[TowerTalk] Narrow Band Filters

Jim Lux jimlux at earthlink.net
Mon Jul 5 14:33:25 EDT 2004


> > A real question is what's the best way to cancel the noise
> sidebands from
> > the TX (I think you can knock the carrier down with more
> conventional
> > techniques).  Since they are random, some adaptive
> canceller with a sample
> > from the TX is probably the best approach.
>
> Random does not matter. The requirement is a tap on the
> transmitter feedline, a time delay, and a phase inversion.
> One it is nulled it is nulled unless the antennas or
> something around the antennas changes. With correct time
> delay, frequency changes of even 5% will  not matter.
>
Which, of course, is exactly what an adaptive canceller is...
(you also need adjustable gain in the canceller..)

The neat thing about doing the cancellation in DSP (assuming you can knock
the Tx carrier down to prevent IM in the front end) is that arbitrary delays
and gains are trivial to implement.  No need for miles of coax and relays...
just bits in memory.  You could probably even buy a single chip that does it
today: the standard over the air digital TV receiver has an adaptive RAKE
receiver/equalizer in it to do multipath combining, with 6MHz instantaneous
bandwidth.  Doing adaptive cancelling over 50 kHz bandwidth would be pretty
straightforward.  The key would be not having a receiver front end with ugly
filter characteristics that you'd have to try to back out later.  You might
even be able to do the processing on a PC (but I'd hate to have to write the
software... real time DSP in the context of Windows... aieeee!!.. but there
are people doing it, and you do get to leverage off the cheap computational
horsepower available.. Dell sells a 2.4 GHz machine for $400)

By the way, all this cancelling of strong interfering sources is fairly
standard stuff in the radio astronomy world (check out designs for things
like LOFAR, which is a low frequency (HF) radio telescope spanning
hundreds/thousands of km).  They want huge instantaneous bandwidth, and the
ability to null out those pesky 41m broadcasters, among other things.  I
don't know that they're doing it in real time yet, though, there's a lot of
interest in it (for detecting transient events... detect the big flash, then
point the array towards it)





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