Chapter 12 12-17
Theory of Operation
Source Theory Overview
The full low band is produced in two subsweeps, to allow addition IF filtering below
3 MHz. At the transition between subsweeps, the source is pretuned and then relocks.
Table 12-2 lists the low band subsweep frequencies at the fractional-N VCO and the RF
output.
Source High Band Operation
The high band frequency range is 16 MHz to 3.0 GHz (or 16 MHz to 6.0 GHz with
Option 006). These frequencies are generated in subsweeps by phase-locking the A3 source
signal to harmonic multiples of the fractional-N VCO. The high band subsweep sequence,
illustrated in Figure 12-5, follows these steps:
1. A signal (HI OUT) is generated by the fractional-N VCO. The VCO in the A14
fractional-N assembly generates a CW or swept signal in the range of 30 to 60 MHz.
This signal is synthesized and phase locked to a 100 kHz reference signal from the A12
reference assembly. The signal from the fractional-N VCO is divided by 1 or 2, and goes
to the pulse generator.
2. A comb of harmonics (1st LO) is produced in the A7 pulse generator. The
divided down signal from the fractional-N VCO drives a step recovery diode (SRD) in
the A7 pulse generator assembly. The SRD multiplies the fundamental signal from the
fractional-N into a comb of harmonic frequencies. The harmonics are used as the 1st LO
(local oscillator) signal to the samplers. One of the harmonic signals is 1 MHz below the
start signal set from the front panel.
3. The A3 source is pretuned. The source output is fed to the A4 sampler. The
pretune DAC in the A11 phase lock assembly sets the A3 source to a first approximation
frequency (1 to 6 MHz higher than the start frequency). This signal (RF OUT) goes to
the A4 R input sampler/mixer assembly.
4. The synthesizer signal and the source signal are combined by the sampler. A
difference frequency is generated. In the A4 sampler, the 1st LO signal from the
pulse generator is combined with the source output signal. The IF (intermediate
frequency) produced is a first approximation of 1 MHz. This signal (1st IF) is routed
back to the A11 phase lock assembly.
5. The difference frequency (1st IF) from the A4 sampler is compared to a
reference. The 1st IF feedback signal from the A4 is filtered and applied to a phase
comparator circuit in the A11 phase lock assembly. The other input to the phase
comparator is a crystal controlled 1 MHz signal from the A12 reference assembly. Any
frequency difference between these two signals produces a proportional error voltage.
Table 12-2 Low Band Subsweep Frequencies
Fractional-N 1st IF Source Output
40.3 to 43.3 MHz 0.3 to 3.3 MHz 0.3 to 3.3 MHz
43.3 to 56.0 MHz 3.3 to 16.0 MHz 3.3 to 16.0 MHz
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