Working with
Mixers/Reverse
Power Effects
Note
Uncoupled operation applies to Option 001 swept signal generators
only.
Uncoupled operation is useful when working with mixers. Figure 1-16
shows a hypothetical setup where the swept signal generator is
providing a small signal to a mixer. The swept signal 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 signal 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 signal 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 dB shift in
the swept signal generator output level. To set the swept signal
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
@
(dB(m)).
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)
@
[ml.
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
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