HP 8901B Operation And Calibration Manual page 190

Model 8901B Operation
Additional Recommendations
0
Re-calibrate the
sensor
when ambient conditions change or approximately every eight
hours.
0
Whenever the source frequency is changed, either select the new frequency manually (in
MHz) or select FREQ to let the instrum ent automatically tune to the signal, then re-enter
the RF Power measurement mode.
Also,
if the tuned frequency moves more that &5 H z
&om the frequency at which the power sensor reference was taken, the
instrum
ent must
be retuned.
0
U s e
the table on the following page,
"Summary
of Special Function 37" for a quick
overview of
all
the codes
that
manipulate the
RF
power cal factors.
HP-IB PROGRAM CODES
%
CAL FACTOR
=
CF
Hz=m
RF
POWER
=
M4
CALIBRATE on
=
C1 LIN results
=
LN
CALIBRATE off
=
C O LOG results
=
LG
DISPLAY FREQ
=
FR h l H z = M Z
ZERO
=
ZR
SAVE CAL
=
SC
WATTS
Units
=
WT
MEASUREMENT TECHNIQUE
The RF Power Measurement
The power sensor produces a small dc voltage which is proportional t o
the power dissipated in the
sensor's input load. The dc voltage is then converted
to
ac, amplified, and converted back to dc. The
Modulation Analyzer's internal dc voltmeter measures the voltage and displays the rea- in terms
of power.
Five ranges of amplification are used to encompass the power sensor's wide dynamic range. Special
Function 10 permits manual selection of the five ranges: Range 1 (10.1 SPCL) has the most
amplification (the most sensitivity), while Range 5 (10.5 SPCL) has the least amplification (the
least sensitivity). The ranges are not specified in terms of absolute power because the sensor type
determines the sensitivity.
The chopper in the power sensor converts the dc voltage (generated by the power sensing element) to
a 220
H z ,
chopped, ac signal. The ac signal is then amplified and rectified by a synchronous detector
whose configuration alternates between a non-inverting
and an
inverting amplifier in step with the
chopper. CThe 220 Hz Multivibrator is the source for both chopping signals.) The result is a rechopped,
dc level proportional to the dc output of the
RF
power sensing element of the power sensor. This
chopping and rechopping method reduces the power meter's susceptibility to dc
d r i f t
and noise.
The dc level from the Synchronous Detector is filtered, to reduce noise then measured by the
Voltmeter. On the more sensitive ranges, filtering is increased to smooth out the large noise
component on the signal and thus stabilize the displayed reading.
RF Power Calibration
With the external power sensor properly connected to the Modulation Analyzer, and the sensor's
FU?
input connected to the CALIBRATION
RF
POWER OUTPUT, calibration begins by first
determining the sensor
type.
(See the Simplified Block Diagram of
F W
Power Calibration.) A switch
at the input of the Voltmeter connects the power sensor's resistor t o the Voltmeter. The voltage
across the resistor indicates, to the Modulation Analyzer, the
type
of power sensor being used.
UNCAL and RECAL annunciators are displayed if the sensor type differs from that used in a
previous calibration.
When the ZERO
key is
pressed, the internal controller sends out a signal
to
the
sensor
module
t o switch out the
RF input to the power sensor and directs the power meter to make
a power
rec.01
FEBSI
R F Power
3-75