Model
8901B
Operation
Frequency Offset Control
FUNCTIONS
Special finction
27
(Frequency Offset Control)
DESCRIPTION
Using an external mixer and local oscillator
(LO)
to
down-convert the input signal, the frequency range
of the Modulation Analyzer can be extended into the microwave region. The Frequency Offset mode
allows the Modulation Analyzer
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, 8672A,
and
8673A.
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
3-71
for
a
procedure for entering calibration factors.)
A
rear-panel FREQ
OFST
TTL OUT connector can indicate
to
external circuits whether the
Modulation Analyzer
is
in the Frequency Offset mode and whether the
LO
is above or below
18
GHz.
Use this feature
to
control external RF switches to route
or
bypass
the input signal between
external mixers.
Use Special finction
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 Modulation Analyzer, 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 Modulation Analyzer.
Key in
27.3
SPCL, then select the
LO
frequency
(in
MHz)
and select MHz to enter the Frequency
Offset mode.
Frequency Offset Control
3-225
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