HP 8560E, Service Manual

Page: 492 / 665

Chapter 9 491
Controller Section
Frequency Counter (8560)
Frequency Counter (8560)
See function block Z of A2 schematic diagram (sheet 4 of 4) in the HP
8560 E-Series
Spectrum Analyzer
Component Level Information.
The frequency counter counts the frequency of the last IF and provides
accurate timing signals for digital zero-spans. The circuit also provides
timing signals to the A3 interface assembly ADC (analog to digital
converter). The nominal input frequency is 5.35 MHz (10.7 MHz
divided by 2). The circuit frequency reference in the frequency count
mode is the 10 MHz reference from the A15 RF assembly. The
frequency reference in digitized zero spans (sweep times
≥
30 ms) is the
4 MHz HPIB_CLK, selected by MUX U704.
In the frequency count mode, U702 prescales the 10 MHz reference by 5
to generate a 2 MHz timebase. This timebase feeds through MUX U704
to programmable-timer U700 CLK2 input. Programmable-timer U700
output (OUT2) is the gating signal (HBKT_PULSE) for performing the
frequency count. The gating time interval is a function of the counter
resolution which may be set between 10 Hz and 1 MHz.
Table 9-2 on
page 490 lists the gate time for each setting of COUNTER RES. The
gate time is the period during which U511 pin 3 is high.
The FREQ COUNT input, A2J7, is gated in U511B by HBKT_PULSE.
The gated signal clocks divide-by-16 counters U703A and U703B. These
counters are cascaded to form a divide-by-256 counter. The MSB of this
counter, CD7, clocks the CLK0 input of U700. The frequency of CD7 is a
function of COUNTER RES as shown in
Table 9-2 on page 490 . If timer
U700 overflows, OUT0 will be set and U701B clocked, generating
CNTOVFLIRQ, which will interrupt the CPU.
If IRQAK2 is high, HBKT_PULSE will clock U701A, generating
FREQCNTLIRQ. Upon receiving the FREQCNTLIRQ interrupt, the
CPU latches the CD0 to CD7 onto the BID bus by setting LCDRD (low
counter data read) low and reading the counter data from the BID bus.
The CPU will also read the data from the timer, U700, by setting
L8254CS and LCNTLRD low, placing the timer data on the BID bus.
The CPU resets U701A by setting IRQAK2 low via the BID bus and
latch U506.