HF weak signal reception in presence of noise

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glovisol
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HF weak signal reception in presence of noise

Post by glovisol » Sat Sep 01, 2018 6:26 pm

Even if I live in the country far from big noise sources, the 80 m band represents a problem with average noise levels of -100/105 dBm throughout day & night. Weak signal reception in presence of fading can be improved with a simple old timer's trick. Set RF gain mid-way (so to make sure you do not reach ADC overload level) and switch AGC off. Like magic the noise level into your headphones (not the meter) will drop the equivalent of two "S" units and listening fatigue will be significantly reduced.

Explanation is simple: when the weak signal approaches the noise level, the AGC will try to keep the output constant, thereby increasing the noise. With AGC off this does not happen. Perhaps a longer time constant in the software for the SLOW position, or even making this time constant adjustable could improve things a bit...

glovisol

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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Sat Sep 01, 2018 7:04 pm

ANOTHER TRICK - UNKNOWN BUT UNBELIVABLE PHENOMENON IN HF BANDS (ONLY TESTED 80 & 40 M)

1) Turn receiver ON and make sure AGC is OFF.
2) Disconnect antenna terminals.
3) Reconnect antenna.
4) You will see the signals on the display and hear them perfectly, BUT THE BACKGROUND NOISE IN YOUR AUDIO WILL HAVE GONE ALMOST AWAY!!

My receiver is RSPduo. Software SDRplay. It seems like some gain parts of the RSP become locked in a special position.

This effects I cannot explain, but provides fantastic S/N in the audio output. Promise I had a glass of wine only for dinner...

glovisol
Attachments
ScreenHunter_227 Sep. 01 21.18.jpg
I am listening to a very weak signal invisible in the display, with virtually no noise in the headphones!
ScreenHunter_227 Sep. 01 21.18.jpg (194.64 KiB) Viewed 3934 times

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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Sun Sep 02, 2018 5:44 am

The explanation of the "low noise" effect should be as follows:

1) With AGC "OFF" and no antenna the system sets for minimum gain: e.g. the noise level internally generated.

2) When you re-connect the antenna the gain stays at the previous level (internally generated noise) and the weak signal is available and readable, provided it is at a level higher or equal to the antenna noise, but the antenna noise cannot be boosted by the AGC action, because AGC is OFF.

3) This mode of operation provides reception similar to that of squelched operation in the FM higher bands by removing most of the noise.


Hope other operators can confirm the effect on this thread.

glovisol
Attachments
ScreenHunter_228 Sep. 01 21.43.jpg
"Noise free" reception with AGC OFF
ScreenHunter_228 Sep. 01 21.43.jpg (201.25 KiB) Viewed 3897 times

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g4enz
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Re: HF weak signal reception in presence of noise

Post by g4enz » Sun Sep 02, 2018 5:47 am

If you find background noise fatiguing, you might want to leave the AGC on but adjust the threshold. This way the AGC still protects you from LOUD signals.
Uno AGC.jpg
Uno AGC.jpg (59.42 KiB) Viewed 3894 times

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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Sun Sep 02, 2018 5:55 am

Hi g4enz, thanks for the suggestion: the fact is that the best improvement is gained with threshold at minimum, which equates to AGC OFF....In effect with AGC OFF, the AGC cursor you mention still controls the RF gain: by using this control as a manual volume gain you even better optimise noise rejection. Of course this operating mode is based on the fact that the ear response is logarithmic, very much so, therefore it is much less sensitive to coherent signal reduction as compared to noise reduction.

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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Sun Sep 02, 2018 12:56 pm

To reiterate and summarise after a lot of listening, this "noise advantage" is very marked as long as the antenna noise is high, in the order of -100 to -110 dBm and becomes less and less significant for antenna noise at or below -125 to -130 dBm. With my setup, Beverage antenna on the 80 & 40 m bands, it makes the difference between copying perfectly an S6 signal on S6 noise and not copying at all.

glovisol
Attachments
ScreenHunter_229 Sep. 02 08.32.jpg
Exploited "noise advantage" on 3.5 MHz
ScreenHunter_229 Sep. 02 08.32.jpg (190.01 KiB) Viewed 3846 times

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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Tue Sep 04, 2018 6:33 am

Another example with AGC + IF AGC in manual and at minimum with perfect copy of S5 signal in S5 noise.
Attachments
Weak signal reception in noise.jpg
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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Wed Sep 05, 2018 3:57 pm

NOISE BLANKER EFFECTIVENESS WITH STRONG IMPULSE NOISE

Noise blanker adjustement when in presence of strong antenna noise plus impulsive noise is extremely critical, not to say impossible, without resorting to manual gain and keeping RF and IF AGC OFF. On the contrary, with the AGC's OFF, Noise Blanker performanceof the RSP receiver is simply exceptional, considering that in most receivers it is just a useless control.

In the enclosed screens you see an S5 signal received on the 80 m band in presence of impulse QRM. Without Noise Blanker nothing can be copied. With Noise Blanker this weak signal can be copied with no problem, while the noise floor has dropped by 10 dB!! The trick is to kep the gain thresholds as low as possible, otherwise controlling the NB slider wil become very abrupt and simply clean off all received signals.

Best performance is acheved by keeping the two AGC thresholds at a very low level and DELICATELY adjusting the NB slider. Also RF gain must be kept as low as possible!

Cheers,

glovisol
Attachments
Noise Blanker ON.jpg
Noise blanker ON - Noise floor: -110 dBm
Noise Blanker ON.jpg (233.13 KiB) Viewed 3645 times
Noise blanker OFF.jpg
Noise blanker OFF - Noise floor: -100 dBm
Noise blanker OFF.jpg (206.71 KiB) Viewed 3645 times

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Mike2459
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Re: HF weak signal reception in presence of noise

Post by Mike2459 » Thu Sep 06, 2018 1:26 am

glovisolL That's an interesting experiment you've done. Very helpful.
I have a question. Any ideas why the noise floor is inversely proportional (more or less) to the RF gain setting? More RF gain lowers the noise floor. In my case it was varied from -116dbm at min RF gain to -135dbm at max RF gain. IF gain was on manual set to 44db, All antennas disconnected at the SDRPlay RSP2, with the measurements taken with the Hi-Z port selected. There is also a disparity between the cursor measurement and the value displayed in the SP2 window i.e. -100dbm at the readout and -116dbm at the cursor. Changing the IF AGC gain has a similar effect but to a lesser degree.

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glovisol
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Re: HF weak signal reception in presence of noise

Post by glovisol » Thu Sep 06, 2018 6:27 am

Hi Mike 2459,

You did not state at what frequency you took the readings. Floor noise variation with RF gain changes with frequency: see Table 1 here
https://www.sdrplay.com/community/viewt ... f=5&t=3352
where significant measurement conditions are given and data was measured with IF AGC ON.

The reason why noise floor diminishes with increasing RF gain is because IF amplification noise figure is significantly worse than RF amplification NF: hence the stronger the RF gain (at low NF) the less the internally generated noise and hence the lower the noise floor in absence of received signal.

In practical use the important factors are antenna signal output level and antenna noise. If you have an antenna with a high output level because it has a large area (like my Beverage antenna) then it is essential to keep all receiver's gains very low: this will prevent overloads, spurious responses and keep noise at bay, not permitting noise to blot out IN YOUR EARS that very weak signal. If you have a low output level antenna, such as a small vertical, then you must increase gain accordingly, to suit these conditions. The RSP receiver provides the reserve gain and resistance to overload necessary to meet most of the operating conditions existing in practice.

I never noticed the difference in cursor's readings, but trust Tech Support will no doubt explain it.

AMENDED 06/09/18, 18:49 UTC
The cursor's readings simply show the level in dBm the cursor is: e.g. this allows one to instantly measure the level of any signal on display while being tuned on another signal.


Cheers,

glovisol

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