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SRS Tech Note
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Carrier suppression in the SG384 RF Sig Gen when using external I/Q modulation
Some customers require high carrier suppression when using the external I/Q modulation inputs. Tests
have shown that the SG384 (with Option 3, External I/Q modulation inputs) can provide more than
70 dB of carrier suppression with careful control of the offset voltage on the external I/Q modulation
inputs.
The photo below shows the power signal envelope of a SG384 in the External I/Q modulation mode. The
carrier is 0 dBm at 3 GHz. The external modulation source (from the BNC output of another SG384) is a
5 kHz, 1 Vpp sine, pulse modulated with a 50% duty cycle at 1 kHz and applied to the I input only. The DC
offsets of the I&Q inputs were adjusted to minimize the carrier feedthrough during the external
modulation "off" time.
The external I/Q modulation circuits are the same in both the SG380 series and SG390 series RF signal
generators, and so these results are expected to hold for all of these instruments. External I/Q
modulation is an option in the SG380 series instruments and a standard feature in the SG390 series
instruments.
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Stanford Research Systems 1-Nov-13
Tel: (408)744-9040