Working with
Mixers/Reverse
Power Effects
Note
Uncoupled op eration applies to Option 001 swept signal generators
only.
Uncoupled op eration 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
0
8 dBm, whichin
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
Leveling Mode Normal
results in
ATTEN = 0 dB, ALC Level =
0
8 dBm. The mixer is driven with an
LO of +10 dBm, and has LO to RF isolation of 15 dB. The resulting
LO feedthrough of
0
5 dBm enters the swept signal generator's
OUTPUT port, go es through the attenuator with no loss, and arrives
at the internal detector. Dep ending on frequency, it is possible for
most of this energy to enter the detector. Since the detector resp onds
to its total input p ower regardless of frequency, this excess energy
causes the leveling circuit to reduce its output. In this example the
reverse p ower is actually larger than the ALC level, whichmay result
in the swept signal generator output b eing shut o.
Figure 1-17 shows the same setup, with uncoupled operation used to
pro duce the same
0
8 dBm output. In this case, ATTEN =
0
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
0
15 dBm undesired
signal. This 17 dB dierence 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 mo de as discussed in this
example, do the follo wing:
1. Press POWER
4
MENU
5
.
2.
Select
NNNNNNNNNNNNNNNNNNNNNNNNNNNNN
Set Atten
and press
4
1
54
0
54
dB(m)
5
. This step do es 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
4
POWER LEVEL
54
2
54
dB(m)
5
. This sets the ALC level to
+2 dBm.
For more information on the ALC or setting power level, refer to
4
ALC
5
or
4
POWER LEVEL
5
in Chapter 2.
1-30 Getting Started Advanced
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