RSA 306 Overview and Teardown

[KF5OBS]

I saw the Signal Path Blog posted his review video before on this forum. On that day I was the other "exclusive" reviewer who released a video. Only The Signal Path Blog, Alan (W2AEW) and I had access to a pre-launch review model.



SignalVu-PC Overview:



The teardown did not reveal any crazy RF blackmagic, unfortunately. But it is still interesting, however.

I would love to receive some feedback on the quality of the close-up shots for the teardown. That was the first time that I used my Canon XF200 together with a macro lense for this purpose.

[Mr Simpleton]

Thanks for the teardown! Have been pondering on what was under the hood.... Now looking at the signal path and what chips there is, *my* guess would be that the MAX2870 is the main first LO and the ADF4360-1 is the second LO.
Both these chips are small wonders, but offers only limited phase noise performance. So Tek has sacrificed good phase noise for small size and low cost. This will also be a drawback if the box is used as an SDR.

But as a field instrument, it is a true marvel, and the only thing that comes close is the Signal Hound BB60C. I would love to have one to play with, but a bit costly just for playing though.

Two thumbs up! One for the video and one for the RSA306

[G0HZU]

Looking at the images it's possible to make some reasonable guesses about the frequency plan.

The ADC looks to be sampling the 140MHz IF2 signal at 112Msps  which makes sense as it centres the wanted alias on 140MHz.(You can the little 112MHz clock oscillator module in the video)

Also, because they use an ADF4360-1 for LO2 this means the first IF must be somewhere around 2400MHz on one range. The other range will exploit the div/2 of the 4360 so the other IF1 range must be around 1300MHz.

So maybe for RF inputs up to approx 2GHz the IF1 is approx 2400MHz and above this it is approx 1300MHz. it may be possible to work out the IF1 frequencies by reading the text on the little IF1 SAW filters.

Below about 40MHz the RF input signal will bypass the dual conversion up/downconverter and will get fed direct to the 112Msps 14bit ADC.

[G0HZU]

Hope nobody me being a bit negative about the design of this analyser (sorry) but a few things jump out at me besides the low cost layout.

I would have expected to see two SAW filters in series at the 140MHz IF with a gain stage inbetween to overcome losses. However this design only uses one. So I would expect the alias rejection to be barely 50dB (probably 45dB) at the IF passband edges. If so then this isn't very good if you want to use this instrument as a general purpose spectrum analyser rather than a signal analyser...  maybe I've missed something here?

For similar reasons I have concerns about the IF1 design. There doesn't look to be much contributing to ultimate filter stopband rejection at IF1. I just see a lumped filter and what looks like a tiny SAW filter at IF1 so I would be looking to test the image rejection here.  It would be nice to see some closeup detailed images of this area. At the moment I can only guess...

It's a neat little analyser but I think the decision to make it USB powered has meant there are some very significant SWAP design compromises here. eg phase noise, image/IF/alias rejection and internal spurious rejection can all be expected to be quite poor compared to other analysers. I'm not sure I believe the -55dBc spurious free description that was mentioned on one video. I wouldn't be surprised if there were some terms at -45dBc.

If someone gets me an accurate frequency plan I can predict where the main int/ext spurious terms will be and how to test for them.

[Mr Simpleton]

Agree that Tek has cut a few to many corners when designing this little box, still not much else on the market for us needing RSA capability and portabiltiy!
See if I can get one to try out...

[G0HZU]

Someone at work has already asked for one of these on demo/loan so I may get to play with one soon.

I also looked up the frequency response of that 140MHz SAW on its datasheet and it appears that the alias rejection could be as poor as about -40dB at the edge of the IF passband. Maybe they have a custom version of this SAW filter but I can't see it being much better than a  standard SAW in this form factor.

Not sure when our demo RSA 306 will turn up but I'll try and get my hands on it for an hour or so at work.

[Lukas]

Agree that Tek has cut a few to many corners when designing this little box, still not much else on the market for us needing RSA capability and portabiltiy!
See if I can get one to try out...

Take a look at software defined radios. They provide similar features.

[daqq]

Thank you for the teardown and the explanation of the RF path!

[G0HZU]

Thank you for the teardown and the explanation of the RF path!

Note that KF5OBS hasn't described the signal path correctly in the video.

He's missed out the whole existence of the first conversion stage in the video and he seems to think it downconverts direct to a 140MHz IF. This is clearly incorrect. The PLL/LO section that confused him part way through the video is actually the LO for the first conversion stage.

I'm hoping that Shahriar from The SignalPath Blog also does a teardown video because it would be nice to see closeup images of the first IF section with its choice of IF1 filter ranges   These got missed in the KF5OBS video.

KF5OBS also says that the ADC samples at 125Msps in the video which is also incorrect. It appears to sample at 112Msps (because of the 112MHz clock oscillator module seen in the video) and this makes much more sense if you are trying for a 40MHz BW centred on a 140MHz IF. This scheme wouldn't work at 125Msps.

I put this info on his youtube channel comments section (plus a load more background info about the signal path) but he has decided to delete all the info from me. Maybe he thinks he is right and I'm wrong?