Agilent Technologies Agilent 4396B, Service Manual

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The comb generator receives the 40 MHz reference signal from the REF OSC and multiples
the fundamental signal into a comb of harmonic frequencies (40 MHz x N). The level of the
harmonics is adjusted in the Comb Generator A djustment.
The phase locked oscillator consists of a 470 MHz to 930 MHz V CO , a phase detector , a mixer , a
pretune D A C and 61 converters . See Figure 11-6. The V CO frequency (F
vco
) is mixed with the
comb generator output in the mixer . The mixer produces multiple harmonics (F
vco
6 40 MHz x
N) through the LPF (low pass lter). The mixer output is compared with the 10 MHz reference
signal in the phase detector . Phase locking imposes the condition of 10 MHz = F
vco
6 40 MHz
x N, and the loop locks to the nearest 40 MHz harmonic satisfying that condition. The initial
frequency (F
vco
) is set to the desired harmonic frequency of 40 MHz x N (N = 12 to 23) using
the pretune D A C. This locks the output frequency (F
vco
) to the desired 40 MHz x N 6 10 MHz
selection of the frequencies listed in T able 11-1 . The polarity of the 10 MHz oset is controlled
by the 61 converters in the loop .
An unlock detector monitors the control voltage to the V CO . When the control voltage is out of
limits , the detector sends the status to the A1 CPU . The A1 CPU causes the message CA UTION:
PHASE LOCK LOOP UNLOCKED to be displayed.
The pretune D A C values are predened by performing the Step Pretune Correction Constants
and are stored in the EEPROM in the A1 CPU .
Multiplier (X 13)
The multiplier receives the 40 MHz reference signal and generates a 520 MHz signal. This signal
is supplied to A3A2 2nd LO and is used to generate the second local oscillator signal. See
Figure 11-6. The 520 MHz signal level is adjusted in the 520 MHz Level A djustment.
A4A1 1st LO
The A4A1 1st LO generates the swept 1st local oscillator signal 2.05858 GHz to 3.85858 GHz
with 1 mHz resolution. The sweep range depends on the start and stop (or center and span)
settings of the analyzer . The signal frequency sweeps between the start frequency + 2.05858
GHz to the stop frequency + 3.85858 GHz.
The 1st local oscillator signal is supplied to the A3A3 source and the A4A2 receiver RF . In
A3A3, the local oscillator signal is used to convert the 2.05858 GHz IF (intermediate frequency)
signal to the 100 kHz to 1.8 GHz RF signal. A4A2 also uses the rst local to convert the RF
input signal to the IF signal.
In addition, the A4A1 1st LO decodes two digital control signals for the A4A2 Receiver RF , and
the decoded signals are supplied to A4A2.
1st Local OSC Circuit
The 1st local oscillator circuit is a phase locked oscillator . The output signal is phase locked to
the FRA C N OSC output signal. The oscillator contains a 2.05858 GHz to 3.85858 GHz V CO , a
phase detector , a 1/4 divider , a mixer , a 1/16 divider , and a single/triple switch. See Figure 11-8.
The single/triple switch is for the single/triple mode and switches the V CO signal to one of the
mixers and the 1/16 divider .
An unlock detector monitors the control voltage to the V CO . When the control voltage is out
of the limits , the detector sends the status to the A1 CPU . The A1 CPU causes the message
CA UTION: PHASE LOCK LOOP UNLOCKED to be displayed.
11-18 Theory of Operation