Should you trust your VSWR meter – linearisation

Should you trust your VSWR meter? asked an interesting question, and Should you trust your VSWR meter – detector linearity discussed a problem apparent in may VSWR meters.

This article illustrates one method of linearisation of the detector response of a practical VSWR meter.

Radio-kits SWR meter

This article contains an analysis of the analogue circuitry of the Radio-kits SWR meter.

The directional coupler at top left contains half wave peak detectors for forward and reflected waves. They are wired to the two compensated op amps at lower right (the connections are not shown on the circuit as the coupler may be remote, follow the terminal designations).

Taking the Rev detector, it is connected to the Rev DC input to the + input of the lower op amp, and the DC load on the detector is essentially R6 10k (the input current of the op amp is negligible).

Importantly, this voltage is nearly a smooth DC value, there is negligible ripple due to the filtering effect of C2 in the coupler.

The output circuit of the op amp includes a peak hold rectifier D7 and capacitor C16.

From the +ve terminal of C16 (the circuit ‘output’), a diode D5 connects to the – input of the op amp, and the load on this diode circuit is essentially R7 10k.

The gain of the op amps is extremely high, so there will be negligible voltage between its + and – inputs, therefore, the output voltage will equal the voltage on the + (and -) input pins plus the voltage drop due to the current flowing in the diode.

The concept of this circuit is that the voltage drop across D5 compensates for the voltage drop across diode D2.

That will only happen if the current in both diodes is the same, and you might note that R6=R7 so the DC component of current flowing in both diodes is the same, but D2 current is nothing like smooth DC, this is not how a half wave rectifier with capacitor input filter works, and so this scheme naively under-compensates for the voltage drop.

Shunting the existing 10k resistors with 2.2k gives a combined value of 1.8k and achieves fairly good compensation from 100W down to 10mW. Obviously this cannot track the pulsing current voltage drop exactly, but can achieve approximate compensation over a usable range.

The end result is that with a stable load of known VSWR=1.2, the meter reads 1.20 with Pfwd 100W down to 1W.