Digital display for Revex W560 directional wattmeter – part 1

The Revex W560 is a dual range VSWR meter that was also sold under other brand names.


The low frequency range is specified as 1.8-160MHz.

This project is for an external digital display to suit the low frequency band of the W560. Whilst this project is for a specific meter, the techniques are applicable more widely.

Above is the schematic of the W560. A 6.5mm TRS jack was added to the back panel, and the T and R connected to the F and R outputs of the low range sensor, bypassing all of the existing display circuitry. It appears that the equivalent source impedance of this connection is quite high and may need buffering.

The question is how the DC output voltage Vo and forward power P are related.

A naive answer might be that \(P \propto V_o^2\). A common proposition is that \(P \propto (V_o+V_{diode})^2\) where Vdiode is say 0.2V for a germanium diode.

Let’s discover what that relationship is by measurement of the real wattmeter, the W560. A series of measurements were made to explore the relationship between the DC output voltage Vo and forward power P.

Above is a plot of the measurements from 0 to 200W.

Above is a plot of three curve fits:

  • a simple square law fit: \(P=2.089*Vo**2\) (** is an exponentiation operator);
  • a simple square law with diode offset fit: \(P=-1.80e-07+2.089*(Vo+-0.000294)**2\); and
  • a full second order polynomial: \(P=1.026e-18+0.883*Vo+1.89*Vo**2\).

The first and second curves are nearly coincident.

Note that the diode offset voltage found by the second curve fit process was a tiny value, and actually of the opposite sign to the common explanation, and suggests the diode offset model often proposed is weak.

In the last case, the first term (Y intercept) is near zero, and can be taken as zero with no significant error.

All of these are easy to implement in the display firmware, the third of these is a better fit and not much harder to calculate than the others.

So, for the range 2-200W, \(P=1.026\text{e-18} + 0.883 Vo+1.89 V_o^2\) is a good expression for conversion of Vo to Power.

Note that there are insufficient data points below 2W to be confident that this curve fit has good accuracy in that range. If we assume that it was good down to 1W, then with Pfwd=100W and Pref=1W, we have a lower limit for accurate VSWR measurement of 1.2 in that scenario. It would be sensible if the digital display did not show VSWR where Pref<1W (at least pending further measurement).

If we allow 5% error in measurement of each of Pfwd and Pref, then we should have low confidence in calculated VSWR greater than 40 and such values should not be displayed.

A work in progress..