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Author Topic: What kind of CW signal would 5 db out of noise be  (Read 3476 times)
K1LEM
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« on: July 06, 2017, 09:14:27 AM »

Recent computer technologies have revolutionized propagation analysis. The world wide skimmers use software to detect CQ de Call sign and then broadcast their detection (skim) to the web with a db out of noise. This allows Dxers to determine if a propagation path is open.
Generally narrow bandwidth allows CW to rank superior in weak or nose situations.
For example a few weeks ago 80 meters from here was open to Brisbane Aus. . My vertical was making the signal swing the 9700 mile trip. So, I wonder what would a 5 db out of noise cw signal sound like? What S reading? weak, fair or fairly good.


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KH6AQ
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« Reply #1 on: July 06, 2017, 09:33:21 AM »

I think the reverse beacon skimmers report SNR in a 2500 Hz noise bandwidth.
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KC4ZGP
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« Reply #2 on: July 06, 2017, 09:53:11 AM »

A 5dB signal above noise would be 3.1622 times louder than noise.

There you go.

I'm on 20 meters phone at 2200UTC. Won't you join me?

Kraus
« Last Edit: July 06, 2017, 10:04:48 AM by KC4ZGP » Logged
PA0WV
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« Reply #3 on: July 06, 2017, 11:06:43 AM »

K1LEM

S9 is mostly defined as 75 microvolt on your 50 ohm antenna input from your receiver. (sometimes 50 microvolt)
S8 is half of that, each S point lower is half voltage of the previous S-point.

That says nothing about the noise. When there is no signal at all you can already have a man made noise level of S8 or higher.

So it is perfectly possible that you receive in a natural surrounding a guy, and he does not detect your signal due to his local noiselevel. Supposed condition path up and path down with equal path damping and both same power in the antenna.

Signal to noise ratio can be defined as the PEP of CW, that is your transmitter output power at key down resulting at the  antenna input of his receiver divided bij the average noise power of the used bandwidth on his rx input or catculated back to the antennainput when the receiver noise is not neglectable.

The average noise power is linear proportional with the bandwidth you listen. 2,5 kHz when in SSB mode of your receiver and small may be 150 Hz  (you really need roughly just somewhat more bandwidth then 4 times the used speed in wpm)

On that bandwidth of 2,5 kHz I can detect minus 5 dB S/N, I have to check that, it is an unchecked result right now.
When you narrow your bandwidth to 150 Hz the average noise power lowers abt 12 dB (2 S points) with the same PEP of the CW.

However in that case you can NOT detect the same weak CW, because the noise has the same pitch as the CW. (preliminary result)

The gain of 12 dB in signal to noise ratio is useless (except for cutting QRM of course) That is because you with your mind are cutting the most of the noise in the CW band. You listen obviously selective.

« Last Edit: July 06, 2017, 11:18:00 AM by PA0WV » Logged

K1LEM
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« Reply #4 on: July 06, 2017, 12:08:57 PM »



  The formula is if I recall signal to noise ratio   S+N/N log 20 = assuming volts are the units used
Space communications EME etc can be heard very well with CW in a narrow pass band. Honest 6 Db S meters that are logarithmic are not there except in lab instruments.

But, you can put attenuation in a HF signal (remove all pre-amps and yes agc)  Then crank in db until you no longer hear that signal. In a practical way, that the effective amount signal is out of noise. Now, I know skilled DSP ears in the brain can have perfect copy with signals that would seem below the noise. So its a tricky question at best
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PA0KDW
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« Reply #5 on: July 07, 2017, 01:33:33 AM »



  The formula is if I recall signal to noise ratio   S+N/N log 20 = assuming volts are the units used


K1LEM I have seen that formula earlier , instead of  10 log (S/N) ( S and N power),
or as you said 20 log of the voltage ratio, but perhaps you can explain me:

When the noise is zero the signal to noise ratio is infinite dB, when the signal is zero the signal to noise ratio is minus infinite in the 10 log (S/N) formula,

but in your formula 10 log{(S+N)/N}
(powerratio) with no signal the signal to noiseratio is 0 dB ; can't become negative.
That is contrary with a signal of minus 5 dB because the CW power is less than the noise .

Quote
  Honest 6 Db S meters that are logarithmic are not there except in lab instruments.

Do you think this statement is true in the case of software defined ham-radio, where the antennasignal is direct from antenne or after a digital multiplier sampled in quadrature over a wide bandwidth with a large dynamic range?

Frans
« Last Edit: July 07, 2017, 01:43:00 AM by PA0KDW » Logged
K1LEM
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« Reply #6 on: July 07, 2017, 04:49:19 AM »



  The formula is if I recall signal to noise ratio   S+N/N log 20 = assuming volts are the units used


K1LEM I have seen that formula earlier , instead of  10 log (S/N) ( S and N power),
or as you said 20 log of the voltage ratio, but perhaps you can explain me:

When the noise is zero the signal to noise ratio is infinite dB, when the signal is zero the signal to noise ratio is minus infinite in the 10 log (S/N) formula,

but in your formula 10 log{(S+N)/N}
(powerratio) with no signal the signal to noiseratio is 0 dB ; can't become negative.
That is contrary with a signal of minus 5 dB because the CW power is less than the noise .

Quote
  Honest 6 Db S meters that are logarithmic are not there except in lab instruments.

Do you think this statement is true in the case of software defined ham-radio, where the antennasignal is direct from antenne or after a digital multiplier sampled in quadrature over a wide bandwidth with a large dynamic range?

Frans

The use of 20 vs 10 log works in any valid equation. The 20 used for voltage or current, 10 in watts. Common forms are dbm which just says db below 1 milliwatt , and.. another common measure is db above or below some standard. FCC uses db microvolt.. they consider a local coverage grade A at 60 dbm.
So we hope that helps? As for the ration Blowing Up with a zero denominator, use sense here if there were Zero noise N=0 of course the equation blows up, but you never have a noise floor of Zero. Even cosmic and thermal noise, shot noise etc are above ZERO.
Hope this helps YOU Wink
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PA0KDW
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« Reply #7 on: July 07, 2017, 10:24:29 AM »



  Honest 6 Db S meters that are logarithmic are not there except in lab instruments.
Quote
Do you think this statement is true in the case of software defined ham-radio, where the antennasignal is direct from antenna or after a digital multiplier sampled in quadrature over a wide bandwidth with a large dynamic range?

Frans


Thanks very much for your fast reply.

Aw that gives me a problem, I am afraid I do not understand the subject completely, perhaps you can clarify my problem:

 Suppose I have an amplifier with a gain of 10 dB when it is loaded with 50 ohm. That is 10 times in power because 10 log 10 = 10 dB and it is 3.16 times in voltage. because 20 log 3.16 = 10 dB, right?

The input resistance of the amplifier is 1000 ohm and the output  is loaded with 50 ohm.

When I put 1 volt RMS on the input the input power is 1 mW so with 10 dB gain the output must be 10 mW

However 10 mW over 50 ohm is 0.707 volt

So 1 volt in the amplifier and 0,707 volt out gives a gain of 20 log (0.707/1)= -3 dB

So the gain is
10 dB according to 10 log(powerratio) and the gain is - 3dB according to 20 log (voltage ratio) ?
according to your cited text :
Quote
The use of 20 vs 10 log works in any valid equation. The 20 used for voltage or current, 10 in watts.

Quote
As for the ration Blowing Up with a zero denominator, use sense here if there were Zero noise N=0 of course the equation blows up, but you never have a noise floor of Zero. Even cosmic and thermal noise, shot noise etc are above ZERO.
Hope this helps YOU Wink

Yes many thanks, but still there is the problem that when you have a signal power S that is half of the noise power N according to the formula 10 log (S/N) the signal to noise ratio is - 3dB and according to the formula 10 log {(S+N)/N}
the signal to noise ratio is 1.8 dB
And the particular fact that I tried to emphasize was that with no or hardly any  signal present according to one formula the S/N ratio is minus a lot of dB's and according to the other 10 log {(S+N)/N} formula always nearly 0 dB. Negative signal to noise ratio's expressed in dB are not possible.

However I sometimes read signal to noise ratio minus 6 dB (in the case signal power is 1/4 of the noisepower).

So I am still somewhat puzzled after your very understandable and practical explanation,

Hope you will give answer on both questions hence also about the S-points in SDR ham radio., because I very much appreciate  taking profit of your acquired knowledge.

Frans
« Last Edit: July 07, 2017, 10:42:53 AM by PA0KDW » Logged
K1LEM
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« Reply #8 on: July 07, 2017, 10:35:13 AM »

 Stop being such an ass. You really don't know much because people of actual learning don't behave like you. Sorry, end of discussion. Sad
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PA0KDW
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« Reply #9 on: July 07, 2017, 10:49:53 AM »

Stop being such an ass. You really don't know much because people of actual learning don't behave like you. Sorry, end of discussion. Sad

? ?  ?
Frans
« Last Edit: July 07, 2017, 10:52:06 AM by PA0KDW » Logged
VK5EEE
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« Reply #10 on: July 10, 2017, 05:52:24 AM »

Wow, reading this thread, I can only assume something is very wrong with K1LEM! Asking a question (out of boredom? already knowing the answer?!) but when someone so kindly gives a lengthy explanation, comes back with his OWN answer to his question, so why ask it in the first place? And then when the other kind guy enters a discussion that is a bit over most of our heads, he is insulted? What are you trying to prove K1LEM? That you are a smart arse? Or clever? Or a time waster? Or an obnoxious character? Or a psychopath? Or all of the above?  Roll Eyes Huh Angry
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Long Live Real Human CW and wishing you many happy CW QSO - 77 - CW Forever
VK5EEE
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« Reply #11 on: July 10, 2017, 05:56:46 AM »

Well looking at recent history of posts you made, it looks like you may be going through something... if so, I hope you soon recover or feel better but let's try not to be so rude, condescending and arrogant to those who truly don't deserve it!
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Long Live Real Human CW and wishing you many happy CW QSO - 77 - CW Forever
KC4ZGP
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« Reply #12 on: July 10, 2017, 07:14:42 AM »


Hey VK5EEE,

Maybe we should ban him from the hardware store.

Oh boy.

Kraus, EB-104 ontwapernwner
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PA0WV
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« Reply #13 on: July 10, 2017, 10:40:18 AM »

Well looking at recent history of posts you made, it looks like you may be going through something... if so, I hope you soon recover or feel better but let's try not to be so rude, condescending and arrogant to those who truly don't deserve it!

Well Lou,
I suppose when I read his texts in this thread, K1LEM is a technician  and studying for general.
So let we help him.

His question was: how does CW sound when the signal to noise ratio is 5 dB.

I made a sample signal for him. Noise flat between 300 Hz till 2800 Hz, Tone of CW 712 Hz. S/N = 5 dB. The .wav file is to be found on

http://pa0wv.home.xs4all.nl/K1LEM/EHAM.WAV

73
« Last Edit: July 10, 2017, 10:48:27 AM by PA0WV » Logged

NI0C
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« Reply #14 on: July 10, 2017, 04:05:02 PM »

Wim,
Nice work on the 5 dB file!  A few years back, AG1LE put up some files of random code groups at various signal to noise ratios down to at least -10 db, as I recall. 
73 de Chuck  NI0C
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