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Old 27th Nov 2022, 12:40 am   #282
regenfreak
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Join Date: Nov 2018
Location: London SW16, UK.
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by G0HZU_JMR View Post
Quote:
I think my problem is more to do with working out the origins and derivations of the linear equations from the first principles in the graphs.
Another way to look at this is to forget about the TOI point and just inject two clean distortion free tones into the analyser. Set the tone level really low to start with. Choose a convenient RBW, say 10kHz and set the front-end attenuator to 10dB and the reference level to -5dBm. On your Siglent analyser, I think that the noise floor (DANL) would be at about -90dBm with these settings if you have the preamp turned off.

In theory, you should be able to keep increasing the input tone levels up to a point where the IMD tones would have the same power as the displayed -90dBm DANL. In practice, it will be difficult to judge where this happens, you could briefly select a narrower RBW to make it easier to see the -90dBm IMD tone level as it begins to compete with the DANL and become visible.

When you reach this point, the difference between the input test tone levels and the -90dBm DANL is the spurious free dynamic range with a 10kHz RBW.
You would obviously have to set the analyser to a span where the phase noise of the analyser doesn't mask the DANL. Maybe try a 5MHz span with 1MHz tone spacing to try and minimise the impact of the LO phase noise. Set the RBW to 10kHz. I'm guessing that your analyser phase noise will be -120dBc/Hz at 1MHz offset. 10kHz RBW is 40dBHz so you should be able to see about an 80dB dynamic range if there was no IMD. However, I think the Siglent will probably have a SFDR of about 73dB with 10kHz RBW because of the IMD it will generate.

Alternatively, if you know the input TOI of the analyser and the DANL from the datasheet, you can predict the SFDR with a reasonable amount of accuracy without having to do any hardware tests. Just use the equations you already have. The equations allow you to work backwards from the published TOI and noise floor data to work out the SFDR at a given RBW setting.

This generally works well for classic spectrum analysers that use a diode pair or diode ring mixer at the front end. I don't know if the Rigol and Siglent analysers will follow the same classic equations. There are solid state devices in the front end of these analysers and the linearity of these devices probably won't be as predictable. Therefore, the tests outlined above might not give the expected results.
Thanks for that. Your numbers are close to what i have got:

At 10kHz , 10Mhz, the front-end attenuator set to 10dB the noise floor. the DANL of the Siglent is measured to be -94dBm with 100 averaging samples.

The phase noise is <112dbc at 1Mhz offset from Sigent's test data.


The Siglent SSA3021X and SSA3032X use HM488 GaAs Schottky diode ring 1st mixer:

https://www.analog.com/media/en/tech...ets/hmc488.pdf


IP3 = +15db, NF = 7, 1db compression = +8dbm.

There is an excellent video on the walkthrough teardown of SAA3021X from EEVblog:

https://youtu.be/fvTfBwRzpdo


I will try to do the above test tomorrow assuming that the two-tone test is done without DUT and LP diplexers.

One of the key things that no one explain is that what is the effect of tone spacing f2-f1? Does it matter it is 100Khz or 1MHz? With 100KHz, I can use much smaller RBW and it is much easier to see the IP3 products as they are often near the noise floor, particularly with DUP with high TOI.


In the attachment, there is a clear, 1:1 linear relationship between DANL and internal attenuation for a particular unknown spectrum analyzer given by Anritsu. I am guessing that the attenuation increment is positive and it is equal to -1x mixer level decrement, hence there is a -1 slope for DANL vs mixer level straight line. Probably I have answered my early question. The y-intercepts for IMD2, IMD3 and DANL straight lines are dependent on the particular model of the analyzer or some kind of frudge factors ...
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Last edited by regenfreak; 27th Nov 2022 at 12:56 am.
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