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RX Voltage vs RX SNR in dB

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Posts: 29
Aug 5th, 2011 at 7:41am  
Can someone clarify the relation between RX  Pointing Voltage & RX SNR in dB. Coz when when we do pointing with iSite its shows values in Voltage while checking with Console with command rx snr, it shows values in dB.

Modem is iDirect EVO X3!
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« Last Edit: Aug 6th, 2011 at 8:50am by Admin1 »  
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Aug 5th, 2011 at 12:42pm  
My guess..

Signal voltage probably refers to total received power. It goes up if the total power increases. The total power comprises the background noise level plus the wanted signal, if present.  The total power level will be higher with a short LNB cable.

RX SNR refers to the receive signal to noise ratio. This represents the quality of the received signal.

Normally the wanted signal will stick up some 6 to 15 dB above the noise floor. What is acceptable depends on the modulation and coding.  If the threshold is 7 dB then operating at 10 dB in clear sky conditions provides a 3 dB link margin for rain fades etc.

Note that it you have your linear polarisation set wrong by 45 deg you will see satellite carriers from both polarisations at the same level.  In this case you will get a very high total received power and zero for the RX SNR.  Smaller misadjustments of either your polarisation or the polarisation of someone else's uplink might similarly degrade your RX SNR, due to interference, while your receiver still shows very high receive power.

I don't know exactly how an iDirect modem measures total power and RX SNR.


Total power might be based on the receiver automatic level control. The receiver has a variable gain amplifier near the front end so that it can adjust the signal level at the demodulator to a steady specific level, regardless of the input level.

The RX SNR displayed value may be derived from the bit error rate, using a look up table or curve. The FEC process involves detecting and correcting most errors. The difficulty of this process can be expressed as the estimated bit error rate.  Since the curve of BER versus RX SNR is known the displayed quality output can be shown as RX SNR in dB.  This is generally accurate with exceptions for high phase noise (e.g. due to LNB power supply problems) or signal distortion (e.g. due to overdriven BUC) where the BER is poor despite the carrier looking good for SNR on the analyser.

Best regards, Eric.

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« Last Edit: Aug 6th, 2011 at 8:49am by Admin1 »  
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