HP (Hewlett-Packard) HP 85660B ユーザーズマニュアル

The Reference Loop is phase-locked to the A22 10 MHz Standard and its phase-locked
outputs (10, 20, 100, and 400 MHz) are used as references for the 

 Loop, the M/N

Loop, and the Heterodyne Loop (part of A6). The 10 MHz signal is also used as an auxiliary

rear-panel output.

The 

 100 MHz VCXO (Voltage-Controlled Crystal Oscillator) is the heart of this loop.

The 100 MHz output signal is 1) routed to the 

 Phase Lock assembly to be used as a

reference for the Heterodyne Loop and as the front-panel CAL OUTPUT signal, 2) routed
to the 

 

 Up Converter to be used as a reference for that phase-lock loop, 3)

multiplied by four to produce the reference signal for the M/N Loop 

 M/N Phase

Detector), and 4) divided by five to produce a 20 MHz reference signal also used by the 
N Loop. The 20 MHz signal is divided by two to produce a 10 MHz signal which is used
as a reference for the 

 and PLL2 phase-lock loops in the 

 Loop and also as the

rear-panel auxiliary output. The 10 MHz signal is also phase compared to the output of the
A22 10 MHz Standard. The resulting error signal is used to tune the 100 MHz VCXO to
phase-lock the Reference Loop.

The frequency output of the M/N Loop is dependent on the front-panel Start
Frequency/Center Frequency and in part, controls the YTO output frequency. This signal is
disabled during sweeps when the frequency span is greater than 5 MHz (fundamental mixing).
An encoded equivalent of the front-panel frequency’s most significant digits are input to the
M/N Loop as M and N numbers. The ratio of the M and N numbers determines the M/N
OUT 

 frequency and are chosen such that the Nth harmonic (same as the divider

number) of 

 tunes in exactly 10 MHz increments as M is changed. There is one 10 MHz

step for each valid 

frequency (M/N ratio) and Nth harmonic (N number). Refer to Table

2 for a complete list of M and N numbers and resulting 

 frequencies. This 10 MHz step

complements the 

 Loop whose tuning range is 10 MHz and step size is 1 Hz. Together,

the M/N loop, YTO pretuning, and 

 Loop are able to tune the 

 YTO from

2000.000 000 to 6199.999 999 MHz in 1 Hz steps.

Whenever a change is made to the Start Frequency/Center Frequency (from the front panel
or remote controller), the YTO is pretuned near the desired new frequency. This is done
by encoded information being sent from the Al2 Front-Panel Interface to the A19 DAC
(Digital-to-Analog Converter) which generates and routes an analog voltage to the A20 Main
Coil Driver. The Main Coil Driver, in turn, generates a tuning current to tune the YTO. This

places the YTO frequency 20 to 30 MHz below the Nth harmonic of the M/N Loop output

 For frequency spans greater than 5 MHz, the YTO frequency is locked at the start

of each sweep with the M/N loop and the 

 loop to a 1 Hz resolution. The YTO signal

 and the M/N Loop output signal 

are mixed in the 

 Sampler (harmonic

mixer) to produce the YTO IF (f

signal. This 20 to 30 MHz signal is phase compared

to the 

 Loop output signal 

 in the 

 YTO Phase Detector producing a dc

error voltage. It should be noted that 

 and 

 are equal when the YTO Loop is

phase-locked. This error voltage is stored in a capacitor, the loop (YTO) opened, and a sweep
taken. The loop is relocked at the start of each sweep. If the frequency span is less than 5
MHz, the YTO Loop is always locked and is forced to sweep by following the swept oscillator

2 Analog Troubleshooting