HP 87114B, Service Manual

Page: 136 / 174

Source Theory
The source group consists of the A3 fractional-N/reference assembly and the A4 source
assembly .
A3 Fractional-N/Reference Assembly
The A3 fractional-N/reference assembly consists of two boards connected together . The
reference board generates the various reference signals used in the analyzer , while the
fractional-N assembly generates a 30 to 60 MHz synthesized signal for use in the A4 source
assembly .
The reference board uses either the internal 10 MHz crystal oscillator or an external 10 MHz
reference signal to generate three reference signals . The 100 kHz signal is used in the
fractional-N V CO phase lock loop . The 10 MHz signal is used by the A4 source assembly .
A 5 MHz signal goes to the backplane , where it is routed to the A2 CPU and A5 receiver
assemblies for use in signal processing.
The fractional-N board is the synthesizer for the source . The 30 to 60 MHz fractional-N V CO
output is used for phase-locking the source LO signal, which in turn is used to generate the
source RF output.
A4 Source Assembly
The source assembly provides two output signals . One signal, which covers the 0.3 to 1300
(or 3000 MHz for HP 8713B/14B) range , is the main source RF output signal. It goes to the
A5 receiver assembly and through an optional AM delay modulator and/or step attenuator
before it gets to the RF OUT (REFLECTION) port on the front panel. The other signal serves
as the internal receiver LO for downconverting narrowband signals . The two signals are
independently phase-locked, and they are separated from one another by the receiver IF
(intermediate frequency) of 27.778 kHz.
Note
The following paragraphs describe an HP 8711B/12B . The HP 8713B/14B is
very similar . See the section \Dierences Between the HP 8711B/12B and
HP 8713B/14B (1300 MHz vs 3000 MHz)" at the end of this chapter .
These two signals are generated from 3 phase-locked V COs , as shown in Figure 6-2. In
normal operation, the RF1 V CO is phase-locked to a 1 MHz reference signal to produce a
stationary 2340 MHz signal. (The 1 MHz signal is derived from the 10 MHz output of the
A3 fractional-N/reference assembly .) Similarly , the RF2 V CO is also phase locked to the
1 MHz reference to produce a stationary signal that is oset from RF1 by the receiver IF of
27.778 kHz. The source LO V CO supplies the LO drive for the two source mixers . It covers a
2340.3 to 3640 MHz range . It is phase-locked to a fractional-N sweeping synthesizer (on the A3
fractional-N/reference assembly). As this phase lock loop sweeps , the main source RF output
is generated as the mixing product of the 2340 MHz RF1 signal and the sweeping 2340.3 to
3640 MHz source LO signal. Similarly , the receiver LO output is generated by the RF2 signal
mixing with the sweeping source LO , resulting in a signal that is oset from the source RF
output by the receiver IF .
6-4 Theory of Operation