Noise limitations in 1-30MHZ range?

[gbugh]

I understand that atmospheric and galactic noise limit how low of signal power can be recieved in the 1 to 30MHz range for AM and SSB but are those limits the same for FM? Isn't there much less FM atmospheric and galactic noise such that the noise within an LNA becomes the limiting factor for FM in the 1 to 30MHz ? I mean, would a reciever with a LO with less than -160db close in phase noise and an untra low noise LNA be able to extract a very weak very narrow band FM transmission even after it is bounced off the F layer? Something kind of like PSK31 but with FM modulation instead of SSB modulation?

 

[jiripolivka]

Noise is a random process, so it has no relation to signal modulation. Above 30 MHz receiver noise is important. Below 30 MHz, galactic noise strongly prevails above receiver noise. Thus it has not much sense to use low-noise preamplifiers as from the antenna, natural noise is >20 dB higher and cannot be reduced in any way.

There are special applications like NMR receivers which are using low-noise preamplifiers as they do not receive external signals.

Signal modulation modes are sensitive to signal-to-noise ratio, therefore a designer must consider system as well as natural contributions to noise. SNR can be improved by adjusting noise bandwidth to match signal spectrum. Especially with FM, input SNR affects the output SNR (after demodulator) in proportion to modulation frequency. Therefore preeemphasis and deemphasis is used to reduce the noise effect.

Your questions indicate you have not seen any textbook on communication technology. Please do. Try ARRL Radio Amateur's HAndbook, it presents all necessary answers.

 

[Darktrax]

I mean, would a reciever with a LO with less than -160db close in phase noise and an untra low noise LNA be able to extract a very weak very narrow band FM transmission even after it is bounced off the F layer? Something kind of like PSK31 but with FM modulation instead of SSB modulation?

If you listen to a receiver on SSB or AM, with the antenna disconnected, you hear noise. If you connect the antenna, and you hear an increase in noise, then the noise figure for the receiver is adequate for you to pick up the smallest signal in that noise. Down at 7MHz, a receiver with a NF as poor as 15dB will still let you hear everything.

FM or any other phase shifting mode demodulators are different. They boost the signal to limiting, and are insensitive to amplitude modulation. BUT, they do require a sufficient carrier-to-noise ratio, like maybe 12dB, so that the demodulator can lock to the phase of the signal. If you pull the antenna out on a FM signal, the demodulator output goes to maximum. The less signal, the more noise, until the demodulator is making maximum racket trying to follow random incoherent noise phases.

When you connect the antenna, and have a little signal, the FM demodulator output gets quieter! A strong carrier that has the demodulator following the phase without error give full quieting. Now any modulation will have a high quality. A competing signal on the same frequency has to get more than about 2.4dB greater to "capture" the demodulator and take over. Same signal has to weaken aout 2.4dB less than the other signal before it loses to the other signal. No matter what the modulation type, if it competes with noise, it gets degraded.

 

[gbugh]

I understand what you are saying, but with an antenna attached and running into an ultra low noise front end, much lower than the usual commercial equipment would have, then with the gain having gone way up trying to detect something, will I hear any atmospheric or galactic type noise before I hear internally generated 1/f noise or component heat generated noise?

Does atmospheric and galactic noise come through with as much power in the form of random FM or PM noise as it comes through as amplitude variation (AM) noise?

The thing I'm trying to understand is if I greatly reduce a front end's internal noise sources of FM and PM noise will I be able to detect external FM or PM signals over the atmospheric and galactic sources better than I could detect weak AM or SSB signals?

thanks,

George
AF5IE

 

[jiripolivka]

I understand what you are saying, but with an antenna attached and running into an ultra low noise front end, much lower than the usual commercial equipment would have, then with the gain having gone way up trying to detect something, will I hear any atmospheric or galactic type noise before I hear internally generated 1/f noise or component heat generated noise?

Does atmospheric and galactic noise come through with as much power in the form of random FM or PM noise as it comes through as amplitude variation (AM) noise?

The thing I'm trying to understand is if I greatly reduce a front end's internal noise sources of FM and PM noise will I be able to detect external FM or PM signals over the atmospheric and galactic sources better than I could detect weak AM or SSB signals?

thanks,

George
AF5IE

Natural noises are generally random, sometimes "colored". Communication systems with AM and SSB can operate with a low S/N ratio but FM systems exhibit a 16-20 dB threshold, below which the demodulator output is noise only.

Your suggestion to communicate through naturally noisy channels was realised after WWII in so-called tropo-scatter systems. Such systems used a high power telegraph (CW) transmission through the tropospheric layer. After traveling thousands of miles, a scattered power may make it to a receiver. Only slow CW transmission was possible.

FM is not suitable as the demodulator below threshold does not offer an output proportional to RF input power. Above threshold, there is again no proportional response but a "full" modulation.

- - - Updated - - -

BTW, all cosmic communication is carried as AM or SSB, now mostly as pulse trains. NO FM.

 

[gbugh]

OK, thanks very much to both of you. I get the picture now. I haven't done much with FM before. I'm always learning more.

 

[Darktrax]

I understand what you are saying, but with an antenna attached and running into an ultra low noise front end, much lower than the usual commercial equipment would have, then with the gain having gone way up trying to detect something, will I hear any atmospheric or galactic type noise before I hear internally generated 1/f noise or component heat generated noise?

Yes - the whole idea of using a low-noise front-end is so that the noise seen in the signal received will indeed be that galactic noise, instead of some greater noise coming from the amplifier. If you have a amplifier noise contribution lower than the natural noise coming into the input, then you cannot do better. Any other signal would have come through stronger, to rise above that noise floor, to be useable.

There is no point in investing in super low noise amplifiers and apply them to receive a noise floor that is so noisy that it delivers a random racket at a much higher level than the amplifier would have given anyway. One can rate the noise contribution from a amplifier by the equivalent noise temperature that a (fictitious) resistance would deliver if it were heated so.

Given (say) a physical temperature of 290K, and a amplifier NF = 2dB, the noise temperature of that amplifier would be 290*(10^(NF/10)-1) = 169.2K. At 400MHz, the incoming noise temperature is about 120K (figures from US Naval Laboratory publication). You can see that this NF is not good enough. You would need a NF of better than 1.5dB to be sure you were hearing the noise floor instead of noise generated by molecules knocking around at 290K in the first transistor.

At 1GHz, the galactic noise at 90° (looking straight up) is 23K, and at 10° above horizon, is about 30K, meaning 0.43dB. There are some LNA that do that, but others resort to cooling. Again, there is no point in spending on better than that, because that is the noise floor. A wanted signal just HAS to be above that to be received by any demodulator.

Be aware that at around 500MHz, the noise from lightning, made-made transmissions, interference from heaters, welders, switches, cable networks, takes over from sky noise as the main noise. It is not all fairly called "galactic".

At 30MHz, it is quieter than (say) 14MHz, and justifies a front-end noise figure of maybe 3 or 4 dB. It won't hurt to have less, but might be unneccesarily expensive.

Does atmospheric and galactic noise come through with as much power in the form of random FM or PM noise as it comes through as amplitude variation (AM) noise?

It is random in both phase shifts and amplitude with nothing to favour either.
One way to make communication possible is by reducing the bandwidth, and hence the data rate. The noise floor just drops away, but this is for both the incoming and the front end contributions. Signals must still be above the noise floor.

If the phase of the signal is not is not random relative to the receiver oscillator, as in coherent CW, there are ways to extract a signal that is even below the random noise floor. Human pattern recognition allows a type of analog fax called "Hellscreiber" to be read in the face of noise above the signal. Non-random frequency-hopping and pseudo-random noise signals masking spread-spectrum methods also can appear to have signals below the noise floor and can work a bit like coherent methods. Much can be just appearance. Artificially raising the noise with pseudo-random does not get around it having to be louder, or less random than the incoming galactic.

The thing I'm trying to understand is if I greatly reduce a front end's internal noise sources of FM and PM noise will I be able to detect external FM or PM signals over the atmospheric and galactic sources better than I could detect weak AM or SSB signals?

Generally no. It is unclear how one could greatly reduce the PM and FM features of random noise to give advantage over AM or SSB any more than that modulation method would have had anyway.

 

[gbugh]

"It is random in both phase shifts and amplitude with nothing to favour either." That is what I wasn't sure about. I understand it wouldn't do any good to make the front end noise figure really low if there is atmospheric or galactic noise anyway. But then I was thinking if the noise favored amplitude more than phase then weak FM might be possible with a very low noise front end. But since you confirm the atmospheric and galactic noise is just as much random phase as amplitude that shoots down the whole idea I had.

 

[Darktrax]

With FM, PM etc. there is the threshold dropout. Generally you need better S/N ratio than for AM to still keep it readable, because of the catastrophic way the demodulators will deliver maximum output following a error phase. There are tricks, called "threshold extensions", and some demods can still be delivering OK even approaching 3dB S/N, but it is not easy, and usually has error correction, redundant coding, and other aids.

In the case of FM, it can get loud and noisy with S/N below 4dB, and is not even "quality" at 6dB. If you have 12dB, you can expect full clear error free, or "hi-fi". So you can make SSB work, extended by human ability to decode the noisy signal, at levels where FM demods will become unusable. When you start using clever multi-phase PSK, the advantage may not be so clear. Either way, it is not something a better LNA can help with.