The process by which the program locks onto an RF signal is defined in the next six sections. Refer to
Figure 06-3.
INITIALIZATION
-
The working table (BANDTP) for band 4
is
cleared and the appropriate table of
constants, used by the program for the particular Band 4 that has been selected, are loaded from PROM in
this area. BANDTP is an area in RAM that is
27
bytes long.
VCO SWEEP
-
This routine steps the VCO frequency by
a
step size stored in BANDTP. After each step,
the VCO frequency is checked for three stop points.
1. Top VCO frequency limit (500 MHz)
2.
Wraparound frequency
3.
Lockout frequency
If the top VCO frequency has been reached and no signal has been found, the program will return to the
supervisor. If the top frequency is reached, and a signal has been detected, then the VCO is set to its low
limit and the bottom range
is
searched until the wraparound frequency
is
reached.
If the wraparound frequency has been reached
(tha frequency at which the last VCO frequency has pro-
duced the strongest IF frequency), then the program will stay at this frequency, and will perform the cen-
tering and harmonic number determination routines.
If a lockout frequency
(a
VCO frequency at which erroneous locking results) is detected, the VCO fre-
quency will be incremented by:
13
STEP SIZE
=
NEW VCO FREQUENCY
and the program will continue from this frequency.
After each VCO step the YIG filter
is
swept to see if a signal
is
detected by the power DAC attenuator. If
a signal was detected, the YIG
is
swept back and forth, and the attenuation
is
increased until the signal is
lost. At this point a new VCO frequency
is
stepped and the process of signal detection continues, thus
leaving the power DAC at the last setting to detect the next highest signal.
CENTERING AND HARMONIC
NUMBER DETERMINATION
-
This routine will determine the har-
monic number of the VCO which is causing the mix product to be in the proper range. (Refer to Figure
06-4
1.
First we obtain the proper step size for the calculation of the harmonic number (N). After the VCO sweep
routine is complete and the VCO frequency
is
set, the incoming signal
is
mixed with a harmonic of the
VCO frequency to produce
a
signal in
a
predetermined passband region. This signal is stepped to the outer
edge of the
passband
(f
step depending on whether the signal is high or low side mixed) by the following
process.
1. Increment the VCO
2.
Power level the IF signal
3.
Center on the signal
4. Test for band limits
These steps are repeated until the edge of the
passband
is
reached.
Scans by ArtekMedia © 2007
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