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TinySA Ultra+ - Signal generator level error

NelHau
 

System: v1.4-192-g73fe677

While checking the output levels of the signal generator, I noticed an odd behavior at signal levels below -70 dBm.

When checked against a bench top spectrum analyzer, the signal levels were measured to be within about 0.5 dBm from -18.5 dBm to -70 dBm. However, when going from -70 dBm to -80 dBm, the output signal was only about -76 dBm. And then farther below that level the measured output levels were also generally off.

I first when down by -10 dBm steps from -50 dBm to the minimum at -115 dBm, and then went up in +10 dBm steps to -55 dBm.

Photos below.

Nelson
Seattle WA



-50 dBm output




-60 dBm output




-70 dBm output




-80 dBm output




-90 dBm output



-100 dBm output




-110 dBm output




-115 dBm output



-105 dBm output



-95 dBm output




-85 dBm output




-75 dBm output



-65 dBm output




-55 dBm output

-50dBm.png
-60dBm.png
-70dBm.png
-80dBm.png
-90dBm.png
-110dBm.png
-110dBm.png
-115dBm.png
-105dBm.png
-95dBm.png
-85dBm.png
-75dBm.png
-65dBm.png
-55dBm.png

 

This offset is due to component spec variations
You can correct this offset using the serial USB command
leveloffset out_switch <correction value>
where the correction value is in dB
--
Designer of the tinySA
For more info go to

NelHau
 

I wonder if there is more going on than simply component spec variations.
?
From the Spec Sheet for the Signal Generator mode, it says:
"Level accuracy +/- 2dB up to 800Mhz between -72dBm and -19dBm, less accuracy below -72dBm, even less accuracy below -110dBm"
?
This seems to imply that perhaps something changes in the Sig Gen circuitry in the -70 dBm area (and below) that affects accuracy.
?
This is right where I was seeing the degradation in the accuracy of the signal output level that I noted in my original post.
?
I may try the suggestion to enter an offset to see if that works, especially for the -80 dBm area where it was off by about 3.5 dBm.
?
Otherwise, I think I will use the TinySA Ultra+ attenuator down to -60 dBm, and then use external attenuators to achieve lower signal levels.? This seems like it could provide better accuracy at very low signal levels.
?
Nelson
 

Nelson
Indeed something changes below -72 dBm.
That is the reason for the parameter?
--
Designer of the tinySA
For more info go to

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