VFO & LO for dummies

Discussions about everything to do with SDRuno
WB5AGF
Posts: 19
Joined: Sat Feb 17, 2018 6:35 am
Location: Garland, Texas (north side of Dallas)

Re: VFO & LO for dummies

Post by WB5AGF » Sat Feb 24, 2018 7:32 am

Mike;

Thanks much for the comments.

Just in the last bit I've come across some things :

- article from this group from over a year ago :
viewtopic.php?t=1431

- and here is something :
https://www.dsprelated.com/thread/567/z ... -receivers

which led me to this :
http://www.cs.tut.fi/kurssit/TLT-5806/RecArch.pdf

Which really has me working to understand how this stuff works (!)

(Also this :
https://www.ieee.li/pdf/essay/quadrature_signals.pdf
but I am really bogging-down ... and having trouble)


Regards;

- Paul, WB5AGF

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Tech_Support
Posts: 491
Joined: Mon Jun 01, 2015 7:00 pm

Re: VFO & LO for dummies

Post by Tech_Support » Sat Feb 24, 2018 9:19 am

Hello folks,
Apart from the lack of a DC spike, there are no fundamental advantages of LIF over ZIF. There is no difference in noise performance or overload performance.

The differences that Mike2459 at LF are all caused by the different frequency plan that is used in LIF mode where the synthesizer frequency has to be offset from the incoming carrier by the IF frequency. At LF, this reduces the prevalence of spurious mixing products that always occur as a result of the double conversion architecture of the RSP.

Spurious (or higher order) mixing products are NOT intermodulation. They are caused by harmonics of the block converter LO generating higher order IF frequencies. These Higher order IFs can then mix with harmonics of the synthesizer to produce and in-band response.

It is easy to determine the difference between intermodulation and a spurious mixing product. With a spurious mixing product, if you change the LO frequency in SDRuno, the apparent frequency of the spurious mixing product will also change. This is not the case with intermodulation products. The other key difference is that spurious mixing products have a 1:1 correspondence with input signal level and intermodulation products do not. On other words, if you apply a 1 dB attenuator in line with your antenna, spurious mixing products will reduce by 1 dB (as will all signals), but intermodulation products will typically reduce by either 2 or 3 dB depending of the type of distortion that causes them.

At LF you can achieve the same effect in ZIF mode as you see in LIF mode, by increasing the LO frequency so that it is further away from the signals that lead to the spurious mixing products. However, many people will find LIF mode the easiest way to get the best performance at LF as it requires little or no 'fiddling about' with the LO to minimise spurious mixing products

Sincerely

SDRplay Tech_Support

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Paul
Posts: 349
Joined: Fri Feb 17, 2017 9:52 am
Location: SW UK

Re: VFO & LO for dummies

Post by Paul » Sat Feb 24, 2018 1:06 pm

I know that I am technically challenged, but having tried low IF mode in the MW band, I found that although the "images" virtually disappeared, the stremgth of "wanted" signals fell dramatically also. Disabling IF agc and increasing IF gain manually was pretty ineffective.
I will look at this again, but so far not an obvious benefit to be had here apparantly.

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Tech_Support
Posts: 491
Joined: Mon Jun 01, 2015 7:00 pm

Re: VFO & LO for dummies

Post by Tech_Support » Sat Feb 24, 2018 2:22 pm

Hello Paul,
There should be no difference in signal power between Low IF and Zero IF modes. One thing to be aware of though is that the IF bandwidth will be different. So for example, if you have set your sample rate to 8 MHz in zero IF mode and your LO to 4 MHz, the spectrum between 0 and 8 MHz will be visible. If you then switch to Low IF mode with a sample rate of 8.192 MHz with the LO still at 4 MHz, the IF bandwidth will only be 1.536 MHz maximum and so signals below about 3 MHz and above 5 MHz will be heavily attenuated.

Sincerely

SDRplay Tech_Support

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Paul
Posts: 349
Joined: Fri Feb 17, 2017 9:52 am
Location: SW UK

Re: VFO & LO for dummies

Post by Paul » Sat Feb 24, 2018 7:03 pm

Thank you Tech Support (once more) for an explanation that I am almost in danger of understanding.
To show that I have paid attention, I have produced two screeshots:
1. zero IF/SR 2 Mhz/Decimation 8
2. Low IF/SR 8.192/Decimation 4
Which confirm the result that you advised - thank you.

https://www.flickr.com/photos/137437642 ... ed-friend/

https://www.flickr.com/photos/137437642 ... ed-friend/

Of course, I am looking at an area of interest and have not confused the LO with Decimation (but I did understand - I think)

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NoelC
Posts: 137
Joined: Sun Nov 27, 2016 11:50 pm

Re: VFO & LO for dummies

Post by NoelC » Sun Mar 04, 2018 11:01 am

By the way, whomever is answering for SDRplay Tech_Support...

It's GREATLY appreciated that you are here discussing practical usage at this level (and thanks as well to the other users here).

Many things I've come to learn and suspect simply by using this software/hardware are being confirmed in this thread.

It truly is a product where the more you understand, the more you can get out of it.

Thanks!

For what it's worth, I most often use Low IF mode (I dislike the Zero IF spike) with Sample Rate 8.192 MHz and DEC 4. With the latest version I can look over HF frequencies zoomed so I can see a 1 MHz bandwidth in the MAIN SP window across my 30 inch monitor (2560 pixels width). At this size it's possible to visually differentiate various kinds of signals and with some care hit frequencies on 1 kHz boundaries with a single mouse clicks. A very pleasant experience all in all.

One thing that would make my user experience more convenient is if the RBW setting would stay where I left it. I find I have to push it up to 65536 for the cleanest display every time.

-Noel

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Tech_Support
Posts: 491
Joined: Mon Jun 01, 2015 7:00 pm

Re: VFO & LO for dummies

Post by Tech_Support » Sun Mar 04, 2018 1:06 pm

Hello Noel,
The RBW is a function of the frequency span and the size of the FFT. We are going to look at maintaining the FFT size as a stored setting in a future release of SDRuno as other people have also requested this feature.

Sincerely

SDRplay Tech_Support

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NoelC
Posts: 137
Joined: Sun Nov 27, 2016 11:50 pm

Re: VFO & LO for dummies

Post by NoelC » Wed Mar 07, 2018 6:31 am

Thank you. 8-)

-Noel

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