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# TRF371109: Gain formula

Part Number: TRF371109

I want to calculate the gain of the I/Q demodulator. I believe the gain is defined as voltage gain from mixer RF input to baseband I or Q output.

For the RF input voltage, I calculated the Vrms using Power in watts = Vrms^2/50 .. for 50Ohm input impedance. The power in watts will be the power at the EVM connector (I am neglecting loss of the PCB trace and the balun). For the baseband output voltage (I or Q), I take the peak voltage and divide by sqrt(2) to get Vrms.... but when I do this, I get gain in the order of 37dB... far short of the typical gain of 52dB. I am using maximum BB gain of 24dB. My RF is 926MHz and my LO is 925MHz, and the baseband filter is set wide open (~15MHz).

What am I missing?

Please provide the exact formula for gain calculation. Thanks.

• You are correct in the gain is defined as Vout(I/Q RMS)/Vin(RMS).

You are correct to convert input power in dBm to Vrms. How are you measuring the BB output voltage? If you are using a scope then are you measuring at the BB output with a high impedance scope and not after the EVM's on-board balun. If so, then I would measure Vpp/2/sqrt(2) to get the Vrms. If you are happening to measure after the on-board balun then there is 12-dB voltage conversion loss. That may be the source of the discrepancy. The balun is intended to allow you to hook up to 50 ohm test equipment. Note, the BB output cannot drive 50 ohms directly so ensure the load or scope impedance is high impedance.

--RJH
• In reply to RJ Hopper:

I am measuring the BB output voltages using NI ADC card (6124)which has very high input impedance.
My measurement points are TP2, TP7 for BBIN-BBIP, and TP10, TP12 for BBQP-BBQN.
For the baseband peak to peak output voltage (Vpp) will be the difference between BBIN&BBIP, or BBQN&BBQP correct?
For the calculated gain of 37dB, I was using the peak voltage of just the BBIN, or BBIP and not the difference between them. This accounts for 6dB. So that makes my gain 43dB. I looked at the TRF371109 datasheet again for gain typical number at 900MHz, and it says 41dB. So I am pretty close now. I think I mistakenly looked at the gain number from TRF371125 which list the gain at 900MHz at 50dB.
• In reply to juswanto wardojo:

Yes, your measurement looks correct now. Good deductions.

--RJH