Hi all....
Listening to JT65 using WSJT-X....and was thinking what is the relevance of the db reading...?
I had a local station coming in at 20 over on the S-meter and the software was giving it a -1 db, the station being contacted showed nothing on my S meter and was given a -3 db on the software.......changing nothing, as in audio levels and such.....why that small of a difference..?
John
WSJT-X db readings
Re: WSJT-X db readings
You didn't mention in your post that the dB reading (yes, capital B because it stands for a man's name) is Signal to Noise Ratio, SNR in JT nomenclature, S/N also.VK2FAK wrote:Hi all....
Listening to JT65 using WSJT-X....and was thinking what is the relevance of the db reading...?
I had a local station coming in at 20 over on the S-meter and the software was giving it a -1 db, the station being contacted showed nothing on my S meter and was given a -3 db on the software.......changing nothing, as in audio levels and such.....why that small of a difference..?
John
So, it doesn't directly measure signal strength in absolute terms, it cannot do that when it is connected to the audio end of the receiver.
What you might not expect is that a very strong signal (20 over on the meter) has such poor SNR... in the first instance, I would look for overload of your PC sound card or the like. If that clips the audio signal, it will create lots of harmonic and IMD products, and there is your noise increase.
Owen
PS: I should mention here that JT software tends to give S/N wrt noise in 2500Hz bandwidth, though it will be typically measured in much smaller bandwidth chunks, averaged and extrapolated to 2500Hz.
Last edited by VK2OMD on Tue Mar 04, 2014 7:35 am, edited 1 time in total.
Re: WSJT-X db readings
Hi all..
Interesting point Owen.....I have tried everything I can to change the dB value of the strong signal but it remains at -1....can't be overloading the PC audio input now.....
even to the point now where I see no signals on the waterfall besides that one local station.....but am still getting plenty of decodes from other stations...some at levels of -4 and -7 that are not visible...
John
Interesting point Owen.....I have tried everything I can to change the dB value of the strong signal but it remains at -1....can't be overloading the PC audio input now.....
even to the point now where I see no signals on the waterfall besides that one local station.....but am still getting plenty of decodes from other stations...some at levels of -4 and -7 that are not visible...
John
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Re: WSJT-X db readings
You have an issue
I can tune into RMV locally and get 13dB
If I disconnect aerial and make rx noisy it drops
Possibly not by any dB ratio
Absolute values seem tp be -29 -28 dB
But if I see those I am using a IEE preamp and NF of less than 0.2db and gain
I cant hear a -29dB signal I can see it
I can hear a -1 db signal
Just
I have no idea what it means but its not directly related to s/s IMO
More SNR which depends on your setup.
I can certainly print out what I cant hear often
2014-03-03 03:02 VK3ZAZ 50.294484 -29 0 QF12ag +43 19.953 VK5KAA PF94iw 421 262
2014-03-03 02:24 VK3RMV 50.294470 +12 0 QF12ah +37 5.012 VK3ZAZ QF12ag 5 3
2014-03-03 01:48 VK3RMV 50.294466 +13 0 QF12ah +37 5.012 VK3ZAZ QF12ag 5 3
2014-03-02 21:56 VK3ZAZ 50.294490 -29 0 QF12ag +43 19.953 VK5KAA PF94iw 421 262
2014-03-02 21:08 VK3ZAZ 50.294485 -27 0 QF12ag +43 19.953 VK2KRR QF34mr
I can tune into RMV locally and get 13dB
If I disconnect aerial and make rx noisy it drops
Possibly not by any dB ratio
Absolute values seem tp be -29 -28 dB
But if I see those I am using a IEE preamp and NF of less than 0.2db and gain
I cant hear a -29dB signal I can see it
I can hear a -1 db signal
Just
I have no idea what it means but its not directly related to s/s IMO
More SNR which depends on your setup.
I can certainly print out what I cant hear often
2014-03-03 03:02 VK3ZAZ 50.294484 -29 0 QF12ag +43 19.953 VK5KAA PF94iw 421 262
2014-03-03 02:24 VK3RMV 50.294470 +12 0 QF12ah +37 5.012 VK3ZAZ QF12ag 5 3
2014-03-03 01:48 VK3RMV 50.294466 +13 0 QF12ah +37 5.012 VK3ZAZ QF12ag 5 3
2014-03-02 21:56 VK3ZAZ 50.294490 -29 0 QF12ag +43 19.953 VK5KAA PF94iw 421 262
2014-03-02 21:08 VK3ZAZ 50.294485 -27 0 QF12ag +43 19.953 VK2KRR QF34mr
Tread your own path
- VK3ZAZ
- Forum Diehard
- Posts: 785
- Joined: Sat Dec 18, 2010 1:46 pm
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Re: WSJT-X db readings
Signal levels reported by K1JT for each WSPR decode are the measured ratio of signal power to average noise power, scaled to a reference bandwidth of 2500 Hz. Within broad limits, this ratio is independent of the measured Rx noise level.
Tread your own path
Re: WSJT-X db readings
Hi all...
Maybe its my set-up.....but wsjt-x does not seem to show dB levels on PSK-reporter......or have I not clicked something...
John
Maybe its my set-up.....but wsjt-x does not seem to show dB levels on PSK-reporter......or have I not clicked something...
John
Re: WSJT-X db readings
The software is reacting to the audio fed into the soundcard. The audio level from the receiver on stronger signals is affected by the AGC threshold and characteristic of the individual receiver ... the AGC creates a a sort of 'compression' effect on strong signals.
(Maybe I didn't explain that very well, but perhaps someone will put it better ... I keep thinking 'audio dynamic range', but that might get misconstrued).
73,
(Maybe I didn't explain that very well, but perhaps someone will put it better ... I keep thinking 'audio dynamic range', but that might get misconstrued).
73,
Bob, ZL1RS in the Bay of Islands at RF64vs
Re: WSJT-X db readings
Hi all...
That is fair enough....but what happens when you reduce input by say turning right back the RF gain...
But it gets back to the point, if this applies to every radio, there is nothing at all consistent about the dB readings..
John
That is fair enough....but what happens when you reduce input by say turning right back the RF gain...
But it gets back to the point, if this applies to every radio, there is nothing at all consistent about the dB readings..
John
Re: WSJT-X db readings
It might help your understanding if you used something simple like Spectran to look at a steady carrier, and did things like turn the preamp on/off, swith the attenuator in/ou, adjust the RF gain by a little, but a lot and observe what happens to the difference in dB between the signal and the adjacent noise (grass) on Spectran... that difference is the S/N ratio. Try this for signals with high and with low S/N ratio... what is different?VK2FAK wrote:Hi all...
That is fair enough....but what happens when you reduce input by say turning right back the RF gain...
But it gets back to the point, if this applies to every radio, there is nothing at all consistent about the dB readings..
John
The S/N ratio you observe depends on signal strength you receive (antenna gain plays into that), and the background noise (your environment affects that a fair bit, but antenna efficiency also plays into it.
Does an inefficient antenna necessarily reduce S/N significantly?
Here is a little exercise to stimulate some grey cells.
http://owenduffy.net/blog/wp-content/up ... rab001.jpg
Above is a screen grab from W4HBK of me last Saturday. (Click on it for a larger image.)
What is the S/N ratio (use the red trace)?
The effective noise bandwidth of the spectrum buckets is 0.25Hz, what would the S/N ratio be if bandwidth was 500Hz?
If CW can be copied by ear with S/N down to about -6dB in 500Hz bandwidth, how much would the signal need to increase to be copyable by ear? Answers down the page.
Owen
What is the S/N ratio (use the red trace)? 15dB
The effective noise bandwidth of the spectrum buckets is 0.25Hz, what would the S/N ratio be if bandwidth was 500Hz? -18dB
If CW can be copied by ear with S/N down to about -6dB in 500Hz bandwidth, how much would the signal need to increase to be copyable by ear? +12dB, 16x (80W since the tx was 5W).
Last edited by VK2OMD on Wed Mar 05, 2014 7:18 am, edited 2 times in total.
Re: WSJT-X db readings
It is trick question... is the inefficient antenna at the tx or rx?VK2OMD wrote: Does an inefficient antenna necessarily reduce S/N significantly?
Receiver:
The answer is no . If you tune in a station on 80m using an efficient antenna, preamps on, RF Gain max, then insert the attenuator that most radios offer (somewhere 10-20dB), it probably doesn't degrade the S/N ratio enough to hear the difference, in fact it might even improve.
Try it! Hint: cover the S meter, you might be influenced by its readings... just concentrate on what you hear, readability.
So, this demonstrates that under some conditions, S/N might not be very sensitive to rx antenna efficiency, or receiver Noise FIgure or sensitivity.
Transmitter:
The answer is yes. An inefficient antenna will reduce the radiation power which reduces S in S/N.
Owen
PS: Efficiency is well defined in engineering as PowerOut/PowerIn, and an efficient antenna system is one where most of the RF energy captured by the antenna is delivered to the receiver, or most of the transmitter power is radiated... ie it has low losses.