THEORY OF OPERATION
Table 2-1. Frequency Coverage Bands
BAND FREQUENCY COVERAGE
HET 0.01 to 14.999999 MHz
Divide-by-32 15 to 31.999999 MHz
Divide-by-16 32 to 63.999999 MHz
Divide-by-8 64 to 127.999999 MHz
Divide-by-4 128 to 255.999999 MHz
Divide-by-2 256 to 511.999999 MHz
Fundamental 512 to 1056 MHz
Three signals are combined in the sum loop to produce a signal that ranges from 480 to
1056 MHz. This signal is divided by factors of 2 to produce the bands in Table 2-1. The
HET band is produced by mixing 80.01 to 94.999999 MHz (from the Divide-by-8 bad)
with 80 MHz to produce 0.01 to 14.999999 MHz.
The three signals that are combined in the sum loop are 576 to 960 MHz in 8-MHz
steps from the coarse loop, 8 to 16 MHz in 1-Hz steps from the sub-synthesizer, and 80
MHz from the FM circuitry. If the sum loop output frequency is below 760 MHz, the
FM signal and the sub-synthesizer signals are subtracted from the coarse loop signal. If
the sum loop output signal is above 760 MHz, the FM signal and the sub-synthesizer
signals are added to the coarse loop signal.
The A2 Coarse Loop PCA contains the reference circuits and generates a 576 to 960
MHz signal in 8-MHz steps. The main reference frequency for the signal generator is a
40-MHz crystal oscillator. This oscillator is phase locked to either an internal 10-MHz
TXCO, or an external reference. Either a 10-MHz or 5-MHz external reference may be
selected by special function. A 1-or 2-MHz reference may also substituted for the
5-MHz reference by setting a switch on the Coarse Loop PCA. The 40-MHz reference
frequency is doubled to 80 MHz. This is used as the local oscillator for the HET band
and is divided down to 20 MHz for use as the reference for the A14 FM PCA.
The coarse loop generates the 576- to 960-MHz signal using a combination of phase
lock and delay line discriminator frequency control circuitry to produce a low phase
noise signal. The delay line is a 125-ns cable contained in the module.
The sub-synthesizer generates a 16- to 32-MHz signal with 1-Hz resolution. This is
further divided on the Sum Loop PCA to 8 to 16 MHz. The sub-synthesizer generates
the fine frequency steps using a modified N-divider loop with a single-sideband mixer
(SSB) in the feedback path. The sub-synthesizer VCO runs from 160 to 320 MHz. The
reference frequency for the loop is 1 MHz, which would normally provide 1-MHz steps
in a conventional N-divider loop. However, by using pulse deletion, which is
controlled by a rate multiplier, the resolution is extended to 10 kHz. Additional
resolution is gained by introducing a 10- to 20-kHz signal in a SSB mixer. This signal is
produced by a gate array, which contains a 14-bit rate multiplier.
The A14 FM PCA also generates an 80 MHz signal that can be frequency modulated.
These signals are combined in the Sum Loop PCA. The first mixer combines the the
sum loop VCO output (the fundamental frequency, 480 to 1056 MHz) with the coarse
loop frequency (576 to 960 MHz) to produce a signal of 88 to 96 MHz. This signal is
subsequently mixed with the 80 MHz signal from the FM PCA to produce 8 to 16
MHz. This is compared with 8 to 16 MHz from the sub-synthesizer to generate a DC
control voltage that locks the loop.
2-3
To Next Page
Loading...
