Working
with
Mixers/Reverse
Power
Effects
Note
Uncoupled operation applies to Option 001 swept CW generators
only.
Uncoupled operation is useful when working with mixers. Figure 1-16
shows a hypothetical setup where the swept CW generator is
providing a small signal to a mixer. The swept CW generator output
is -8
dBm,
which in Leveling Mode Normal results in
ATTEN
=
0 dB, ALC Level
=
-8
dBm.
The mixer is driven with an LO of
+lO
dBm,
and has LO to RF isolation of 15 dB. The resulting LO
feedthrough of -5
dBm
enters the swept CW generator’s OUTPUT
port, goes through the attenuator with no loss, and arrives at the
internal detector. Depending on frequency, it is possible for most of
this energy to enter the detector. Since the detector responds to its
total input power regardless of frequency, this excess energy causes
the leveling circuit to reduce its output. In this example the reverse
power is actually larger than the ALC level, which may result in the
swept CW generator output being shut off.
Figure 1-17 shows the same setup, with uncoupled operation used to
produce the same -8
dBm
output. In this case,
ATTEN
= -10 dB,
ALC Level
=
+2
dBm.
The ALC level is 10 dB higher, and the
attenuator reduces the LO feedthrough by 10 dB. Thus the detector
sees a
+2
dBm
desired signal versus a possible -15
dBm
undesired
signal. This 17 dB difference results in a maximum 0.1 shift in the
swept CW generator output level. To set the swept CW generator to
the attenuator uncoupled mode as discussed in this example, do the
following:
1. Press POWER (MENU).
2.
Select Set
Atten
and press
(iJ
@
IdBo).
This step does two
things, it uncouples the attenuator from the rest of the ALC
system, and it lets you set an attenuator value, in this case, 10 dB.
3. Press [POWER LEVEL)
@
[dB(m)).
This sets the ALC level to
+2
dBm.
For more information on the ALC or setting power level, refer to
(ALC)
or [POWER LEVEL) in Chapter 2.
l-30 Getting Started Advanced
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