Al Clark said:
An analog notch filter may be fine for this application if the following
conditions are actually true:
1. Temperature is stable, so that the filter will not drift.
Once the site is turned up, the temp remains fairly constant.
Big fear here is latter on this winter when outside temps are -20 and lower,
may take a bit longer to stabilize.
2. The interfering signal is actually 400 Hz. My guess is that it has a
fundamental at 400 Hz and also some significant harmonics.
It is 400hz power that is making the whine. We use 400hz power on the
electronic equipment at the site, except of course the comm gear which is
your basic 120.
The harmonics
may be a bigger problem than the fundamental, since hearing is more
sensitive at higher frequencies (1-2kHz).
I would agree with this outlook in most other situations. But being that
this is the noise of the 400hz power, these harmonics are not so bad.
I would look at the signal with an FFT analyzer (You can probably find a
PC program on the web). This information will suggest an appropriate
solution.
Looked at with a Spec A, as well as one of those level meters that gives
freq's and levels that are used to find rf hotspots to comply with OSHA
regs.
These might be:
1. A simple notch filter as discussed.
2. Multiple notch filters.
For the remote locations these would be best option, as I have restrictions
on electrical supply, being these are very remote locations.
3. Automatic notch filters using adaptive filtering.
4. Comb filters.
Active devices are kind of limited due to the scaricty of electricity at
site.
Cutoff is to high to properly knock down the 400. This was also my first
choice of trying to find a solution for this problem, which has existed
since before I came here.
All of these solutions are easily addressed with a DSP and I have used
each of these methods for similar noise problems. The best solution
depends on the specifics.
I agree, active would be nice, but electrical supply is very limited.