Calculating VSWR using dual Linear-in-dB RMS Detecto

Question: If I’m using a device such as AD8364 or HMC1030 to measure VSWR, how do I actually get from output voltage (or ADC code) to VSWR?

Answer: The short answer is that the return loss in dB is proportional to the differential voltage that appears between OUTP and OUTN (i.e. OUTP-OUTN). It’s important to treat this voltage as a differential voltage. So you need to sample it with a ADC that has differential inputs that can handle the common mode voltage ( 2.5 V).

The detector must be calibrated. Let’s say that Channel A is going to measure forward power and Channel B is going to measure Reverse power. So you would start by setting the input power (∆P_IN1) to both channels to be equal. So ∆P_IN1=0 dB This will give a differential output voltage (OUTP-OUTN) which will be close to 0 V with a common mode level of 2.5V. So you measure this voltage or ADC output code. Let’s call it V1.

Next you reduce the power level on Channel B by, say 20 dB. So now your are emulating a return loss of 20 dB. So  a ∆P_IN2 of 20 dB will give you another differential voltage of V2.

So the slope is going to be given by the equation:

Slope = (V1-V2)/(∆P_IN1-∆P_IN2)

The unit for this will be Volts (or codes) per dB. This slope value should be stored for use when the system is operational.

When the system is operational,  the return loss can be calculated using the equation

Return Loss = (OUTP-OUTN)/SLOPE

Since the reflected power will always be less than or equal to the incident power on the antenna, (OUTP-OUTN) will always be positive.

VSWR is calculated using the equation

VSWR = (10^(RL/20)+1)/ (10^(RL/20)-1)

Let’s consider an example. During calibration, we first apply two equal input powers. This yields a differential output voltage of 0 V. Next we reduce the input power to channel B by 20 dB. This yields and OUTP voltage of 3.5V and an OUTN voltage of 1.5 V. So the differential voltage is 2V.

So the slope comes out to be

Slope = (0-2)/(0-20) = 0.1 V/dB. This is the calibration coefficient which must be stored.

Now with the system in operation, we measure a voltage on (OUTP-OUTN) of 1V. This converts to a return loss of

RL = (1V)/(0.1V/dB) = 10 dB

And the VSWR is given by

VSWR = (10^(10/20)+1)/ (10^(10/20)-1) = 1.9