Waterfall display slightly offset from actual signal strength display


Srinivas Chennupaty
 

I have been happily SWLing on my S3. Noticed that there is a fairly consistent offset on the display waterfall on signal strength and the actual signal peak on the signal display. 
I have attached a screenshot which shows this. 
any suggestions? i don't have the GPS hooked up. is that the issue? 

BTW the receiver is so fantastically sensitive and my environment is so noisy, that I have had to use multiple front-end isolators in front of the receiver. 
but it is one superb piece of kit.. 
now if only i can find the filters in stock :-)

73,
KC7FSY


GLENN NUTSEY <glennnutsey@...>
 

Hi

It would seem that you are off centre by 15hz. 

Are you not just off tuning or do you have SAM running?

An Broadcast would be a frequency round up 6050, 6070, 9015 etc...

Thanks 

Glenn


On 22 May 2021, at 17:43, Srinivas Chennupaty via groups.io <chennu@...> wrote:

I have been happily SWLing on my S3. Noticed that there is a fairly consistent offset on the display waterfall on signal strength and the actual signal peak on the signal display. 
I have attached a screenshot which shows this. 
any suggestions? i don't have the GPS hooked up. is that the issue? 

BTW the receiver is so fantastically sensitive and my environment is so noisy, that I have had to use multiple front-end isolators in front of the receiver. 
but it is one superb piece of kit.. 
now if only i can find the filters in stock :-)

73,
KC7FSY
<Screenshot 2021-05-22 093902.png>


GLENN NUTSEY <glennnutsey@...>
 




On 22 May 2021, at 18:27, Glenn BT <glennnutsey@...> wrote:

Hi

It would seem that you are off centre by 15hz. 

Are you not just off tuning or do you have SAM running?

An Broadcast would be a frequency round up 6050, 6070, 9015 etc...

Thanks 

Glenn


On 22 May 2021, at 17:43, Srinivas Chennupaty via groups.io <chennu@...> wrote:

I have been happily SWLing on my S3. Noticed that there is a fairly consistent offset on the display waterfall on signal strength and the actual signal peak on the signal display. 
I have attached a screenshot which shows this. 
any suggestions? i don't have the GPS hooked up. is that the issue? 

BTW the receiver is so fantastically sensitive and my environment is so noisy, that I have had to use multiple front-end isolators in front of the receiver. 
but it is one superb piece of kit.. 
now if only i can find the filters in stock :-)

73,
KC7FSY
<Screenshot 2021-05-22 093902.png>


GLENN NUTSEY <glennnutsey@...>
 

It would also seem that you are not receiving the full AM signal with conditions. 

Manually tune and it should be spot on with AM and the radio will track the signal therefore move the received frequency in SAM. 

Hope I’m not telling  you something you already know. 

Glenn. 


On 22 May 2021, at 17:43, Srinivas Chennupaty via groups.io <chennu@...> wrote:

I have been happily SWLing on my S3. Noticed that there is a fairly consistent offset on the display waterfall on signal strength and the actual signal peak on the signal display. 
I have attached a screenshot which shows this. 
any suggestions? i don't have the GPS hooked up. is that the issue? 

BTW the receiver is so fantastically sensitive and my environment is so noisy, that I have had to use multiple front-end isolators in front of the receiver. 
but it is one superb piece of kit.. 
now if only i can find the filters in stock :-)

73,
KC7FSY
<Screenshot 2021-05-22 093902.png>


Ron Hunsicker
 

Srinivas:  I see something similar with my S3. 

So we are not comparing apples and oranges:  I have the OCXO version and it runs 24/7, so it should have stabilized.  Also, I have a GNSS antenna attached and am advised that I have a lock. 

I just tuned to WWV on 10 MHz.  When I expand the main waterfall, the red arrow head is centered to the right of the carrier (higher in frequency) by something less than 10 Hz, but more than 5 Hz.  When I expand the IF waterfall, it appears similarly offset to the right.

I think that this is an interesting observation rather than an annoying or concerning one.  I have to keep reminding myself that the S3 is a radio, not a laboratory instrument.  Consider that, at 10 MHz, 7 Hertz is 7 parts in 10 million; less than a part per million.  (By all accounts, less than 1 part per million of almost anything will not cause cancer.)

Remember slide rule dials?  Remember tuning with a piece of white cardboard with small marks on it noting the known (We hoped!) locations of BBC, Radio Moscow, HCJYB, and VOA stations and interpolating between these marks?  The mark was probably 5 KHz wide, so, at 10 MHz, 5 parts in 10,000; 1 part in 2000. 

And we were happy.  Modern radios have spoiled me!


Ron Hunsicker


Neil Smith G4DBN
 

Hi Ron, how wide are the FFT bins you are using?   On the DUO at max zoom on the main waterfall, with a bin size of 11.7 Hz, the peak is around 6Hz off.  On the IF windows at 2.9 Hz bins, it is around 1.5 Hz off. The reason is just that the peak for my 10.00000000MHz signal is "somewhere" in a bin that happens to be from (perhaps) 10000000 Hz to 10000011.7 Hz instead of 9999994.65 to 10000005.35 Hz, when it would appear to peak at 10.000000 exactly.

IF window, showing peak at about 1.5 Hz HF of the 10.000600 Hz point (I'm using CW offset of 600 Hz). Sometimes, you'll find the peak is in the middle of a bin, but on average it will be around 6Hz off from the true frequency simply because that's how the maths work. (or "how the math works" for those who use the singular shortened form of "mathematics").

You should be able to calculate the bin edges and make any necessary adjustments.  However, if you really want to know the exact frequency, and your radio is locked to a precision 10 MHz reference, you can use Spectrum Lab on a long integration FFT (262144 bins perhaps) to check your PC sourd card clock calibration against a precise tone generated from your reference, then use Speclab to look at the actual frequency:

That is monitoring drift of an OCXO at 1kHz offset and 512k bins in Speclab. The DUO shows it at 1006 Hz offset approximately.

Neil G4DBN

On 23/05/2021 13:09, Ron Hunsicker via groups.io wrote:
Srinivas:  I see something similar with my S3. 

So we are not comparing apples and oranges:  I have the OCXO version and it runs 24/7, so it should have stabilized.  Also, I have a GNSS antenna attached and am advised that I have a lock. 

I just tuned to WWV on 10 MHz.  When I expand the main waterfall, the red arrow head is centered to the right of the carrier (higher in frequency) by something less than 10 Hz, but more than 5 Hz.  When I expand the IF waterfall, it appears similarly offset to the right.

I think that this is an interesting observation rather than an annoying or concerning one.  I have to keep reminding myself that the S3 is a radio, not a laboratory instrument.  Consider that, at 10 MHz, 7 Hertz is 7 parts in 10 million; less than a part per million.  (By all accounts, less than 1 part per million of almost anything will not cause cancer.)

Remember slide rule dials?  Remember tuning with a piece of white cardboard with small marks on it noting the known (We hoped!) locations of BBC, Radio Moscow, HCJYB, and VOA stations and interpolating between these marks?  The mark was probably 5 KHz wide, so, at 10 MHz, 5 parts in 10,000; 1 part in 2000. 

And we were happy.  Modern radios have spoiled me!


Ron Hunsicker


Ron Hunsicker
 

Neil, thanks, an explanation that I found interesting--"Oh, that's why it does that!"

For what I do, other than looking odd at high resolution, the offset creates no difficulty.  In AM mode, it really does not matter and in sideband I tune by ear.  (What's the likelihood that my 7 parts in 10 million is more accurate than the frequency control of their machine?)

If you are willing to continue to be helpful, what is an "FFT bin?"  The vertical lines in the graphical part of the IF display?  Something else?


Ron

p.s., I'm a "math, science, and engineering guy," not a "maths, science, and engineering guy!"


Neil Smith G4DBN
 

On my main waterfall screen, I have this, showing 11.7 Hz per point (what I called FFT bin size):

and on the IF screen on my SW2, is says 2.9 Hz/point

The fast fourier transform calculation takes the full bandwidth of the digital stream coming from the FPGA front end and makes a calculation of the amount of energy in a specific frequency range (the "bin size" I mentioned) over a specified time period.  The math(s) behind it is one of the wonders of the world. There are subtleties around the overlap with adjacent bins.

Usingf MATLAB or Octave, you can play with the parameters, but a simple example is if you generate a couple of tones at 50 Hz and 120 Hz, then add noise, like this:

Fs = 1000;            % Sampling frequency                   
T = 1/Fs;             % Sampling period      
L = 1500;             % Length of signal
t = (0:L-1)*T;        % Time vector
S = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t);
X = S + 2*randn(size(t));
plot(1000*t(1:50),X(1:50))
title('Signal Corrupted with Zero-Mean Random Noise')
xlabel('t (milliseconds)')
ylabel('X(t)')

you get a signal like this:

and it is hard to see the two frequencies from that time-domain signal.

However, if you run a fast fourier transform on the signal at the sampling rate, you can calculate the amount of signal in each increment of frequency:

Y = fft(X);

P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);

f = Fs*(0:(L/2))/L;
plot(f,P1)
title('Single-Sided Amplitude Spectrum of X(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')

and you end up with a spectrum that you can use to create the intensities of the display on one line of a waterfall.  The size of each bin depends on how fast you are taking the samples.  The faster you sample, the wider each bin becomes.

The whole subject area is fascinating, and the math(s) gets intricate, although the solutions in sotware have probably had more academic effort expended on improving the speed and efficiency of processing than almost any other set of algorithms.

I'm sure someone who knows this stuff better could offer a more cogent explanation.  There is a nice fun read at https://www.analog.com/media/en/training-seminars/design-handbooks/Software-Defined-Radio-for-Engineers-2018/SDR4Engineers_CH02.pdf if you have a spare couple of hours!

Neil G4DBN


On 23/05/2021 16:41, Ron Hunsicker via groups.io wrote:
Neil, thanks, an explanation that I found interesting--"Oh, that's why it does that!"

For what I do, other than looking odd at high resolution, the offset creates no difficulty.  In AM mode, it really does not matter and in sideband I tune by ear.  (What's the likelihood that my 7 parts in 10 million is more accurate than the frequency control of their machine?)

If you are willing to continue to be helpful, what is an "FFT bin?"  The vertical lines in the graphical part of the IF display?  Something else?


Ron

p.s., I'm a "math, science, and engineering guy," not a "maths, science, and engineering guy!"
-- 
Neil
http://g4dbn.uk


Srinivas Chennupaty
 

Ron _ Fascinating explanation! Thank you!! the technology outstrips my capabiilty :-) 

On Sunday, May 23, 2021, 06:02:53 AM PDT, Neil Smith G4DBN <neil@...> wrote:


Hi Ron, how wide are the FFT bins you are using?   On the DUO at max zoom on the main waterfall, with a bin size of 11.7 Hz, the peak is around 6Hz off.  On the IF windows at 2.9 Hz bins, it is around 1.5 Hz off. The reason is just that the peak for my 10.00000000MHz signal is "somewhere" in a bin that happens to be from (perhaps) 10000000 Hz to 10000011.7 Hz instead of 9999994.65 to 10000005.35 Hz, when it would appear to peak at 10.000000 exactly.

IF window, showing peak at about 1.5 Hz HF of the 10.000600 Hz point (I'm using CW offset of 600 Hz). Sometimes, you'll find the peak is in the middle of a bin, but on average it will be around 6Hz off from the true frequency simply because that's how the maths work. (or "how the math works" for those who use the singular shortened form of "mathematics").

You should be able to calculate the bin edges and make any necessary adjustments.  However, if you really want to know the exact frequency, and your radio is locked to a precision 10 MHz reference, you can use Spectrum Lab on a long integration FFT (262144 bins perhaps) to check your PC sourd card clock calibration against a precise tone generated from your reference, then use Speclab to look at the actual frequency:

That is monitoring drift of an OCXO at 1kHz offset and 512k bins in Speclab. The DUO shows it at 1006 Hz offset approximately.

Neil G4DBN

On 23/05/2021 13:09, Ron Hunsicker via groups.io wrote:
Srinivas:  I see something similar with my S3. 

So we are not comparing apples and oranges:  I have the OCXO version and it runs 24/7, so it should have stabilized.  Also, I have a GNSS antenna attached and am advised that I have a lock. 

I just tuned to WWV on 10 MHz.  When I expand the main waterfall, the red arrow head is centered to the right of the carrier (higher in frequency) by something less than 10 Hz, but more than 5 Hz.  When I expand the IF waterfall, it appears similarly offset to the right.

I think that this is an interesting observation rather than an annoying or concerning one.  I have to keep reminding myself that the S3 is a radio, not a laboratory instrument.  Consider that, at 10 MHz, 7 Hertz is 7 parts in 10 million; less than a part per million.  (By all accounts, less than 1 part per million of almost anything will not cause cancer.)

Remember slide rule dials?  Remember tuning with a piece of white cardboard with small marks on it noting the known (We hoped!) locations of BBC, Radio Moscow, HCJYB, and VOA stations and interpolating between these marks?  The mark was probably 5 KHz wide, so, at 10 MHz, 5 parts in 10,000; 1 part in 2000. 

And we were happy.  Modern radios have spoiled me!


Ron Hunsicker


Paul Jones
 

Sam,

It just looks like you aren’t tuned correctly, set the step to 1khz, that will eliminate the .157 off your signal then your should be spot on, even without the gos hooked up the radio is very accurate. It’s just a tuning issue I believe 

Paul EladUSA 


On May 23, 2021, at 09:02, Neil Smith G4DBN <neil@...> wrote:



Hi Ron, how wide are the FFT bins you are using?   On the DUO at max zoom on the main waterfall, with a bin size of 11.7 Hz, the peak is around 6Hz off.  On the IF windows at 2.9 Hz bins, it is around 1.5 Hz off. The reason is just that the peak for my 10.00000000MHz signal is "somewhere" in a bin that happens to be from (perhaps) 10000000 Hz to 10000011.7 Hz instead of 9999994.65 to 10000005.35 Hz, when it would appear to peak at 10.000000 exactly.

<dbjfibjhoallilci.png>

IF window, showing peak at about 1.5 Hz HF of the 10.000600 Hz point (I'm using CW offset of 600 Hz). Sometimes, you'll find the peak is in the middle of a bin, but on average it will be around 6Hz off from the true frequency simply because that's how the maths work. (or "how the math works" for those who use the singular shortened form of "mathematics").

You should be able to calculate the bin edges and make any necessary adjustments.  However, if you really want to know the exact frequency, and your radio is locked to a precision 10 MHz reference, you can use Spectrum Lab on a long integration FFT (262144 bins perhaps) to check your PC sourd card clock calibration against a precise tone generated from your reference, then use Speclab to look at the actual frequency:

<mkclmmnkdciemikc.png>

That is monitoring drift of an OCXO at 1kHz offset and 512k bins in Speclab. The DUO shows it at 1006 Hz offset approximately.

Neil G4DBN

On 23/05/2021 13:09, Ron Hunsicker via groups.io wrote:
Srinivas:  I see something similar with my S3. 

So we are not comparing apples and oranges:  I have the OCXO version and it runs 24/7, so it should have stabilized.  Also, I have a GNSS antenna attached and am advised that I have a lock. 

I just tuned to WWV on 10 MHz.  When I expand the main waterfall, the red arrow head is centered to the right of the carrier (higher in frequency) by something less than 10 Hz, but more than 5 Hz.  When I expand the IF waterfall, it appears similarly offset to the right.

I think that this is an interesting observation rather than an annoying or concerning one.  I have to keep reminding myself that the S3 is a radio, not a laboratory instrument.  Consider that, at 10 MHz, 7 Hertz is 7 parts in 10 million; less than a part per million.  (By all accounts, less than 1 part per million of almost anything will not cause cancer.)

Remember slide rule dials?  Remember tuning with a piece of white cardboard with small marks on it noting the known (We hoped!) locations of BBC, Radio Moscow, HCJYB, and VOA stations and interpolating between these marks?  The mark was probably 5 KHz wide, so, at 10 MHz, 5 parts in 10,000; 1 part in 2000. 

And we were happy.  Modern radios have spoiled me!


Ron Hunsicker