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Re: Frequency Counter

Subject:  Re: Frequency Counter
  Date:   Sat, 10 May 1997 14:15:31 -0700 (PDT)
  From:   "Edward V. Phillips" <ed-at-alumni.caltech.edu>
    To:   tesla-at-pupman-dot-com
    CC:   ed-at-alumni.caltech.edu


"These are basically a hi-Q tuneable tank circuit
(tuning fork) which can be made to ring when excited by a RF pulse
train, continuous or not, at the frequency to which they are tuned.
A mechanical dial pre-calibrated at the factory then tells you the
frequency, you just tune the dial like tuning a radio receiver and
peak up on the highest meter reading.  If you do not have a wavemeter
you can use a communications receiver with the RF gain turned way
down low and a very short, or no antenna wire connected.  A portable
battery operated receiver is best as it cannot be confused by RFI on
the local AC mains which will also be there from the Tesla coil
device."
        I use a wavemeter here, with plugin coils to cover range
from 200 kHz (I don't make big coils for a small attic) to 500 kHz.
Use an old-fashioned 115 ma Weston thermogalvanometer (therocouple
xxx thermocouple milliameter) to read the current in the tank circuit.
Works fine - calibrated it against my Q-meter, which in turn was
calibrated with counter.  As a matter of interest, I also tried
using a small (60 ma) pilot lamp as a cheaper (and expendable)
tuning indicator.  Doesn't really work because the high peak currents
in the circuit break down (spark over) the tuning capacitor at
about the power level where the lamp lights satisfactorily.
        There has been talk here about using incandescent lamps for
ammeters.  Works fine.  If you want to get quantitative, you can
use two lamps, one heated by the circuit under test and the second
heated with DC or 60 cycles (if you have a suitable AC ammeter). 
By adjusting the current in the test circuit for equal apparent
brightness of the two lamps you can get a pretty decent quantitative
measurement.  Synlvania used to make (long, long ago) a tube in
which two identical filaments were provided, for use in just this
way.  I have a gadget here I use to measure the power of microwave
oscillators which uses the same principle, except that the feedback
is automatic, with a dual photocell and op amp arrangement. 
Unnecessarily
complicated, but works fine.
Ed