(4) Automatic Method.
The automatic method, illustrated in figure
12, makes it possible to obtain instantaneous direct readings of unknown
swept oscillator frequency (f) is applied to a mixer and harmonics are
generated and mixed with the input signal. A 1 MHz signal from the counter
time base is used as a phase detector and locking frequency for the output
signal of the mixer. This circuitry will phase lock the proper harmonic (N)
of the swept oscillator (f) with the input signal (F) at a precise 1 MHz
offset.
The input signal (F) is also applied to a second mixer which has the
same swept oscillator signal (f) but with the addition of a 1 kHz signal
from the counter. When the phase detector locks the swept oscillator, the
signal in this second mixer will be N (f + 1 kHz) which is mixed with the
input frequency (F). When the offset is exactly 1 MHz, then F = fN 1 MHz.
The output of the second mixer is 1 MHz plus N kHz. The 1 MHz signal from
the counter is mixed with the output of the second mixer and the resultant
signal is N kHz. The mixed signal (N kHz) is then applied to a digital N
computer circuit along with the 1 kHz signal from the counter. This circuit
then divides the N kHz by 1 kHz and the result is N pulses. The N pulses
are gated in conjunction with the counter time base signal.
This gating
extends the time base to N times the time base switch position.
This
extension multiplies f x N 1 MHz so the counter can read out the unknown
input frequency directly with the 1 MHz offset.
As you can see, the electronic frequency counter is an extremely
versatile instrument and can be a valuable tool for you to use in your day
to day calibration tasks. In order for you to maximize use of electronic
counters, obviously you must first fully understand the multiplicity of
applications for which they were designed.
A basic counter consists of an input circuit for signal amplification
and shaping, a gate circuit which passes a selected portion of the input
signal and drives the reset circuits, an oscillator which establishes the
counting interval, and the frequency multipliers and dividers which allow
the gate time to vary.
An electronic counter may be used for frequency, period, totalizing,
ratio, time interval, and multiple-period averaging measurements.
High
frequency measurements are accomplished by the prescaling, heterodyne,
transfer oscillator, and automatic methods.
Of these, the transfer
oscillator method is probably the most commonly employed.
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