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Calibrating the low output level

Using one of the many calculators you can find on the internet the relation between dBm and Vp-p is quick to calculate. An example calculator can be found at https://www.random-science-tools.com/electronics/dBm-Watts-volts.htm
For the calibration of the low output we find that an output level of -25dBm equals 35.6mVp-p in a 50ohm system and we can use this relation to calibrate the low output level using a scope.

Measurement setup

This calibration requires firmware 1.2-47 or later.
Connect the high output to the input of a scope terminated with 50ohm. This can be done using a F-F-M tee where the cable from the tinySA output connects to one side of the tee and a 50ohm load is connected to the other side of the tee. The M side of the tee is directly connected to the scope input.

Enable the tinySA CAL OUTPUT at 30MHz and set it to HIGH INPUT mode to avoid disturbances from the high connector. The cal output will provide an exact -25dBm signal. On the scope we will a somewhat distorted square wave with an amplitude of 35mVp-p.

The small deviation can be explained by the harmonics present in the cal output.
Now connect the scope to the tinySA low output and set the the tinySA in LOW OUTPUT mode, 30MHz and a level of -25dBm. Using the scope we measure (as an example) 30mVp-p

Calculating and setting the correction

Using the calculator we convert 30mVp-p to dBm and find the output level to be -26.5dBm. To correct the output level (which is -1.5dB too low) we use connect to the tinySA over USB and issue the commands

leveloffset low output -1.5
saveconfig 1234

to change the output level calibration setting and store the data so it survives a reset.

Verifying the correction

Using the scope we measure again the Vp-p of the output and now we measure 35.6mVp-p which matches perfectly an output level of -25dBm.

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Page last modified on March 07, 2021, at 06:52 AM