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VHF/UHF Receiver noise

SteDavies

Jul 10, 2015
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Hi, Does anyone know why the circuit noise in a VHF or UHF receiver is so high when compared with a short wave or lower frequency receiver? Also, why does a carrier quieten the noise - is it just because the automatic level control kicks in, or is there real noise reduction in the circuits? Thanks.
 

ramussons

Jun 10, 2014
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The "No carrier" noise depends on the type of demodulator. In general, Shortwave or AM demodulators are Amplitude Modulation detection, whereas the VHF / UHF are Frequency Modulation detection.
By the basic working principle, FM discrimination (demodulation) gives white noise when there is no carrier. There is a mathematical explanation for this, maybe someone else can explain it better.
 

SteDavies

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Thanks ramussons, I was beginning to think that no-one was going to reply! That makes sense and if anyone has a mathematical explanation then that would be gratefully received.
 

davenn

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Sep 5, 2009
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@SteDavies

hmmm strange

an FM receiver should be quieter than an AM receiver regardless of frequency
The reason is that specially manmade noise has AM modulation and this gets cut out
because of the standard function of an FM receiver

I can tune my amateur radio from very low HF freq ~ 200kHz to as high as 1300MHz
That's right across LF, MW, HF, VHF , UHF and into SHF
I can switch between AM and FM on any freq within that range.
There will ALWAYS be more hiss noise out of the speaker on AM than on FM when you are not receiving
a radio station signal. And even if you are receiving a signal, as you get further from the transmitter,
the AM transmitter signal will get noisier where the FM stays clear till the drop out range is reached

so I'm not really sure how you are testing or seeing the result that you are ?


PS sorry for a slow response ... have been really ill over the last 5 days ... haven't been overly active online

cheers
Dave
 

ramussons

Jun 10, 2014
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Most FM detectors have a "Squelch" mode when there is no carrier to prevent a blast of noise. AM detectors do not have this.
The noise from a AM detector is mainly "crackling" while that from the FM is a "hiss"
 

SteDavies

Jul 10, 2015
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Thanks for that Dave. My experience of VHF and UHF is that, unlike longer wavelengths, you always get a massive burst of noise from the electronics when there is no carrier; hence the need for the squelch that ramussons mentioned. This applies to everything from television sets and quality stereo FM receivers to mobile transceivers and walky-talkies. With my Hi-Fi stereo FM receiver, if the signal cuts out due to transmitter interruption, I get a massive burst of noise through the speakers and have to rush to turn it down. Analog TV sets always did the same thing. Longer wavelength receivers never needed a squelch, and I'm wondering why this is peculiar to VHF/UHF FM; and ramussons suggestion that it is the FM detector made sense. It's interesting though that your communications receiver doesn't do it, even when using the FM detector.
 

davenn

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that's not the reason for the squelch

no you did mention component noise, that's a whole different ball game to random RF noise appearing at the
antenna. Component noise will be the same in any sort of receiver for a similar component list

This is called thermal noise. it is initially reduced by using good quality components.
metal film resistors, low noise transistors and FETs,

when into radio astronomy, amateur radio moon bounce etc we even go to more extremes to decrease that
thermal noise by cooling the electronics down using liquid nitrogen etc

Dave
 

SteDavies

Jul 10, 2015
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I've read that 90% of VHF/UHF receiver noise is generated by it's own electronics. You can test this by pulling the ariel out of an analogue TV and noting that the noise drops only very slightly. Are you familiar with the noise that I'm talking about Dave - I mean from TVs, FM stereo tuners, walky talkies etc?
 

davenn

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yes ... its what I referred to in my previous post and as I said, it isn't just VHF/UHF its any band anywhere in the RF spectrum
 

poor mystic

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FM is almost noise-free where a carrier is present, but when no carrier is within lock range the PLL will hunt, this creates awful noise in both the bass and treble of the audio output. This applies regardless of the radio band, since the noise is in the demodulation technique rather than in the signal.
With AM demodulation, noise is usually less horrible because it is mostly in the lower part of the audio spectrum.
 

ramussons

Jun 10, 2014
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I don't think the frequency range has anything to do with the hiss from a fm demod with no input. Back in the late 70's there were cordless phones using fm on the 1700 KHz band.
Disable the squelch and see the result. I just confirmed it.
 
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