Gain Reduces When Centering Signals In HF

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jkreski
Posts: 1
Joined: Fri Jan 25, 2019 4:08 am

Gain Reduces When Centering Signals In HF

Postby jkreski » Fri Jan 25, 2019 4:16 am

Looking at my screen, seeing a strong signal in right 1/3 of screen. I click on it - strong clear audio. I click the Center button and it centers the signal on the screen and the signal strength seemingly drops. Is there some setting to adjust for this? It did not seem to do this some time ago. I do run a Windows 10 PC, a powered USB 3 hub with 7 ports. That sports three RTL-SDR dongles and on connection to Ham Radio Deluxe and my Yaesu FT 857d.

One dongle sports a SpyVerter to convert HF to a signal the dongle can tune. Really works well with 150' or so wire antenna and 9:1 Balun. The other two dongles connect to an external TV and FM beam antenna with a rotor and TV pre-amp. Ideas? :?
Last edited by jkreski on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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g1hbe
Posts: 481
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Gain Reduces When Centering Signals In HF

Postby g1hbe » Fri Jan 25, 2019 10:30 am

By moving the signal to the centre, you are putting the received signal on the same frequency as the Local Oscillator. You can confirm this by looking at the on-screen display in the top right of the big spectrum display. If the VFO frequency and the LO frequency are the same, that's the problem. This causes a partial cancellation of the signal and can also cause distortion. All zero-IF SDR's do this. You should always receive with some spacing between VFO and LO.
If you don't like this arrangement you can select 'low IF', which gets rid of the central LO frequency. However it does place limits on bandwidth and sample rates.

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Andy

mark.
Posts: 51
Joined: Tue Oct 03, 2017 9:14 pm

Re: Gain Reduces When Centering Signals In HF

Postby mark. » Fri Jan 25, 2019 6:12 pm

I can confirm the issue. VFO and FREQ are separated by 80khz. I monitored an unmodulated carrier 1460khz with zoom all the way out which put the signal at the right 1/3 of the screen. Mouse pointer at the very top of the signal spike and numeric display on the left of the main screen matched at -50db. But when I choose to zoom in all the way and the signal is centered onscreen, the text on the left stays at -50db but the signal spike and the graph now display -55db.

So even with VFO and FREQ set apart I have the issue. I noticed months ago when I was testing my homebrew BCB notch filter but wasn't too worried because I'm not doing any lab precision measurements with my SDRPLAY.

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sdrplay
Posts: 949
Joined: Wed Jan 07, 2015 7:58 am

Re: Gain Reduces When Centering Signals In HF

Postby sdrplay » Fri Jan 25, 2019 7:05 pm

The power measurement is made within the VFO bandwidth of the SP2 spectrum and not the bandwidth of the input spectrum (SP1 display). For an accurate power measurement, use the power reading and not what the cursor number says in the SP1 display.

Best regards,

Andy

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