Specifications
Speed-Up Circuitry In addition to the phase-lock and bounding circuitry, there is also cir
-
cuitry to help speed the locking of the loop. The settling time of the main
loop and its phase detector is very slow. Frequency lock detect circuitry
emits pulses to help get the source main VCO locked on frequency
quicker. Once on frequency, the speed-up circuit’s contribution drops out
and the normal phase lock circuitry locks up the loop the rest of the way.
In this way, the loop locking occurs more rapidly, but the stability of the
loop is not compromised.
However, for this speedup to work, the polarity of the loop must be cor
-
rect. If on the wrong side, then the speedup circuitry only acts to push the
VCO even farther off, even faster. Thus a hierarchy of control must be es
-
tablished. The bounding correction circuits must be strong enough to
counter the contribution of the speedup circuits, but should not be so
strong that the loop ends up banging back and forth between its two ex
-
tremes.
Frequency Bands
and Switched
Filters
Although the main oscillator produces an output of 800 to 1600 MHz only,
the module output frequency covers 10 MHz to 3 GHz. There are four dis-
tinct bands of operation to achieve the desired frequency range. If the de-
sired output frequency is between 10 to 400 MHz, the circuit is in the het-
erodyne mode. The VCO output is mixed with a heterodyne oscillator
(typically 1200 MHz). Passing the VCO output through a divide-by-two
circuit produces frequencies between 400 to 800 MHz. For 800 to
1600 MHz, the VCO signal passes directly. To achieve 1600 to 3000 MHz,
a doubler is used.
To improve harmonics and spurious performance, output signals are
passed through switched filter sections. A given frequency mode (Hetero-
dyne, divide-by-two, through, or multiply-by-two) may have one or more
switched filter band sections. The switching for the frequency generation
modes is combined with the control for the switched filter section.
Heterodyne
Oscillator
In the heterodyne mode, a heterodyne oscillator is used to mix with the
main VCO outputs of the source and LO to generate frequencies less than
400 MHz. This VCO is the same type as that used for the main and offset
loops. The VCO has an 800 to 1600 MHz range, but will typically be tuned
to 1200 MHz. The main VCO is tuned higher than the fixed heterodyne
oscillator, so that positive steps in system frequency will correspond to
positive steps in the main VCO. To avoid mixing spurs, the heterodyne
VCO is allowed to step down as far as 900 MHz. The circuitry for the
phase-locking of this heterodyne oscillator is similar to the offset VCO
locking. The heterodyne VCO signal is also passed along out of the mod
-
ule to be supplied to the optional source for use in its heterodyne mode.
THEORY OF OPERATION SOURCE MODULE
MS462XX MM 2-7