regulator by quite a bit. I'd expect the output impedance of this regulator to be very low even wih the 8R current sense resistor. This is because the feedback is tapped back after this resistor. So the output impedance is probably a fraction of an ohm at AF frequencies. So any internal noise produced by this reg will get forced onto the 12V rail and I'm not sure that adding caps will help much in removing this noise because they are fighting the low Z output of the regulator and the cap ESR could be greater than the reg impedance. However, I'd be surprised if the LM723 reg noise on the 12V rail could cause the significant problem that you are seeing. It's an old PSU regulator chip that I have only ever used here at home so I don't have any experience in terms of analysing its performance in any detail.
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Old 2nd Aug 2018, 12:57 am   #24
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Here's some info that is a bit more definite and it may be of some interest. See below for a phase noise plot I took of one of my old Marconi 2019 sig gens. I rarely use these old generators anymore and so this was an excuse to fire them up and show what they can do.

The 2019PN.gif plot below is at 2.9MHz and the phase noise is measured with my Tek RTSA in signal source analyser mode. The Tek analyser has a typical phase noise floor around -136dBc/Hz in this mode so it is of limited use here but it can show that the 2019 has low phase noise close to carrier.

At a 150Hz offset it shows the 2019A has a noise level of -100dBc/Hz and this is about 20dB better than the Marconi 2022 at a similar test frequency. Your 2018 should be similar to my 2019.

Also below is a plot of the first LO of an old Cobra 148GTL-DX (mk2) CB radio called 148PN.gif. This is the first LO (at about 17MHz) when measured on channel 19 on the now legal mid band AM block. This LO will probably have similar phase noise as your JRD-515 at a 10kHz offset at about -120dBc/Hz but you can see how noisy it is when tested close to carrier at a 50-100Hz offset. It typically shows -55dBc/Hz and this is much noisier than I'd expect the close in noise of a healthy JRD-515 at these offsets. But that is just a guess.

But this radio does not show any funny noise effects on a scope on the AF out with ten cycles shown on a scope. It also sounds quite clean when beating with a test tone from a sig gen. These old CB radios did always sound very nice on SSB receive and transmit and they are a similar vintage to your JRD-515 because they were mainly manufactured around 1981 to 1982. You can also see the fairly significant 10kHz PLL reference sidebands on the CB radio LO plot at 10kHz offset with harmonic spurs at 20kHz, 30kHz etc.
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Old 2nd Aug 2018, 1:40 am   #25
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

For completeness I dug out my little 2022C sig gen and measured it at 2.9MHz on the Tek SSA so it can be directly compared to my 2019 plot. See below for the plot and it is very similar to my previous library plot of a works 2022 at 10MHz. The phase noise is typically 20dB worse than the 2019 down at this test frequency.

At higher test frequencies there is much less between the 2019 and the 2022 in terms of phase noise performance and they are probably quite similar up at UHF. Both of my 2019s look quite noisy once in the doubled range above 512MHz and the 2019 is at its very best across the 2-30MHz range where the benefits of the frequency division really help reduce the phase noise. Across 2-30MHz the 2019 is better than a lot of modern sig gens in terms of close in phase noise. But up at UHF the 2019 is quite a poor performer in terms of close in phase noise so I rarely use mine these days.

My 2022C has been repaired a few times and it has some dodgy looking OM345 alternatives fitted to it. But the phase noise still looks fairly normal for a 2022 for most of the frequency range. I think both of my 2019 sig gens and my 2022 could benefit from a decent service to replace a few tired electrolytic caps as they seem to have more spurious terms on them than I expected to see and some of them look to be harmonics of 50Hz.
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Old 2nd Aug 2018, 5:59 am   #26
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Hi Jeremy, Yes, the MC4044 does have a burst of erratic behaviour in its transfer function, centred on zero degrees difference, right where it tries to lock a second-order loop. In the day, a lot of users of the part didn't understand the issue and Motorola weren't exactly forthcoming. Later, it got identified and talked about. The usual countermeasure was a little analogue offset from a resistor from a power supply, to give the PSD something to oppose. Inevitably this added output needed from the PSD meant bigger ref freq sidebands, and a new route for PSU noise to FM the output.

The two Marconi sig gens exemplify instruments for two different markets. Basic repair operations are OK with a simple, cheaper generator, typically a single frac-N loop in recent designs. It'll let you trace signal paths, check demodulators are working and generally do alignment. A more expensive instrument is needed to get lower phase noise to allow performance measurements to be made. Adjacent channel rejection ratio is usually the most demanding along with measuring selectivity down at the skirts. The third grade of sig gens are the ones intended for phase noise measuring sets, price and weight go up rather disproportionately!

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Old 2nd Aug 2018, 9:01 am   #27
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Jeremy

How long do you ideally need WAV files of the demod AF output to be please?
What would be the shortest period, in secs, or mins?
The are attachment difficulties, and I would like to be able to post here, just for everyone's interest and or curiosity.

rad50

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Old 2nd Aug 2018, 10:32 am   #28
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Here you go.

I may have to call the gent I sold them to
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Old 2nd Aug 2018, 8:22 pm   #29
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Quote:
How long do you ideally need WAV files of the demod AF output to be please?
Something like 5 seconds would be ample. If this is still too big then cut it down to just two seconds.
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Old 3rd Aug 2018, 6:22 am   #30
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Hi Jeremy,

AF WAV files for you, all about 5 seconds.

The file names contain the important info.

Two NRD515 receivers RX1 (with most FM) and RX2.
One iCom 7600, quite new.

Two sig gens, Marconis 2018, 2022.

RF Freq was 10.5MHz, centre of the NRD515 VFO 1 MHz tuning "range".

NRD515 set to USB, 6 KHz filter, slow AGC.
iCom set to USB 3.5KHz filter, mid AGC.

Recorder was program Audacity on an ASUS laptop running on battery for isolation.
Record rate 44.1 KHz.
Microphone input.
Mono.
Record level was -10 dB as on Audacity recording bargraph.
AF output from receivers was transformer coupled, balanced above earth.

I have included three extra files for control or comparison.
One the NRD515 on AM.
The other two are for interest, an iCom 7600.

File list

NRD515 RX1 10MHZ5 SSB 1090HZ 2018 44K1 5SEC.wav
NRD515 RX1 10MHZ5 SSB 1090HZ 2022 44K1 5SEC.wav
NRD515 RX2 10MHZ5 SSB 1090HZ 2018 44K1 5SEC.wav
NRD515 RX2 10MHZ5 SSB 1090HZ 2022 44K1 5SEC.wav

ICOM7600 10MHZ5 SSB 1090HZ 2018 44K1 5SEC.wav
ICOM7600 10MHZ5 SSB 1090HZ 2022 44K1 5SEC.wav
NRD515 RX1 10MHZ5 AM 1000HZ 2018 44K1 5SEC.wav

Because of forum attachment limitations, I have uploaded the files as "pseudo PDF".
You will have to download, then edit the file extensions (only) to WAV.

Thank you

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Old 3rd Aug 2018, 6:26 am   #31
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Hi again Jeremy

the extra three "WAV" files, for interest if you get time.

For casual readers, see previous post.

thanks
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File Type: pdf ICOM7600 SSB 1090HZ 2022 44K1 5SEC.PDF (437.4 KB, 105 views)
File Type: pdf NRD515 RX1 10MHZ5 AM 1000HZ 2018 44K1 5SEC.PDF (448.2 KB, 109 views)
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Old 3rd Aug 2018, 10:41 am   #32
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

this is proving to be a very interesting read.

I do like the older radio stuff. Just finished a Trio 2M multimode ste, and halfway through an old "Belcom" 11/10m radio thats proving to be a handful.

All sets that I wanted when they were new but couldnt afford on a paper round
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Old 3rd Aug 2018, 11:52 am   #33
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Well, there's definitely a difference between RX1 and RX2. RX1 has a slight roughness to it, whereas RX2 is smooth.
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Old 3rd Aug 2018, 1:35 pm   #34
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Thanks for posting up the wav files. So far, I've only had a chance to have a quick look at the spectrum of RX1 and RX2 from the 2018 but both have a fair bit of 50Hz modulation on the 1090Hz signal. These appear as sidebands either side of the wanted signal. It looks like RX1 has much higher 1kHz reference sidebands compared to RX2 but there are so many 50Hz related sideband tones and harmonic tones that it's hard to make it out for sure. But the 'ref sideband' does appear exactly 1kHz above the wanted signal.

The loop bandwidth of the Digital VFO appears to be about 80-100Hz which is probably right. However, the level of the close in VCO/divider noise is about the same as the Cobra SSB CB radio which is a bit disappointing. But it's the 50Hz and 100Hz sideband terms that stand out the most. There's also a rising bump in the noise towards very low frequencies but this might be in your test setup rather than the radio.

I'll look in more detail later and post up a few spectrum plots.

Quote:
Hi Jeremy, Yes, the MC4044 does have a burst of erratic behaviour in its transfer function, centred on zero degrees difference, right where it tries to lock a second-order loop. In the day, a lot of users of the part didn't understand the issue and Motorola weren't exactly forthcoming. Later, it got identified and talked about. The usual countermeasure was a little analogue offset from a resistor from a power supply, to give the PSD something to oppose. Inevitably this added output needed from the PSD meant bigger ref freq sidebands, and a new route for PSU noise to FM the output.
Hi David, I've not used the MC4044 as it is so old although someone at work must have used one in the past because this part is on our older CAD parts list at work. It looks to be quite a slow device so I guess this will make the dead zone wider and more problematic because the propagation delays in the IC will be longer. So this might explain why the divider noise is higher than I expected to see. But then again these two radios may be faulty in this respect. The RX1 radio does look to have higher reference sidebands and slightly higher 50Hz modulation than RX2.

The n times 50Hz sideband terms could be getting in from lots of places including the PD reference or maybe on the digital supply. I can try and play the wav file into the Tek RTSA to see if the tones are AM or FM but I might not get time to do this today.
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Old 3rd Aug 2018, 2:34 pm   #35
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

At work, nearly all the stuff we do goes in manpacks or vehicles so I don't have much experience wrt PLL design for AC mains powered gear. However, could the VCO in the Digital VFO use a tuneable magnetic core and does it therefore need to be carefully screened as per the original factory assembly to prevent mains pickup from a nearby transformer?

However, I wouldn't think this was a likely cause of the problem because the amount of modulation seems to be very high, much more than I'd expect to see. So maybe it could be 50Hz modulation on the main frequency reference multiplied up via the 1kHz PD reference signal. There is a big 20*log(Division Ratio) multiplication here for the amplitude of any low freq FM on the reference so you don't want any 50Hz FM on the 1kHz PD reference or it will multiply up in amplitude a lot and paint itself onto your Digital VFO via the action of the PLL. There is a 20dB cleanup in the div/10 prescaler so if this is FM on the Digital VFO then it must look really bad on the actual 24-34MHz VCO (before it gets divided by 10).
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Old 3rd Aug 2018, 5:37 pm   #36
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

There are all sorts of problems that create 50Hz sideband combs. Earth loops, bad reg design, drying out electrolytics, bad joints.

I'd start with a capacitor fest in the appropriate areas then cleaning and tightening screws etc.

David
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Old 3rd Aug 2018, 5:45 pm   #37
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Here's a screenshot of what the Cobra SSB CB looks like with an SSB test tone of about S9 on USB. This radio was one of the better radios of its type and was often praised for its good quality audio on transmit and receive. This is a 13.8VDC mobile radio so it isn't going to have 50Hz sidebands on it but it is still worth showing it as a comparison.

You can see it has no sidebands of any kind on it and the second harmonic of the AF tone is quite low. However, the AGC system on these Cobra CB radios is a bit aggressive and it tends to clamp the signal to noise ratio even for big signals. So the overall noise floor is not as good as your NRD-515. Also the VCO and PLL divider noise is slightly worse than your radio. I think the 50Hz related components on your NRD-515 indicate there is a (fixable?) problem somewhere.
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Old 3rd Aug 2018, 6:18 pm   #38
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Here are the wav file based plots of the Icom 7600 radio. Very nice...

The first plot on the left shows that the Marconi 2022 signal generator is clearly the weak link here with quite a bit of added noise and spurious terms. But this would still sound very good to a listener I think. The plot on the right using the Marconi 2018 signal generator really shows how clean the Icom 7600 radio is.
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Old 3rd Aug 2018, 9:21 pm   #39
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Here's the spectrum of the wav file for RX1 in AM mode. It looks a lot better with much lower spurious terms. However, there is a spurious term at exactly 1kHz (when I zoom in) and this is most likely to be from leakage from the 1kHz PD signal. But this is low enough to not really be an issue.

It must have been a bit of a battle for the designer to get adequate 1kHz ref sideband suppression because the ratio between the loop BW and the 1kHz ref frequency is barely more than 10. The large variation in division ratio in this loop can't help here either. Maybe the need to keep lowish ref sidebands is why there is no 1Meg(ish) bleed resistor in the circuit for the Digital VFO loop filter?
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Old 4th Aug 2018, 9:46 pm   #40
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

I only had time to have a quick look at the wav files on the RTSA today. The RX1_2018 file looks to have mainly 50Hz FM on it with about +/- 11Hz peak deviation. The RTSA can demodulate the signal for both AM or FM and it looks like FM modulation with a pointy sinewave (almost a sawtooth) at 50Hz.

This is a lot of FM and it must be a lot worse on the 24-34MHz VCO itself. The VCO gain is 2MHz/V so you only need a few uV of interference on the VCO tuning line to modulate the VCO and begin to see the effect on the signal on a spectrum analyser. However, the 12V supply is double regulated with linear regulators so it's unlikely to get in here unless something was very wrong in the PSU and even then I'd expect to see a bigger effect at 100Hz rather than 50Hz.

There is the VCO coil itself plus some inductors in the VCO circuit for the Digital VFO and these can act as pickup devices for stray magnetic fields. Is there any way you might be operating the radio on top of or near something with a large magnetic field? Maybe next to a large PSU/transformer or a charger or fan?

I'm used to seeing similar effects from DCDC converters in some of our highly compact (HF though microwave) receiver designs at work but these have VCO tuning gains of hundreds of MHz/V and even then the FM effect is much less. Maybe -60dBc worst case at the switching frequency of the DCDC converter. But you are seeing -24dBc after a divide by 10.
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Regards, Jeremy G0HZU
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