HP 8560A, Service Manual

Page: 242 / 450

Al3 Second Converter
The Al3 Second Converter down-converts the 3.9107 1st IF to a 310.7 MHz 2nd IF. The
converter generates a 3.6 second LO by multiplying a 600 MHz reference.
filters remove unwanted harmonics of the 600 MHz driving signal. First IF and 2nd LO
signals are filtered by cavity filters.
Second IF Distribution Amplifier (P/O A15)
The SIFA (S IF Distribution Amplifier) amplifies and filters the second IF.
(Option 001 instruments provide the pre-filtered signal at the rear-panel’s 2ND IF OUTPUT.)
Factory select attenuator
ensures that the gain provided by the SIFA is 12
The external mixing input from the front-panel’s IF INPUT connector is also directed through
the SIFA. A dc bias is placed onto the IF INPUT line for biasing external mixers.
Third Converter (P/O
The third converter down-converts the 310.7 MHz IF to 10.7 MHz. A PIN-diode switch selects
the LO signal used. For normal operation, a 300 MHz LO signal is used. The signal is derived
from the 600 MHz Reference PLL. During signal identification (SIG ID ON), the 298 MHz
SIG ID Oscillator is fed to the double balanced mixer on alternate sweeps.
Flatness Compensation Amplifiers (P/O A15)
The flatness compensation amplifiers amplify the output of the double-balanced mixer. The
amplifier’s variable gain (8 to 32
compensates for flatness variations within a band. Band
conversion loss is compensated by step gain amplifiers in the IF Section.
Control for the amplifiers originates from two on the A3 Interface assembly. (DAC
values are interpolated approximately every 17 MHz based on data obtained during the
Frequency Response Adjustment.)
flatness-compensation control circuitry converts the
RF GAIN voltage, from A3, into two currents: RF GAIN1 and RF GAIN2. These currents
drive PIN diodes in the flatness compensation amplifiers.
Synthesizer Section
The first LO is phase-locked to the instrument’s internal 10 MHz standard by four See
Figure 6-5.
The Reference PLL supplies reference frequencies for the instrument. The three remaining
tune and phase-lock the LO through its frequency range. To tune the LO to a particular
frequency, the instrument’s microprocessor must set the programmable feedback dividers (N)
and reference dividers (R) contained in each PLL.
Sweeping the First LO
The analyzer uses a method called Lock and Roll to sweep the first LO (All YTO). This
applies to all frequency spans and involves phase-locking the analyzer at the start frequency
during the retrace of the sweep. The sweep ramp, generated on the Al4 Frequency Control
assembly, is applied to either All main coil, All FM coil, Roller Oscillator
Main Oscillator, or Roller Oscillator Offset Oscillator. The frequency/span
relationships are as follows:
General Troubleshooting 6-37