On units equipped with the Power Measurement Option (02), accurate frequency correction factors are
stored in the
counter'smemory. This allows absolute power calibration of the video amplifier output.
Once the
YIG filter is tuned to the input signal, the appropriate harmonic number (N) and VCO frequency
(FVCO) are selected to produce an IF frequency (FIF) at approximately 125 MHz. An approximation
of the input signal is found by using:
The IF frequency produced in the mixer is amplified by the high gain IF amplifier and sent to the count
chain
(A106). The IF threshold detector (A201B) insures sufficient IF amplitude for count accuracy.
OPERATION
First the YIG filter is stepped, (in 64 MHz steps), from its low to high limits. During this search the RF
detected output is fed, through
a
microprocessor controlled step attenuator to a threshold detector. After
each step the threshold detector is checked. If triggered, the search mode is halted until the amplitude of
the signal is determined. This is done by stepping the filter back and forth through the signal and stepping
the attenuator until the signal is attenuated below the threshold. The counter then returns to the search
mode to look for any larger signals. After searching the entire frequency range, it returns to the largest
signal and begins to center the
YIG filter precisely on the input frequency. See Figure 4-6 for
a
simp-
lified diagram of Band 3 operation. For more detailed descriptions of Band 3 operation
see
Figures 4-7
through Figure 4-1 1.
The centering process consists of
slowly stepping the YIG filter down (in 2 MHz increments) until
a
level
of 3-6 dB below the peak
is
reached. This frequency
is
stored and the process is repeated from the other side
by stepping the filter up in 2 MHz steps. The average of the two frequencies obtained
is
the center of the
passband. This
is
the frequency which
is
used to determine the N and FVCO.
After centering, N
is
determined from
N
=
Fyl~
-
125 and then rounded up to the next highest integer.
500
From this, FVCO is calculated using FVCO
=
Fyl~
-
125. Should this yield FVCO
<
400 MHz, then
N
FVCO is recalculated using FVCO
=
FYIG
+
125.
N
Since
Fyl~ is only approximately equal to FIN, the IF frequency will not be exactly 125 MHz. Therefore,
the next step in operation
is
a
VCO frequency adjustment to shift FIF into the middle of the IF passband.
VCO frequency correction is achieved by counting
FIF and changing FVCO by
+
FIF
-
125. If the error
N
is large enough to be outside the IF
passband (IF threshold is not triggered) then
a
series of steps (shifting
the IF in
f
20 MHz increments) are taken until the signal falls within the passband.
Once the VCO corrections have been made, the converter has acquired the signal and the counter is ready
to count and display the input frequency.
Scans by ArtekMedia © 2007
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