Topband: noise

[Tom Rauch]

> Audio nulling .. that is interesting. This should have 
> been easy to test as
> if he tuned both rx's to the same active frequency it 
> should have nulled the
> audio..

> The  theory was that the noise (static) received by both 
> radios
> would cancel in  the primary, leaving a clear (or nearly 
> so),
> static-free signal in his  headphones.   Interesting....

I've loooked at this extensively over the years. (Ssince 
1969 or 70!)

Here are the problems:

We have to define the noise. If it is impulse noise with a 
short duty and is idential in time period on all frequencies 
we can just blank it and be done. We will only lose a little 
bit of receive signal if the nosie duty cycle is very low. 
This is a large part of why I could work JA's on 160 through 
the LORAN from Ohio. I built a phase compensated noise 
blanker that sampled LORAN from the same antenna I was 
using. I delayed the main receiver through a long delay line 
so I had time to turn the receiver off quickly but smoothly 
during each LORAN pulse (by disconnecting the antenna), and 
I was already turning the receiver back on as the pulse 
arriving at the receiver was stopping.

I could blank a 50 over nine LORAN signal to noise floor, 
but ONLY when the LORAN signal sample was on the same 
frequency and same antenna. The key to success was the pulse 
was very short and I adjusted time delay so the blanking 
system disconnected the receiver antenna system just as each 
pulse arrived and reconnected it just as the pulses stopped 
without adding switching clicks. It took about 5 vacuum 
tubes and some semiconductors (diodes) and a long length of 
coax to do that.

If the noise is NOT short duty with long rest period we 
cannot blank it without eating up considerable signal. If 
the nosie is a long duty cycle we have to subtract the noise 
without subtracting signal at the same rate, or we have to 
add the signal without adding the noise at the same 
rate.....and this only works under **very** limited 
criteria.

1.) We CANNOT phase cancel the noise on one frequency with a 
receiver on a different frequency or with a receiver having 
greatly different bandwidth or phase charateristics.

2.) DSP's will only work if the signal is detectable through 
the noise. In other words the DSP, when selectivity is 
applied, has to be able to clearly tell what is signal and 
what is noise. Unfortunately the human brain does that just 
as well when optimum filters are used before reaching our 
ears, so the DSP can generally only enhance a signal when an 
experienced weak signal operator can already copy it.

3.) The system can't subtract random "white" noise from 
random "white" noise. It can only eliminate it by adding 
selectivity or adding the coherent signal. This requires two 
antennas, and winds up being no better than phasing two 
antennas. Each antenna has to have a good pattern and about 
the same S/N ratio to double the signal while the system 
adds the noise randomly. In other words this is the same as 
making the antenna bigger by phasing the two antennas.

It doesn't matter if we do this at RF or at audio. I can 
combine the audio output of two phase-locked identical 
receivers. The receivers have to share common local 
oscillators and BFO's and have similar filters and stable 
phase delay. Then, by changing audio phase, I can add the 
signal while the random noise adds and subrtacts randomly. 
The problem is the end result is absolutely no different 
than changing the phase at the antenna inputs and using ONE 
receiver. It is the same thing.

As a matter of fact an HP vector voltmeter measures phase 
after running two different inputs through identical 
receivers using a common local oscillator and it is exactly 
the same as measuring the signals before conversion to the 
low IF frequency!

The problem on 160 is even over a space of several hundred 
feet, phase of signals become random. It is impossible to 
add two skywave signals with fixed phase compensation and 
have them stay in phase very long even under the most stable 
conditions.

To eliminate the consant tuning and fussing to keep two 
antennas several hundred feet apart running in-phase for 
signals, I simply use stereo audio from the two identical 
phase-locked receivers and let my brain process the signal. 
This has been up on my website for years now.  Unless the 
receivers are perfectly phase locked and nearly identical 
the full advantage is never reached.

While a DSP could do the processing, it could only do it if 
the signal is sorted from the noise enough that phase can be 
determined. At that point any of us who are any good at 
copying a signal that is below noise could already hear the 
signal clearly so the DSP is no help.

73 Tom