There's lots of case radiation to see to several GHz with a probe, presumably PLL products.
I searched for the final LO specifically and the most prominent clean narrowband signal I found was 750.000MHz. This one is quite loud.
Looked at the conducted ports again. The RF input is the most fruitful. Two components observed as follows, SA zero span ...
(Centre f) ... Sig 1 ... Sig 2
(100MHz) ... 915 ... 2025
(200MHz) ... 1013 ... 2070
(1000MHz) ... 1815 ... 2475
The frequencies are approximate since recorded from a broad span.
If the observed 750MHz is the final LO, final IF is 60MHz. Does the -60MHz spur fit that? ... it's not immediately obvious to me but I'll have a ponder on the possible mechanism.
Here are my thoughts so far (bear in mind, I’m not very experienced in defining frequency plans for triple conversion receivers, but anyway):
We do not know the first IF, nor the 3rd, but we do know the 2nd to be 810MHz.
The first IF has to be above the highest input frequency, so I’d say it has to be at the very least 3.6GHz (hopefully a good deal higher than that).
To me it’s quite obvious the problem cannot be in the first mixer, if only because the spur has a fairly constant offset to the input signal (within 2MHz) and is present for the entire input frequency range. Any unwanted mixer product would only fall within the first IF for a very narrow range of input frequencies. Furthermore, I would rule out that it’s a mixing product with any harmonic of the input signal, as various different signal sources have been used with quite similar results, and the HP8642B used by rf-loop has provided a decent spur suppression even though this RF-generator of all things exhibits rather high harmonic distortion.
So let’s look at the 2nd mixer.
We have to make an assumption for the first IF and this is important, as this determines the 2nd LO frequency and the spurious mixer products in turn.
Since we obviously want the 3rd LO to be easily derived from the 2nd LO, it should indeed be a 4:1 ratio as suggested by G0HZU and we need to consider the 3rd IF at this point as well.
And here the troubles start. A 3rd IF of 30MHz could explain an unwanted image response, but that’s not what we’re seeing here. It rather looks like the spurious frequency moves twice as fast as the signal frequency, hence it has to be a 3rd (or 4th?) order intermodulation product, so the 3rd IF could be anything but 30MHz and 60MHz would be rather plausible.
Also, with the frequency plan posted by G0HZU, we’d never get out of the reverse frequency position of the IF signal, but that does not change with IF frequency, we would rather have to give up the 4:1 ratio for the 2nd and 3rd LO.
Having played with the numbers a bit (unfortunately I don’t have any fancy software for that), I just didn’t find a solution yet. Independent of your measurements, I’ve landed at 60MHz IF3 and 750MHz LO3 at some point, but still didn’t get anywhere. Because of the symptoms, I firmly believe that it has to be some unwanted mixing product with the 2nd harmonic of the 2nd IF signal. A 4th order mixing product looks good at first glance, but then I believe the spur would appear 30MHz off the carrier and not 60MHz.
In any case I am still puzzled and would not rule out that it’s not a problem with the 2nd IF filter, but maybe the 3rd mixer. Maybe they have used a double balanced mixer initially and thought they swap it out for a single diode – LOL. Just kidding…