What is "LO Plan Frequency"

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KE2KB
Posts: 18
Joined: Fri Dec 02, 2016 1:11 am

What is "LO Plan Frequency"

Postby KE2KB » Sun Dec 04, 2016 11:13 pm

Hi;
I'm new to SDR, but not to Amateur radio. That said, I have never heard of "LO Plan Frequency", so I am trying to figure out what it is in regards to the setting in SDRUno 1.1 with SDRPlay RSP1. I do know that LO is Local Oscillator, and do understand basic receiver theory (I used to know more, but have forgotten a lot of it by now...). I have found by experimentation that setting to Auto isn't always the best option. When I manually change to one of the other settings, I get a lower noise floor, especially on the HF bands.

Thanks
Frank - KE2KB
Last edited by KE2KB on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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13dka
Posts: 136
Joined: Sun Feb 14, 2016 8:40 am

Re: What is "LO Plan Frequency"

Postby 13dka » Tue Dec 06, 2016 12:16 pm

The RSP initially had a gap between 370 and 420MHz with the standard LO frequency of 120MHz. To make reception possible in that range anyway, they later introduced a scheme to change the LO frequency when you tune through this range. 144MHz gives you gaps from 250 to 255MHz and 400 to 420MHz, 168MHz gives you a gap between 250 and 265MHz. Leaving this on "Auto" gives you automatically full coverage.

Edit: Forgot to add... The reason why you can change this in first place is that shifting the LO frequency can help mitigating some overload and imaging issues, those may get shifted someplace else as well.

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KE2KB
Posts: 18
Joined: Fri Dec 02, 2016 1:11 am

Re: What is "LO Plan Frequency"

Postby KE2KB » Wed Dec 07, 2016 4:31 pm

Thanks for explaining that. I wish I could shift the LO in my Yaesu FT60 and VX150 when listening to "out of band" services like police/fire, and railroads. Sometimes I get really bad overload from my local police transmitter, which causes their signal to desense the receiver, making it very difficult to hear anything on the RR band.
Adding the 152Mhz PAR filter to the feed really helped for the pagers, but I might be looking for another filter for 155.7Mhz, which is the output of the local police repeater, located only a couple blocks from my home.

On the SDRUno SP1 display, I can see the overload when it occurs. The entire noise floor jumps up many db. I'm not sure if changing the LO would help that situation, but I'll try it next time I am listening there.

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