indication while adjusting the fundamental fre -
quency of the 540A.
(2) Use a medium non-repeating Gate Time (0.1 sec.)
and Infinite Display Time. Press and release
RESET button at instant of optimum 540A tuning
and read the 540A frequency on counter. This
allows the reading to be made at a precise moment
and this frequency will be displayed until a new
reading is to be made. ~
a. If the frequency of the signal to be measured is known
approximately, select a harmonic at this frequency whose
fundamental lies between 100 and 220 megacycles i. e.,
divide the frequency by a number to give a quotient within
the fundamental frequency range of the 540A. For example,
if the unknown signal lies between 1 and 1.2 kilomegacycles,
the tenth harmonic of a fundamental frequency between 100
and 120 megacycles will produce a zero-beat, or the sixth
harmonic of a frequency between 162 and 200 megacycles
can be used, etc.
b. Using the COARSE VERNIER tuning control, tune the 540A
to the fundamental frequency determined above and very
carefully adjust the control until a response is seen on the
oscilloscope screen. Any response that is seen indicates
that some harmonic of the 540A fundamental frequency is
sufficiently close to the frequency of the unknown signal
that their difference frequency is within the bandwidth of
the oscilloscope amplifier. Tune as close to zero-beat as
is conveniently possible with the COARSE VERNIER control.
C. Using the FINE VERNIER tuning control, reduce the differ-
ence-frequency response on the oscilloscope to as close to
zero-beat as the stability of the measured signal will allow,
(see step f). Absolute zero-beat will be obtained when the
oscilloscope trace disappears into the horizontal line.
Various looped patterns are obtained as the 540A is tuned
slightly away from the measured frequency. Any patterns,
such as illustrated in Figures 8, are sufficiently close to
zero-beat for most measurements, and are more practical
to use than the absolute zero-beat.
If an input signal connected to the ATTENUATOR INPUT is
too weak to produce an easily read response on the oscil-
loscope, it can be connected directly to MIXER INPUT B
instead, with an 18 db increase in signal strength.
d. Read the number displayed on the 524B Electronic Counter,
adding the correct Mixing Frequency indicated on the 525B
Frequency Converter .
2 -7 |
ACEC
