Our second faulty SSA3032X returned under warranty to Germany today.
Now 9 weeks from initial purchase, but we like this instrument, so waiting on a replacement ... third time lucky? Hope so.
Our second faulty SSA3032X returned under warranty to Germany today.
Now 9 weeks from initial purchase, but we like this instrument, so waiting on a replacement ... third time lucky? Hope so.
Fingers crossed!
In a situation like this, at least by now, a good distributor should test any new instrument for this specific flaw before sending it to you (or anyone else for that matter). Have you negotiated something like this?
Here is some things about 60MHz below carrier spur.
Here is case when carier moves slowly up.
If slowly move carrier frequency up this spur move down until it hop up 2.5MHz. After jump up it again continue down until it have moved 1.25MHz and again hop up (if carrier continue moving up).
It moves like zig-zag. Moves slowly down same amount what carrier moves up. Until it reach limit where it need hop up.
These movements are roughly (not measured enough accurately) 1.25MHz down following carrier but opposite direction, and then 2.5MHz hop and again start moving down following carrier but opposite direction.
I have not opened my SSA for look what happen if take bottom and top aluminium block out and try do some GND contackt-seal trick for reduce possible leak between blocks. Area between IF3 and 810MHz saw filter looks "terrible" if think high grade isolation but I have not opened it for look aluminium wall between these blocks. Also between these blocks GND is cutted for air gap but.... is it possible also that this aluminium block do some "fun" things and then every SA is different. I dso not like idea that there is just plain aluminium surface "contacting" (or not) with PCB GND surface "randomly" here and there and mostly just near screws (if they are tightened right. But how can tighten chinese soft screws...
In professional equipments there is mostly also good flexible rf-seal in this kind of places between aluminium blocks and PCB.
Attached images may clarify how this spur frequency acts relative to carrier frequency.
Note that for this 10MHz span 60MHz image I have rised 120MHz carrier level so that spur level rise for better visibility.
Other image show same carrier but now with -30dBm mixer level and there can see spur is under -70dBc.
I think the frequency hopping is due to the fact that the analyzer does the analysis based on an FFT. If FFT is involved the LO probably is set at fixed intervals.
You can try and deselect FFT, and see if the frequency hopping still occurs...
I really don't think anyone should take their analyser apart yet but if anyone is brave enough to take their analyser apart then please be VERY careful not to flex the big PCB because you could easily cause cracks inside the many ceramic SMD caps on the main board. Even a tiny amount of flex/stress could damage lots of capacitors. Not good!
We don't know what is causing the variability in rejection. It could be they changed something on the PCB or it could be a metalwork/isolation issue. It could just be bad luck with how the stopbands interact when two SAW filters are connected in series like this. Probably best to wait until RFDUK gets the third analyser to try out.
The zigzag nature of the spurious as described by TurboTom in post #13 and by rf-loop is exactly what I'd expect to see because the analyser is in FFT mode for all the screenshots I have seen so far.
I think the frequency hopping is due to the fact that the analyzer does the analysis based on an FFT. If FFT is involved the LO probably is set at fixed intervals.
I think the frequency hopping is due to the fact that the analyzer does the analysis based on an FFT. If FFT is involved the LO probably is set at fixed intervals.
I think this is why rf-loop added the text “Because this is frequency hopping sweep spectrum analyzer…” to his 2nd image
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Btw. I have asked my friend for some more tests and it’s now confirmed that the spur is related to the input signal in a linear way. He has tested this for mixer levels of -40dBm - -20dBm.
So it actually would be some sort of image response and I think the concerns voiced by rf-loop, regarding poor contact (or no contact at all) between aluminum and the PCB traces sound very plausible to me. Trying to contact aluminum usually is a true nightmare…
I consider the cover issue only valid if Siglent changed from the machined parts to completely die-cast configuration without any post-processing. CNC machining is so accurate nowadays that even if they only machine the surface mating with the PCB, shielding performance should be good enough that there wouldn't be the inaccuracies present that we are observing on the recent instruments. If Siglent doesn't employ any post-processing of the cast components, okay. But since machining of the covers only requires any dated three-axis-mill, I don't think they will risk the problems induced by using bare, die-cast shields. Anyway, I may be proven wrong - which would be a relly stupid move on behalf of the manufacturer... . I would be curious to see a recent SSA being torn down for ananlysis...though I wouldn't recommend anyone to do this.
Cheers,
Thomas
Whatsoever, I've got to agree with nctnico that only a shielding problem without any additional change in hardware would not be very likely to have so much effect to change the image frequency rejection by 20dB on some machines.
It looks like many peoples have more like audio experience than rf.
It looks like many peoples have more like audio experience than rf.
So instead of starting to insult 'peoples' Have you as a Siglent dealer, asked Siglent for a reaction on this issue ?
I openly admit that I'm more experienced with digital, lower frequency and high power stuff than with RF/microwave circuitry but I tend to have quite a good "gut feeling" and I'm curious and brave (stupid?? ) enough to tear down my SSA for science's sake.
So please have a look at the attached photos. First of all, I found the RF PCB in my SSA to be warped round about 1mm in the centre -- probably this has no detrimental effect when it's sandwiched between the shielding plates. The fingerprints on the PCB don't originate from me but must have been there when the instrument left siglent's production line -- I handled the PCB only by the edges.
By the way, the lands of the shields completely cover the "isolation slots" as shown.
I removed the PCB, apparently it's exactly the same configuration as in Dave's teardown. And now I put some standoffs in the places where the long screws bolt the RF assembly to the chassis in order to install the RF PCB completely witout any shielding.
So please have a look at the screenshots... (full span is without input signal, just for comparison and to visualize the effect of the missing shields). I guess this rules out the (suggested lack of) shielding as the cause for the 60MHz spur one for all times.
Cheers,
Thomas
So please have a look at the screenshots... (full span is without input signal, just for comparison and to visualize the effect of the missing shields). I guess this rules out the (suggested lack of) shielding as the cause for the 60MHz spur one for all times.