TinySA miniature spectrum analyser

 

 I bought this little instrument to help look for interference sources. A portable radio works OK but it doesn't give you a complete picture of the noise problem. I've used one to look at our washing machine lately and I heard all sorts of jingly tunes emanating from it, but until now hadn't appreciated the size of the noise problem. I'm guessing the noise is not only radiated but also gets into the house mains wiring. I also found that it doesn't matter whether the machine is actually in operation or in an ostensibly off state. The interference is much the same.

As my amateur band of chief interest is 80m I set the TinySA to scan from 3MHz to 4MHz and the picture above shows the results. The 10 graticules cover 1MHz so the pulse widths indicated are about 100KHz wide with an amplitude ranging over 30dB.

What else can the thing do.. I'd read it works as a signal generator so connected it to my SDRPlay and fiddled around with the settings and you can see the result below. The frequency is 70KHz. Initially I saw a huge amount of overloading because the default was +7dBm output, but cranking it down to a sensible -40dBm the result was pretty good.

 

 

 

 Here are the settings.

Selecting these is dead easy because the screen is touch sensitive and poking it with the plastic stylus that came with it you can choose whatever you want

 

 

 

 At the other end of the spectrum, the highest frequency reached is 960MHz and here the modulation is NBFM. Some overloading is present and the best attenuation accepted is -38dB so an external attenuator would be advisable. I guess the TinySA would make a good beacon for VHF/UHF aerial testing.

The official spec refers to low band output as "sinus" but the high band as square wave. Output level is said to be +/-2dB and whether that includes the usually understood 3dB overall bandwidth figure needs to be checked.
 Next, I decided to see how the TinySA worked with my homebrew noise source. After all, that was the reason for building one a few months back. The results were very surprising because in the process I discovered my favourite BNC to BNC cable was faulty. Below are the results and I've included the shots where the cable was playing up (I think there's a bad screen ground at one end of the cable)..

 

 

 

 

 

 

 Top left. Scan zero Hz to 200MHz with noise base at -90dBm connected to a 50 ohm terminator. Top right as before but with cable fault evident and broadcast signals present (eg FM stations around 100MHz).

 

Centre left. Scan as before but connected to a 30MHz low pass filter and noise source turned off. Centre right with noise source turned on. Noise level about -50dBm.

 

Bottom left as above but displaying cable fault. In many applications the faulty cable may not have been apparent and indicates any cables used in an experiment should be carefully tested. Ref level was set to Auto.

 Next some pictures showing a similar test using a different cable (not perfect). Scan reduced to to 50MHz and reference level set to manual 0dBm throughout.

 

 Scan 50MHz with 50 ohm termination

 As before with noise source on.

 As before but with filter connected, noise source off.

 Noise source on but with RBW reduced to 3KHz hence baseline 10dB down.

 Finally a word on cables. I used really ancient BNC cables and BNC to PL259 adaptors to fit the old 30MHz low pass filter with a BNC to SMA adaptor to fit the TinySA. The larger adaptors showed signs of losses no doubt because they weren't tight fitting. Photos are pretty poor because I used a handheld camera without flash. I think the TinySA can produce jpeg files if I used the software offered by the manufacturer and if so I'll repeat the tests later (see below).
 

 If you're interested in seeing a scan of the same low pass filter using my DSA815TG, then here it is. This uses a tracking generator rather than a noise source but results are pretty close with attenuation showing up at around 50dB with the TinySA and 60dB with the Rigol but with overall shape very similar. Click the picture to see more, then read on below...

 

 Now a couple of interesting pictures. I'd found problems with the cables I was using so instead connected the noise source, low pass filter and TinySA together with a series of adaptors . I also used software (TinySA-App.exe) to save the scans. The results were much better as you can see below. Up to now I haven't found out about averaging the trace, but having seen a u-tube video I suspect the TinySA either has an automatic averaging method or possibly over 10 samples?

 

 

 

 Something rather odd, but consistent with physics (a relationship between noise and bandwidth) can be seen. The output power recorded above right in the shortwave band is registering around -35dBm which is very much higher than the figure without the low pass filter. Below are pictures showing a direct connection between the noise source and the TinySA without the low pass filter.. first with noise off and second with noise on and clearly the noise power is registering around -55dBm or 20dB lower than can be seen using the low pass filter. The effect of using a noise source instead of a tracking generator results in quantitative results which at first are puzzling but nevertheless, for filter experiments and receiver alignment, the use of a noise source is OK as long as qualitative results are acceptable (eg. the shape of the filter response). In fact these two pictures show a 60dB attenuation at 40MHz which is consistent with the Rigol result shown above.

 

 

 

 The scans above were made with my homebrew noise source and I was interesred to see how this compared with the Nooelec noise source, albeit using a 10dB attenuator because the total noise power output from this is higher than the maximum rating of the TinySA.

The first two were made with the scan from zero to 50MHz with noise OFF then ON. As you can see the baseline is around -95dBm rising to -40dBm (with a 10dB attenuator fitted)
 
   

 I then repeated the tests with the scan set from zero to 500KHz. The baseline varied and the noise level was arounf -40dB once the frequency had risen to 100KHz which is the lower spec for the TinySA. I also tried the scan in the UHF range but it seemed that the noise source was oscillating somewhere around 500MHz so I didn't bother.
 
   

 In summary, the TinySA together with a noise source would be ideal for testing and adjusting filters, producing similar results to a spectrum analyser and tracking generator. The smoothness of the curves is dictated by the built-in averaging circuitry and the bandwidth setting. This latter is normally automatic but it can be set manually bearing in mind the lowest settings will be pretty slow. The end result will not match that of an expensive spectrum analyser with its much better averaging circuitry.

 pending

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