Markus:
The circuit does not require that the noise be on your power line -- in fact
it is preferable that it is not on that line.
What it does do is to use your power line to supply the 120 Hz clock for the
mutivibrator pulse generator. The pulse output of the multivibrator circuit
controls a circuit that is in series with the RF input to the receiver and
separate from the multivibrator pulse generating circuitry.
The timing of the start of the generated pulse used to cut off the RF input
to the receiver is adjusted using the PHASE potentiometer so that when the
radiated line noise reaches your antenna and is heading for your receiver
the pulse generated by the multivibrator circuit will "cut" off the RF
starting at the beginning of the spike on the RF line. The WIDTH
potentiometer adjusts the duration of the cut off period so that the
incoming spike is over before the RF reception is restored.
The beauty of the circuit is that you can adjust the start time and the
duration of the RF cut off so that only that part of the incoming RF that
contains a noise spike is affected.
All of the circuitry is 'outside' of your receiver so there is no need to
modify the receiver and, in fact, you can use the receiver noise blanker as
well as the external one.
Because there may be more than one source of radiated noise it is quite
feasible to have two of the multivibrator pulse generator circuits each
driving the same RF gating [cut off] circuit. Each multivirbrator has its
own PHASE and WIDTH controls so you can adjust them independently to blank
two different sources.
My personal preference is to use the circuit to drive and external RF gating
circuit rather than to cut into the receiver and connect it to the internal
noise blanker circuit of the receiver. The internal noise blanker might be
needed to remove electric fence noise which I would suspect is not synched
with the power line 60 Hz. I don't have such noise so I don't know if this
is true.
You mention a "noise canceller" and I am guessing that might refer to a
device which uses a sense antenna to provide noise that is 'cancelled' using
a unit such as the DX Engineering NCC-1 or the MFJ-1026. I have used both
and have the NCC-1 presently. The 'cancellation' process nulls both the
noise and the desired signal -- especially if they are in the same general
direction. I use a preamp to restore the signal level after the nulling
process but that still leaves weak signals with a S/N that I wish was
better.
Like the RX antenna rule -- more than one method for eliminating noise is
desirable.
The best method of course is, as you state, eliminate the noise at the
source.
Tod, K0TO
> [mailto:topband-bounces@contesting.com] On Behalf Of Markus Hansen
> Subject: Re: Topband: Noise
>
> Hi Tod. I built this cct many years ago and found it unsatisfactory
compared to the noise blanker in my receiver. It requires a high level of
> power line noise spikes on the power line coming into your shack before
it will trigger the blanker diodes and that
> caused a lot of IMD. Put a scope on your 110 V line into you
> QTH and see if you can see any power line spikes on your 110
> volt line. If not the noise you are receiving may be quite
> some distance away some your QTH being radiated instead of
> induced along the AC line into you shack.
>
> Of course the best solution is locate the power line noise
> source and try to get it fixed.
>
> The next best solution is a noise canceller which is inserted
> between the receiver and the receive antenna and it doesn't cause IMD.
>
> Markus VE7CA
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160 meters is a serious band, it should be treated with respect. - TF4M
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