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Model 8902A
Operation
Frequency Offset Control
FUNCTIONS
Special Function 21 (Frequency Offset Control)
DESCRIPTION
Using an external mixer and local oscillator (LO) to down-convert the input signal, the frequency
range of the Measuring Receiver can be extended into the microwave region. The Frequency
Offset mode allows the Measuring Receiver to keep track of the frequencies involved in the down-
conversion so the user can be freed from this task. This feature operates for frequencies
up
to 42
GHz.
The full complement of instrument measurements can be made on the down-converted microwave
signal (with the same signal restrictions that apply in the normal frequency range). For best
measurement accuracy, the external LO should be a source free from microphonics, residual FM,
and drift since these signal impurities translate directly on to the down-converted signal. Use of a
synthesized signal source is recommended. Examples of suitable signal sources are the HP 8340A,
8612A, and 8613A.
The RF Power measurement mode uses a separate table of frequencies and calibration factors in the
Frequency Offset mode. Frequency entries can range from 0 to 110 GHz. (Refer to
RF
Power on
page 3-75 for a procedure for entering calibration factors.)
A rear-panel FREQ OFST TTL OUT connector can indicate to external circuits whether the
Measuring Receiver is in the Frequency Offset mode and whether the LO is above or below 18
GHz. Use this feature t o control external RF switches to route or bypass the input signal between
external mixers.
Use Special Function 27 to enter and exit the Frequency Offset mode and to enter and display the
LO frequency.
PROCEDURE
To down-convert a microwave input signal into the range of the Measuring Receiver, set an external
microwave signal source to a level suitable as an LO for the external mixer. Set the frequency of the
external LO source to within 1300 MHz of the microwave input signal. Connect the external LO
source to the L port of the external mixer. Connect the microwave input signal to the appropriate
input port of the external mixer. Connect the output port of the mixer to the RF INPUT connector
or to the input of a sensor module connected to the Measuring Receiver.
Key in 27.3 SPCL then select the LO frequency (in MHz) and select MHz
to
enter the Frequency
Offset mode.
rcwIFEBRUARY86
3-255
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