Hi Frank,
A friend of mine was having a QRM problem on 160 meters, which he
mistook as a problem with the really strong station's signal. I wrote
him the following explaination of how most noise blankers work. This may
help you understand the problem, and why a radio with a narrower first
IF filter suffers from this problem less. The strong signal was from
N7JW, and there was probably nothing at all wrong with his signal.
Here is how I explained it:
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Noise blanker circuits typically sense the signal level in the first
IF. This fairly wideband signal is detected and the peaks of it are used
to actuate the blanking of some later IF or audio stage. Since strongest
signal inside this wide bandwidth is usually 60 Hz powerline arcing
noise (especially on 160 meters) the 60 Hz pulse train coming out of the
noise blanker's noise detector then mutes a later stage in the receiver,
thus relieving you of having to hear a lot of the noise. (I should have
said 60 or 120 Hz, since many power line arcing problems occur both on
the positive and negative peaks of the 60 Hz power waveform) When some
other signal inside that wide IF bandwidth becomes stronger than the
powerline noise, the blanking pulses stop, and the muting of the noise
stops. So you start hearing what your noise blanker usually mutes.
Try turning off your noise blanker and see if you hear
"zzzzzzzzzzzzzzzzzzzzzzzz" all the time. Check out the S meter reading
of "zzzzzzzz" when N7JW is not transmitting. If N7JW's signal is a lot
stronger than "zzzzzzzzzz" (when you are tuned to him) then the noise
blanker interaction I have described is likely happening. The fact that
you hear the problem over about a 60 kHz range really makes me think
this is the problem. First IF filter bandwidths in multimode receivers
are often just a little bit wider than necessary for the widest mode the
receiver works on, which is usually FM. See if turning on your 20 dB
attenuator makes the problem reduce in severity, or perhaps occur over a
slightly narrower frequency range.
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You said that you have this problem "whenever there is a strong signal
almost anywhere on the band". I'll bet that this is true whenever there
is a strong signal inside your first IF filter bandwidth. If the rig you
are using has wideband FM as one of the modes it can receive, then your
first IF filter bandwidth may be greater than 75 kHz wide. You can test
this by seeing whether tuning about 40 to 50 kHz away from any really
strong signal allows the noise blanker to work again on the 60 or 120 Hz
power line noise. It may be difficult to determine your first IF filter
bandwidth this way, because when you move 50 kHz to get one really
strong signal out of that bandpass, another one has moved into it.
I do not know the conversion scheme of every Ten-Tec radio. So I cannot
tell you which ones are the best. What I can tell you is that the OMNI
VI radios have a first IF filter bandwidth (a two pole monolithic
filter) of about 15 kHz. Most rigs that operate in multi modes which
include wideband FM will have a much wider first IF filter, although
there may be exceptions in which the FM detector uses a completely
separate IF that goes around the first IF filter used for the other
modes. You cannot fit 8 pounds of stuff in a 5 pound bag, so if it is a
super compact multimode radio, it probably has just one really wide
first IF filter.
I hope you find this helpful.
DE N6KB
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