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Message-ID: <5.1.0.14.0.20030430120518.00ade070@gemini.herts.ac.uk>
Date: Wed, 30 Apr 2003 12:44:04 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
In-reply-to: <006301c30e7e$5e4c4230$1b00a8c0@dellboy>
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Subject: LF: Re: LF tuning meter
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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