The RF Preamp Explained for FlexRadio Systems' SDRs

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The RF Preamp Explained for FlexRadio Systems' SDRs


The RF Preamp Explained for FlexRadio Systems' SDRs

System Dependencies
Minimum PowerSDR Version:
1.x
Applicable Hardware: All FlexRadio Systems SDRs

Content provided by: Gerald K5SDR (FlexRadio Systems Engineering)

Hopefully, I can shed a little light on the subject regarding the behavior of the RF preamp in FlexRadio transceivers. Here are a few facts:

1.) Traditional "legacy" radios measure the AGC gain and convert the gain to S Units. I remember my old 706 that showed a higher S Meter reading if I turned the RF gain down, rendering the S meter totally useless for determining signal strength. The S meter readings would change if I turned the attenuator or preamp on/off. That means that the S meter was totally meaningless for measuring signal strength at the antenna terminals.

2.) The FlexRadio products (transceivers) measure the signal strength at the antenna terminals in the same way that a spectrum analyzer does. If you change the preamp settings (turned on or to a higher value depending on the transceiver), the noise floor goes DOWN and the signal level stays the same. Remember that when you engage the preamp, you have not changed the signal level at the antenna terminal. You have, however; improved the signal to noise ratio with the preamp and therefore the noise will go down in reference to a fixed signal level. The previous statement holds true in all cases except one. That is if the radio is antenna noise limited.

In the above mentioned exception, atmospheric noise is a real signal just as the desired information carrying signal is. The only way to improve the SNR in this case is by narrowing the bandwidth, match the filtering to the characteristics of the desired signal or adjusting the AGC-T gain to provide maximum SNR.

In summary, an accurate S Meter must show the same signal strength reading no matter what the preamp, attenuator, or gain settings are set for. If the preamp is turned on, the spectral noise floor goes down while the signal stays the same on the Panadapter.

The Necessity to Calibrate the RX Level after a New Version of PowerSDR 1.x is Released

FLEX-5000/3000: If you are using a FLEX-5000 or FLEX-3000, it is fully calibrated at the factory and the data is stored to EEPROM inside the radio, so there is usually no need to perform a RX Level calibration. When you start PowerSDR for the first time; it automatically imports or pulls the calibration data from EEPROM into the PowerSDR database. Anytime you delete the PowerSDR database, it will re-import the FLEX-5000 calibration data from the EEPROM into the new PowerSDR database automatically.  On occasion, it may be necessary to recalibrate the RX level and this is easily done since the FLEX family of SDRs have an internal signal generator to perform the calibrations.

SDR-1000: If you are using a SDR-1000 you must calibrate the transceiver’s RX Level using an external signal generator of known frequency and intensity. If you move to a new version of the software with a new database, you must recalibrate the SDR-1000 unless you import your old database. It is preferred that you always create a new database with every new version of PowerSDR you install in order to get the most optimal default settings that may have changed between releases.


This KB article may reference additional files that are available on the FlexRadio Systems web site Downloads page. Please use the URL(s) below to download the referenced materials.

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KB Source Document(s):

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Last Modified:Friday, June 26, 2009
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Type: INFO
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