From: Ernest Landi <Ernie.Landi@sv.sc.philips.com>
Subject: 2.7 KHz Filters
Date: Friday, January 09, 1998 4:45 PM
I think I have a good reason why one cannot increase the bandwidth of
an SSB filter much beyond 2.4 KHz. The filter not only suppresses the
unwanted sideband from the balanced modulator, it also adds to the
carrier suppression. A common method to generate both LSB and USB is
to move the carrier frequency to the high or low side of the filter.
Assuming for example that a 2.4 KHz filter was centered at 9.000 MHz,
a LSB signal could be generated by setting the carrier to 9.0015 MHz.
This would give an audio passband of 300 Hz to 2700 Hz. For a USB
signal, the carrier would be moved to 8.9985 MHz. In each case the
carrier would be 300 Hz from either the upper or lower edge of the
filter. Any increase in filter bandwidth will reduce this margin,
resulting in less carrier suppression and even opposite sideband
I think my Drake T-4xb and the old Atlas radios used separate SSB
filters for USB and LSB. In this case the bandwidth could be increased
as long as the edge closest to the carrier was held fixed.
There are be other schemes to get around this, but the key is that the
carrier has to always be on the same side of the filter, that way the
filter edge next to the carrier is fixed and the farther edge can be
moved out for wider audio bandwidth.
73, Ernie WA7THF
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