Dear LF Group,
Thanks for the feedback on the tuning meter. Thanks to G3NYK who has
pointed out that the "V" and "A" range switch markings were shown the wrong
way round on the circuit diagram - I have corrected that in the most recent
version of the file sent out.
It was pointed out that, instead of separate RF volts and current ranges,
one could subtract voltage from current, and have a second centre-zero
meter showing "R > 50 ohm, R < 50 ohm". I'm sure this would work fine, but
I didn't do it because I felt it was more useful to have the separate
ranges for diagnostic purposes - so you can see directly if there is any
output voltage from the TX, whether the load is drawing any current,
measure power, etc. Having the scales set so that equal meter deflections
are obtained with the correct load resistance is almost as convenient - you
don't need to adjust the loading as often as you adjust the resonant frequency.
The meter could be used unmodified in a 75ohm system, the only difference
would be that there would not be equal deflections on V and A scales with a
matched load. Of course, if you have a 75 ohm system, the ranges can be
altered so that the FSD of the V scale is 75 times that of the A scale.
The phase detector output could be used to drive a servo tuning motor for
an automatic tuning system. It would be a good idea to RF low-pass filter
the DC output from the detector if you wanted to do this.
Cheers, Jim Moritz
73 de M0BMU
|
