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

functions as a programmable frequency divider and a phase/frequency detector. The

divider is programmed by the Al5 Controller to divide the input frequency by the
divide number

down to 5 MHz. This is then compared with a 5 MHz reference in the

phase/frequency detector. The detector output, after amplification and integration, tunes the
frequency of the

voltage-controlled oscillator (VCO).

The

divide number

always falls in the range between 3.60 and 13.97. The integer

part is coded in four binary bits while the fractional part (0.00 to 0.99) is coded in two 4-bit
BCD bytes.

U5 constitutes a 3-stage limiting amplifier. U5 is an ECL triple-line receiver which changes
the input to the proper amplitude and dc level (approximately

V low and

V high) for

driving the subsequent divider.

is a D-type flip-flop which divides the limiting amplifier

output by 2.

and

are latches which store the divider programming number. The number is clocked

into the latches from the instrument bus with LCK4.

U12 is a 4-bit binary counter which is programmed with the integer part (3 to 13) of the

divide number

In operation it is clocked by the output of

and counts down to

1. On the next clock pulse, the programming number is loaded into the counter, and the cycle

repeats. Loading time is sensed by

when the count is at 2. The next clock pulse causes

to change states which enables the loading of U12 on the subsequent clock pulse.

The fractional divide works on the pulse-swallowing principle. Consider a divider that divides
by N for P-A output pulses and by N 1 for A output pulses. Note that the total output
pulses is P. The number of input pulses to get P output pulses is
N(P-A) (N + l)A = NP A.
Therefore, the divide ratio is the number of input pulses divided by the number of output

pulses which is equal to N •l- A/P. In this case P is 100, so N represents the integer portion of
the divide number while A represents the fractional part times 100.

An example best illustrates how the fractional divide works. If the

divide number

is 8.57, then

and

The integer divider normally divides by N each divide cycle.