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Mode1 8901B
Service
Power Reference Oscillator Assembly (A32)
Since the Power Meter uses an open-loop measurement technique (as opposed to a dc substitution
technique), an independent power reference is required to calibrate (that is, determine the sensitivity
of) the external Power Sensor. The Power Reference Oscillator produces a 50 MHz RF signal which is
set, during calibration of the reference, to deliver
1
mW to a 50R load. The output of the oscillator is
accurately controlled by the Automatic Leveling Control (ALC) Loop. A Level Detector a t the output
of the oscillator senses the peak RF level. The level is compared to
a
stable, dc reference by the Level
Error Amplifier. The error is amplified and fed back to the level-control circuitry in the oscillator to
correct the error.
RF Input Assembly (A15)
The RF Input Assembly is the instrument's front end. It receives the RF input signal and attenuates
it to an optimum level for the Input Mixer.
The RF level is sensed by the RF Level Detector. The output of the detector is buffered by the Detector
Amplifier and applied to the Voltmeter. The Controller uses the RF Level detector when automatically
setting the RF Attenuator and when making RF Level (Special Fbnction 35) measurements. The RF
Level Detector senses the peak of the RF voltage including AM envelope peaks.
The Overpower Detector compares the detected RF level with a reference. If the RF level (with AM
envelope) exceeds
1
W, the Overpower Protection relay is de-activated (opened) and latched. Pressing
any key will reset the relay.
If the instrument is tuned to a frequency greater than 10 MHz, the 5.25 MHz High-Pass Filter can be
switched in to eliminate low-frequency signals on the input which can pass directly into the IF. Special
Function 3 controls the selection of the 5.25 MHz High-Pass Filter (as well as the IF filter).
The Input Attenuator consists of one 10
dB
pad and two
20
dB
pads for a range of 0 to 50
dB.
The
RF path
is
switched between the thru-lines and attenuator pads by RF relays as determined by the
Controller.
Input Mixer Assembly (A17)
The Input Mixer Assembly converts the RF input signal to the IF.
Part
of the
IF
filtering is included
in this assembly. In the automatic tuning mode, the Local Oscillator (LO) is tuned so that the LO
frequency minus the signal frequency equals the IF. Using manual tuning, it is also possible to tune
the
LO so
that the
IF
responds to the image; that is, when the signal frequency minus LO frequency
equals the IF. In this case, the phase of the FM and OM is inverted.
The Input Mixer has two modes of operation.
(1)
For input signals in the range 2.5 to 1300 MHz, the
Input Mixer down converts the input signal to the
1.5
MHz or the 455 kHz IF. (2) For signals below
2.5 MHz, the Input Mixer passes the signal directly into the IF. (Down conversion can be extended
below 2.5 MHz using the 455 kHz IF and manual tuning.) The normal operating signal level is less
than -16 dBm for AM and -6 dBm for FM and @M.
The LO signal for the Input Mixer comes from the LO Dividers through the LO Amplifier.
(The next
t w o
paragraphs apply
to
23148
to
26368.)
The IF frequency response is determined by the IF Filters and the IF amplifier in the A18 IF Amplifier
Assembly. The 455 kHz Wide Bandpass Filter in the A17 Input Mixer Assembly determines the
response of the 455 kHz IF. The 455 kHz Wide Bandpass Filter is switched in automatically for input
signals in the range of 2.5 to 10 MHz. When the 455 kHz IF
is
chosen, the 455 KHZ IF annunciator
lights.
Automatic IF filter selection provides the optimum IF frequency and
IF
filter selection for each
measurement mode. The frequency response
of
the IF filter
is
determined by the RF input blocking
capacitors (not shown), the 4 MHz Low-Pass Filter, and (principally) the 2.5 MHz Low-Pass Filter
Service Sheet BD2
8D-17
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