HP 8560E Service Manual page 499

Controller Section
16 MHz Harmonic Filter (8560EC)
16 MHz Harmonic Filter (8560EC)
The 16 MHz Harmonic Filter generates a 16 MHz signal through a
series of stages, consisting of a filter and a comparator. The 10 MHz
reference signal from the A15 RF assembly is first prescaled by 2.5 to
yield a 4 MHz signal with a 20 percent duty cycle. This prescaling is
performed within U35. The 4 MHz signal is then passed, first, through
a high pass filter, and then, through a bandpass filter at 16 MHz. The
high pass filter consists of R85, C122, and L14. The bandpass filter
consists of L19 and C139. The filter basically filters the fourth harmonic
of the 4 MHz signal to generate a 16 MHz signal. The resulting signal is
then passed through comparator U34A to generate a 16 MHz square
wave. Three more stages, consisting of a bandpass filter followed by a
comparator, further filter the signal so that a clean 16 MHz signal
results. The 16 MHz signal which is the result of these successive
stages of filtering is output at pin 10 of U34. U35 buffers this signal to
provide the 16 MHz clock for the CPU. In addition, divide-by-two flip
flops are located within U35, which generate 8 MHz and 4 MHz signals.
498 Chapter 9