SDRUno 1.33 and RSP1; RF gain slider confusion

[blinkerParticipant]

With my RSP1 and older SDRUno versions, use of the RF gain control slider and LNA button was intuitive and linear. The new 1.33 has no LNA button, only a RF slider with 4 positions. Increasing RF gain results in nonlinear gain with the noise floor increasing, decreasing, then increasing again. Can someone please explain what the 4 RF gain positions represent?

The manual for 1.33 has no details on the RF gain control.

Thanks!

[HandymanUKParticipant]

I have wondered about this as well, seems strange that decreasing the gain makes the noise floor raise.

[Andy2Participant]

It’s worth reading app note AN008. It contains the following:

Note: When IF AGC is turned on the overall gain will not vary much with RF Gain adjustments as the IF gain will tend to compensate to maintain an optimum level to the ADC.

As the RF gain control is right at the input to the RX, turning the gain down will add noise as the later gain stages add more gain to keep the ADC working at max efficacy. This will cause the observed noise level to rise. I’s also worth noting that the S-meter maintains its reading of incoming signal strength regardless of RF gain setting.

[blinkerParticipant]

Thanks for the app note info. I see the same results with AGC off. Here’s some data:

RSP 1 with SDRUno 1.33; AGC off; antenna input grounded (no signal)

4 (Highest)RF gain: noise floor at -125.6 dBm; S meter 0.5
3 RF gain: noise floor at -105.7 dBm; S meter 3.5
2 RF gain: noise floor at -115.9 dBm; S meter 2.0
1 (Lowest) RF gain: noise floor at -92.5 dBm; S meter 6.0

My current theory is that the RF gain positions represent the following:
4 LNA on; RF gain high
3 LNA off; RF gain high
2 LNA on; RF gain low
1 LNA off; RF gain low

Comparing 1 and 3, the noise floor is consistent with Andy2’s explanation of later stages adding noise to keep the ADC working efficiently. The S meter reading is correlated to noise level since there is no signal.

[Author]

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