(2) Heterodyne Method. The heterodyne method, illustrated in figure
10, is based on mixing known reference frequencies until the difference
frequency is within the direct measuring range of the electronic counter. A
harmonic generator produces all the harmonics of 10 MHz.
A harmonic
selector cavity is manually tuned until the selected harmonic, mixed with
the input, produces a difference frequency that is fed through an amplifier.
The output of the amplifier is applied to the counter.
A level meter
indicates when the harmonic selector has reached the proper reference
frequency. To find the frequency being measured, the reference frequencies
are added to the electronic counter display. This addition usually involves
nothing more than placing one or two digits before the counter reading.
Figure 10.
Heterodyne method.
(3) Transfer Oscillator Method. A method that provides an extremely
wide measuring range with counter accuracy is illustrated in figure 11. A
transfer oscillator is used with a 50 MHz electronic counter. The transfer
oscillator method compares harmonics of a fundamental frequency with an
unknown high frequency.
When you are performing a measurement the
fundamental frequency is adjusted to the point where one of its harmonics
has the same frequency as the input signal. This is accomplished by beating
harmonics against the input
10
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