Agilent Technologies 8753ES, Руководство по обслуживанию

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12-14 Chapter 12
Theory of Operation
Source Theory Overview
Source Theory Overview
The source produces a highly stable and accurate RF output signal by phase locking a YIG
oscillator to a harmonic of the synthesized VCO (voltage controlled oscillator). The source
output produces a CW or swept signal between 300 kHz and 3 GHz (or 300 kHz and 6 GHz
for Option 006) with a maximum leveled power of +10 dBm. The source’s built-in 70 dB
step attenuator allows the power to go as low as
−
85 dBm. (The step attenuator is standard
on 8753ES analyzers, and available as Option 004 on 8753ET analyzers.)
The full frequency range of the source is produced in 14 subsweeps, one in super low band,
two in low band, and eleven in high band. The high band frequencies (16 MHz to 3 GHz) or
(16 MHz to 6 GHz for Option 006) are achieved by harmonic mixing, with a different
harmonic number for each subsweep. The low band frequencies (300 kHz to 16 MHz) are
down-converted by fundamental mixing. The super low band frequencies (10 kHz to
300 kHz) are sent directly from the A12 reference board to the output of the A3 source
assembly. This band is not phased locked nor does it use the ALC. It is the basic amplified
output of the fractional-N synthesizer.
The source functional group consists of the individual assemblies described below.
A14/A13 Fractional-N
These two assemblies comprise the synthesizer. The 30 to 60 MHz VCO in the A14
assembly generates the stable LO frequencies for fundamental and harmonic mixing.
A12 Reference
This assembly provides stable reference frequencies to the rest of the instrument by
dividing down the output of a 40 MHz crystal oscillator. In low band operation, the output
of the fractional-N synthesizer is mixed down in the A12 reference assembly. (The 2nd LO
signal from the A12 assembly is explained in
“Receiver Theory” on page 12-27 .) The A12 is
also the origin of the super low band portion of the analyzer’s source.
A7 Pulse Generator
A step recovery diode in the pulse generator produces a comb of harmonic multiples of the
VCO output. These harmonics provide the high band LO (local oscillator) input to the
samplers. In low band and super low band, the operation the pulse generator is turned off.
A11 Phase Lock
This assembly compares the first IF (derived from the source output in the A4 sampler) to
a stable reference, and generates an error voltage that is integrated into the drive for the
A3 source assembly.