The RF Preamp Explained for the FLEX-5000 and SDR-1000 The ...

The RF Preamp Explained for the FLEX-5000 and SDR-1000The RF Preamp Explained for the FLEX-5000 and SDR-1000System DependenciesMinimum PowerSDR Version: 1.xApplicable Hardware:FLEX-5000 and theSDR-1000Content provided by: Gerald K5SDR (FlexRadio Systems Engineering)Hopefully, I can shed a little light on the subject regarding the behavior of the RF preamp inFlexRadio transceivers. Here are a few facts:1.) Traditional "legacy" radios measure the AGC gain and convert the gain to S Units. I remembermy old 706 that showed a higher S Meter reading if I turned the RF gain down, rendering the Smeter totally useless for determining signal strength. The S meter readings would change if Iturned the attenuator or preamp on/off. That means that the S meter was totally meaningless formeasuring signal strength at the antenna terminals.2.) The FlexRadio products (transceivers) measure the signal strength at the antenna terminalsin the same way that a spectrum analyzer does. If you change the preamp settings (turned on orto a higher value depending on the transceiver), the noise floor goes DOWN and the signal levelstays the same. Remember that when you engage the preamp, you have not changed the signallevel at the antenna terminal. You have, however; improved the signal to noise ratio with thepreamp and therefore the noise will go down in reference to a fixed signal level. The previousstatement 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 desiredinformation carrying signal is. The only way to improve the SNR in this case is by narrowing thebandwidth, match the filtering to the characteristics of the desired signal or adjusting the AGC-Tgain to provide maximum SNR.In summary, an accurate S Meter must show the same signal strength reading no matter what thepreamp, attenuator, or gain settings are set for. If the preamp is turned on, the spectral noise floorgoes 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 ReleasedSDR-1000: If you are using a SDR-1000 you must calibrate the transceiver’s RX Level using anexternal signal generator of known frequency and intensity. If you move to a new version of thesoftware with a new database, you must recalibrate the SDR-1000 unless you import your olddatabase. 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 changedbetween releases.FLEX-5000: If you are using a FLEX-5000, it is fully calibrated at the factory and the data isstored to EEPROM inside the radio, so there is no need to perform a RX Level calibration. Whenyou start PowerSDR for the first time; it automatically imports or pulls the calibration data fromEEPROM into the PowerSDR database. Anytime you delete the PowerSDR.mdb database, it willre-import the FLEX-5000 calibration data from the EEPROM into the new PowerSDR databaseautomatically.This KB article may reference additional files that are available on the FlexRadio Systems web siteDownloads page. Please use the URL(s) below to download the referenced materials.An Adobe Acrobat Reader may be required to open the file. You can download Adobe Acrobatfrom here.KB Source Document(s):NoneReferencedLast Modified By: Administrator, Thursday, July 31, 2008http://kc.flex-radio.com/KnowledgebaseArticle50144.aspxThursday, June 18, 2009