HP 8901B Operation And Calibration Manual page 224

Operation Model 8901B
COUNTER/
fw
CONTROLLER
-
OSCl
L
L A T O
R
RF Input Bequency Measurement Block Diagram
COMMENTS
lnput/Output Status
When the instrument is measuring input frequency, the output signal at MODULATION
OUTPUT/AUDIO INPUT represents the last modulation measurement made as indicated by the
MODULATION OUTPUT annunciator.
Relative Measurements
If
RF frequency is to be displayed relative to a reference, enter the value as a ratio reference using the
RATIO
key. (Refer
to Ratio on page 3-215.)
Limiting Low-Level Signal Measurements
When the
E r r o r
Message Disable Control special function is in the automatic selection mode (Special
Function 8.0), very low-level signals can be counted without generating error messages (Error 03 for
example). However, other Special F'unction 8 codes enable error messages at much higher signal levels.
(Refer to Error Message Disable Control on page 3-233.)
Centering the IF Does Not Affect Frequency Measurements
It
is important to note that when
FREQ
is selected, the IF need not be exactly centered to make valid
frequency measurements. A frequency is displayed as long as a signal is present in the IF. Using the
kHz (up arrow) or kHz (down arrow) functions will not change the displayed input frequency even
though tuning has been altered. If desired, the frequency error function can be used to indicate tuning
error if desired.
3-112 RF Input Frequency