Signal to Noise

[Space Varmint]

Hi, I would like to have a discussion about this subject, signal-to-noise. I got some questions and I would like to report some of my findings as well that may be informative to others. Some of it may be speculative because I am using crude equipment to the best of my ability. Here is what I think:

I got a 40 watt transistor that I am using as a transmitter final amplifier into the antenna. I have had as much as 25 watts output but my signal to noise ratio went up to 2-to-1 ratio or 1 to 2:1 commonly refered to as 2:1 ratio.

Unfortunately I was probing around in the mixer section and made some changes in the filtering and I cannot achieve the same results now. That's OK I'm not done yet. My output signal is derived from two different frequencies combined in a mixer. So from there I am amplifying and filtering all the way to the output.

So my questions and apparent observations are this. I believe that spectral purity is paramount and directly proportional to the amount of signal that will be received by the antenna. Does this sound right? What I have noticed is that some frequency components such as parasitics and harmonic energy will be reflected by the antenna impedance. This can be seen as spurious radiation being kicked back onto the scope probe when reaching the maximum acceptable output.

In building power amps, I have more experience with class C type power amps where allot of the signal can actually be produced by the amplifier itself, triggered by an input signal, and so spectral purity is not that hard to achieve because we are not interested in bringing forth linear modulation intelligence from lower stages where in class A operation we wish to preserve and amplify all necessary intelligence which can bring forth other undesired parasitics.

Your thoughts and inputs are greatly appreciated.

Thanks, Frank

 

[Nigel Goodwin]

Signal to noise is really a function of a receiver, NOT a transmitter - if your tranmitter is that poor perhaps you shouldn't be using it?.

 

[Space Varmint]

Signal to noise is really a function of a receiver, NOT a transmitter - if your tranmitter is that poor perhaps you shouldn't be using it?.

Nigel...lol. I'm going to try to be polite. Don't let me catch you on the street, karate or no karate...lol.

Is that why we all have the all important Signal-to-Noise/Power Meter sitting our our desks with our ham stations?

 

[crutschow]

Is that not a SWR/Power Meter?

 

[Space Varmint]

Is that not a SWR/Power Meter?

Granted it's not SINAD but really that is part of the question. Are parasitics being a different frequency sent back do to impedance mismatch. I hope you are not agreeing with Nigel that signal to noise is not a function of a transmitter? For one thing, if that were true the FCC would have nothing to do.

There is much more to it than that. A spectrally pure note will have all of it's power focused on the frequency in question and therefore will travel much further. Certainly noise will effect SWR but what is a good way to detect it with minimal tools such as a scope,SWR meter & freq. counter?

 

[audioguru]

If your power output is not a pure sine-wave then it is distorted with harmonics. The harmonics cause interference at their frequencies.
A class-C amplifier is very distorted and needs a good lowpass filter at its output to attenuate its harmonics. A class-A amplifier does not have much distortion so its harmonics are low.

If the transmitter's output impedance does not match the antenna's impedance then some of the signal will be reflected off the antenna and will return to the transmitter causing a standing wave.

 

[Mikebits]

Granted it's not SINAD but really that is part of the question. Are parasitics being a different frequency sent back do to impedance mismatch. I hope you are not agreeing with Nigel that signal to noise is not a function of a transmitter? For one thing, if that were true the FCC would have nothing to do.

There is much more to it than that. A spectrally pure note will have all of it's power focused on the frequency in question and therefore will travel much further. Certainly noise will effect SWR but what is a good way to detect it with minimal tools such as a scope,SWR meter & freq. counter?

I have never heard a SNR number like 2:1, no idea what that means.
I think your confusing spectral purity with Signal to noise ratio. This measurement is more useful to specify a receivers sensitivity over a specified bandwidth. See article link below:
**broken link removed**

 

[flat5]

"I have had as much as 25 watts output but my signal to noise ratio went up to 2-to-1 ratio or 1 to 2:1 commonly refered to as 2:1 ratio."

I don't understand your terminology. Noise? Are you saying you have harmonic distortion that is half of your output power?
or half of your audio signal is noise?

As Nigel said (sorta) s/n is a measure of received signal audio/background audio. SINAD is further defined as you know from pager and hand held repair work.

Transmitters do have an audio noise floor but the FCC does not care about that (on the ham bands). They are concerned with RF noise only and I don't understand what you mean about 2:1. 2:1 SWR I understand but you don't mean that. Right?

 

[Space Varmint]

Signal to noise applies to anything where there is a desired frequency. It could apply to motor controls. Signal to noise. Let me break this down for you. I am after a desired frequency. Any other frequency or spurious response etc to include audio hum....is noise.

Now do tell me that spectral purity has nothing to do with transmitter efficiency. Gee whiz.

My question was supposed to be, since the frequency of the antenna is cut to resonate at a specific frequency, do you think it is possible that parasitics, whatever they might be would be reflected because they do not match the impeadance of the antenna & transmission line? Might I see this as spurious radiation?

Come on radio experts out there.

 

[Space Varmint]

Signal to noise applies to anything where there is a desired frequency. It could apply to motor controls. Signal to noise. Let me break this down for you. I am after a desired frequency. Any other frequency or spurious response etc to include audio hum....is noise.

Now do tell me that spectral purity has nothing to do with transmitter efficiency. Gee whiz.

My question was supposed to be, since the frequency of the antenna is cut to resonate at a specific frequency, do you think it is possible that parasitics, whatever they might be would be reflected because they do not match the impeadance of the antenna & transmission line? Might I see this as spurious radiation?

Come on radio experts out there.

 

[audioguru]

Your transmitter is supposed to transmit a pure RF sine-wave that has no distortion (no harmonics and no parasitics).
If your transmitter has harmonics that cause radio and TV interference then the FCC radio cops will shoot you and chop off your head.

 

[Space Varmint]

You know what? I may have resolved this. It looks like it might be my output transformer. It was an act of congress to change it, but I did and the reflected energy does have a different nature to it. I thought I could correct the impedance mis-match in the output network but apparently not enough anyway. I think I'm moving in the right direction.

Thanks guys.

Oh, and Nigel..? No hard feelings. I'm just joshing with ya. You know that. You take your pot shots at me all the time.

 

[Space Varmint]

OK, I saw it! It was from the modulator carrier. It was 4 MHz super imposed on the output. I used a high pass filter to remove it. Looks nice

 

[Space Varmint]

Yeah, I got 20 watts out with the carrier mixed in. When I clean it off I only got 5 watts but it is pure and pretty 7MHz.

The SWR with 5 to 6 watts pure 7MHz output is 1.1 to 1.

 

[Space Varmint]

A question for you intellectuals out there.....

Does spectral purity have anything to do with "Singnal to Noise"?

Just curious as to the type of answers I will get.

 

[Nigel Goodwin]

Yeah, I got 20 watts out with the carrier mixed in. When I clean it off I only got 5 watts but it is pure and pretty 7MHz.

The SWR with 5 to 6 watts pure 7MHz output is 1.1 to 1.

As already mentioned, SWR is nothing to do with signal to noise, it's just a measure of how well your aerial is matched.

 

[Mikebits]

When discussing transmitters, forget about signal to noise. This ratio is usually discussed as a reference point of the ambient noise + thermal noise of a system within a givien bandwidth. Typical noise floor of a system is like -127 dBm so a 10 dB signal above this noise floor is significant. Now when you compare a -127 dBm noise floor to a 4 watt carrier signal then this number become insignificant.

Transmitter specs are not discussed in terms of Signal to noise, rather intermodulation products and spurious output.

Transmitters should not have any spurious output, nor should they contain IMD products. This discussion is silly in terms of the terminology that you have chosen to use as it does not apply to transmitters. The noise you are talking about is noise that your design has created, this is not what signal to noise refers to.

Also, your antenna match should not be designed to match spurious response, as there should be no spurs to start with.

If you really want to test your system performance then I recommend that you read up on two tone testing. This can be done with simple equipment.

 

[audioguru]

A question for you intellectuals out there.....

Does spectral purity have anything to do with "Singnal to Noise"?

Just curious as to the type of answers I will get.

No.
Spectral purity is very low harmonics because the distortion is low.
Noise in an electronic system with a fairly high output power is so low that it is difficult to measure.

 

[Space Varmint]

No takers?

Excuse me for being rude but I think I need to thump a few heads.

For you geniuses out there. Noise is any undesired component and it applies to all electronics.....period. Signal to noise is only prevalent in receivers because it it is so critical to it's performance. I have NEWS!! It is crucial in any system you wish to put together. What you want to bet, I can take your printer design, and introduce a small frequency at numerous locations and F the Mo...up! Get it?

 

[flat5]

Noise is a very general term. Science does not like general terms.
Use correct terms (words) to discuss the specific type of noise you want to talk about.