HP 3457A - 8-37. Frequency Counter; 8-38. Floating Common Microprocessor

enabled by input multiplexer U507b, pin 9. Multiplexer U507b selects one of three inputs as the
counter enable input (U506, pin 7). The three inputs are either Slope Control A from U508a pin 1,
Slope Control B from U508d pin 13 or the frequency gate signal from U503b pin 9. This time becomes
the time interval that is measured. Upon completion of a time interval measurement, the data present
in U506 is read by the MPU (US501) and combined with the internal MPU counter (U501, pin 14) to
determine the total time interval measured.
8-37. Frequency Counter
The AC assembly, A3, provides a convenient front end for frequency measurements, The ranging
capability can accept input signals over a wide dynamic range of 10mV - 300 volts and frequency
measurements can be made on the function selected by the FSOURCE command. First, an A/D
measurement is performed to determine the optimum range, voltage or current, for the frequency
measurement, This provides input signal conditioning without user intervention.
The A/D converter has external digital hardware used for run-down slope TIME INTERVAL
measurements. Since both frequency and A/D results can not be performed simultaneously, the
frequency counter utilizes this same hardware to accumulate a portion of its result. Frequency is
defined as CYCLES/SECOND. This can be interpreted as ZERO CROSSINGS/TIME INTERVAL and
is precisely what the multimeter has done.
One
of
the
inputs
to
the
prescaler
counter
U506
is
the
Frequency
Gate
signal.
This
is
selected
through
U507b.
Notice
that
the
inputs
of
U506
are
the
Q
outputs
of
another
counter
U505.
U505
is
also
a
prescale
counter
but
it
prescales
MPU
(U501)
internal counter
TI
at
USO1
pin
15
while
US06
prescales
MPU internal counter
TO
at
U50!
pin 14.
These
two
counters
convert
the
frequency
of the
FREQ
sig-
nal
input
from the
A3
assembly
to
an
appropriate
count.
At
the
start
of
a
frequency
conversion,
MPU
USOI
outputs
a
measurement
gate
(USOI
pin
16)
of
ap-
proximately
0.5
second.
With
the
measurement
gate
signal
set,
flip-flop
U503b
will
respond
with
its
Q
(FREQ
GATE)
output
going
high
at
the
next
rising
edge
of
the
FREQ
IN
signal
at
US03b
pin
Il.
The
FREQ
GATE
signal
enables
both
the
time interval (U506)
and
zero
crossing
(US05)
counters
as
long
as
the
MPU holds
the
measurement
gate
signal
high
(U501
pin
16).
Counter
U505
therefore
measures
the
number of zero
crossing
during
the
gate
and
counter
U506
measures
the
"actual" measurement
gate
in-
terval with
0.5uS
resolution.
Frequency
is
computed
from
these
results. This technique
yields
a con-
stant
measurement
resolution
of
Ippm
independent
of
the
input
frequency
with
an approximate
2
readings/second
conversion
rate.
8-38. Floating Common Microprocessor
The floating common MPU US501 is responsible for all aspects of the Analog Section operation. It con-
trols all the measurement dynamics and adds the calibration correction to the finished reading. For ex-
ample, during the A/D conversion process, MPU 501 is completely occupied with control and interac-
tion with A/D hybrid USI1, input hybrid UI01 and external logic. This maintains a constant conver-
sion time. For each measurement configuration, the Al Main Controller sends the appropriate calibra-
tion constants to US01. These constants are used to correct the measurement just completed.
Analog section set-up is performed through serial registers in essential areas. Communication to these
registers
is
accomplished
through
a
common
serial
data
path,
SDO
(Serial
Data
Output)
generated
by
US501. Individual registers are addressed to receive data by clocking data only to the desired register
through the SDCn control lines which are also generated by USOI. There are five control lines,
SDC2-SDC6, that address serial registers. The register number, the name of the register and the com-
ponents switched are as follows.
HP 3457A Multimeter
8-17