How important is the quality of the mixer in the tinySA. #tinysa
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The mixer in the tinySA is a relative expensive part and it is obvious to see if cost can be reduced by using for instance an active gilbert cell mixer such as the IAM81008. One big advantage is the active mixer will often have conversion gain (+9dB for the IAM81008) instead of a loss (about -7dB for the ADE-1) so you can save on the cost of the LNA.
In evaluation the performance differences I looked at three characteristics. 1: The noise floor at a certain resolution bandwidth and nothing on the input 2: The maximum signal that can be measured without being compressed (1dB compression point) 3: The 3rd order Input Intermodulation point. This is somewhat complex to explain but quantifies the ability to measure two signals without generating artifacts by unwanted mixing of the two signals. So I did quick comparison of the ADE-25MH and a IAM81008 The IAM 81008 based tinySA has in total 16dB less loss in the signal path so I used 16db attenuation for all measurements. This -16dB can be reduced to get more sensitivity (like an LNA) but reducing will also imply more distortion (lower IIP3) and lower 1dB compression point. Lets first look at the noise floor when measuring the power calibration signal from the tinySA itself. The measured signals are at the same level but the ADE-25MH based tinySA(purple line) has a bit less noise versus the IAM81008 based tinySA (blue line), but not a big difference The 1dB compression point was equal at 0dBm for both versions version. This confirms the -16dB attenuation used with the IAM81008 gives a fair comparison Next is the IIP3 measurement The important signals are at 5MHz and 8MHz. These are the unwanted intermodulation products of the two input signals at 6MHz and 7MHz. The numbers a the top (IIP3=9.5dBm) relate to the blue line of the IAM81008. The purple ADE-25MH line has lower signal levels for 5MHz and 8MHz and the calculated IIP3 is at least 4dB better so even with 16dB attenuation the IAM does not perform better then the ADE-25MH. Reducing the attenuation makes the performance quickly much worse Last step is to establish the noise level And this shows the biggest performance difference. The IAM81008 (blue line) has a 10dB high noise floor compared to the ADE-25MH. This difference will depend on the selected RBW as the difference is less in the first measurement shown. Due to the high sensitivity of the IAM81008 the tinySA is also more susceptible to stray radiation from other sources. The big bump at 44MHz is an alias of a mobile phone cell tower signal nearby and the FM broadband stations around 90MHz also have more impact. This was a rather quick measurement but it provides some insight in why the first mixer in a spectrum analyzer is so important. -- HBTE Files section:?/g/HBTE/files Erik, PD0EK |
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Probably a really dumb question but did you try the "H-Mode" mixer? Not sure what the frequency range of that topology is and it probably won't be any cheaper since it requires transformers (unless you scavenge those from some equipment like old ethernet cards) but it should give better figures, or at least that is what I read and probably misinterpreted. On Mon, 6 Jan 2020 at 18:24, <erik@...> wrote: The mixer in the tinySA is a relative expensive part and it is obvious to see if cost can be reduced by using for instance an active gilbert cell mixer such as the IAM81008. One big advantage is the active mixer will often have conversion gain (+9dB for the IAM81008) instead of a loss (about -7dB for the ADE-1) so you can save on the cost of the LNA. |
