VFO & LO for dummies

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buzzme258
Posts: 6
Joined: Mon Feb 19, 2018 12:38 pm

VFO & LO for dummies

Post by buzzme258 » Wed Feb 21, 2018 6:07 pm

Noobie here to the world of shortwave radios.

Trying to get my head around what exactly VFO and LO are/do and how they interact. Pretty sure that VFO stands for "variable frequency oscillator", and LO stands for "local oscillator". If that's correct, that's about all that I know. It seems like that to use SDRuno, you need to have at least a working knowledge of what they are.

Any help or links to knowledge would be appreciated. "LO" only has 2 characters, so a search here came back invalid.
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sdrplay
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Re: VFO & LO for dummies

Post by sdrplay » Wed Feb 21, 2018 7:03 pm

One relates to the hardware (LO) - this is the centre frequency that is ALWAYS used by the hardware to tune to the chunk of spectrum you want to work with. The Sample rate defines the amount of spectrum bandwidth that you can see centred around the LO frequency.

The VFO is a frequency that you can tune to within SDRuno WITHIN the available spectrum as defined above. It's the VFO that is used to demodulate the signal that you are interested in.

It's always a good idea to separate the VFO from the LO. Because of the way the spectrum is produced, there often will be a visible spike at the centre of the spectrum. Tune away from that with the VFO so that it doesn't interfere with the audio demodulation.

Note that there are 2 ways that the spectrum can be produced from the hardware (Zero IF and Low IF) - Zero IF supports the widest bandwidth (2 to 10 MHz) but has the "DC" spike I mentioned. Low IF doesn't suffer from the "DC" spike but can only be used at smaller bandwidths.

Hope that helps,

Best regards,

SDRplay Support

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buzzme258
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Re: VFO & LO for dummies

Post by buzzme258 » Wed Feb 21, 2018 9:32 pm

Thanks for the quick response. I'm impressed.

So, does VFO = "variable frequency oscillator"?

and does LO = "local oscillator"?

Tell me more about what "LO LOCK" does? The manual touches on it, but quickly goes over my head. I'm sure that as I use it I will understand it better. :oops:

Thanks
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sdrplay
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Re: VFO & LO for dummies

Post by sdrplay » Thu Feb 22, 2018 3:08 am

You are right on the definitions.

LO LOCK does exactly that. It locks the LO frequency so now when you scroll with the mouse wheel you will see the VFO move up and down the spectrum, but the LO frequency does not move.

So from the hardware perspective, nothing is changing. You are just selecting the VFO frequency to use within the bandwidth already specified.

We use that in the band framing. When you press a band button on the keypad, the entire band (where possible) is framed and the LO is locked to the centre of the band. Now you can tune up and down the band without the entire spectrum shifting and potentially taking you out of the desired band.

Best regards,

SDRplay Support

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WB5AGF
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Joined: Sat Feb 17, 2018 6:35 am
Location: Garland, Texas (north side of Dallas)

Re: VFO & LO for dummies

Post by WB5AGF » Fri Feb 23, 2018 9:44 am

(I don't mean to hijack this thread ... this is the closest discussion I've found that seems to potentially be discussing something thats got me puzzled.)

SDRplay support said :

"Note that there are 2 ways that the spectrum can be produced from the hardware (Zero IF and Low IF) - Zero IF supports the widest bandwidth (2 to 10 MHz) but has the "DC" spike I mentioned. Low IF doesn't suffer from the "DC" spike but can only be used at smaller bandwidths."

So when should we use Zero IF and when should we use Low IF ?

If I was to be (for instance) tuning the 40M ham band, and not needing to be seeing several MHz above-and-below where I'm listening, is there an advantage to be configured in Low IF Mode other than the lack of the LO sitting in the middle of the displayed frequencies as is the case when running Zero IF ?

(Here's something that has just occured to me.)
What if I'm in Zero IF Mode and (to listen to the 40M ham band) I manually set-and-lock the LO to (for instance) 6.9 MHz ... then just tune the 40M ham band (7.0 MHz - 7.3 MHz) ? In that situation is there any practical difference compared to running in Low IF Mode ?

- Paul, WB5AGF

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sdrplay
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Re: VFO & LO for dummies

Post by sdrplay » Fri Feb 23, 2018 11:06 am

If you want to see more than 1.5/2MHz of spectrum at a time then you will need to use ZeroIF mode.

There are currently only 3 sample rates supported in LowIF mode (0.5 MHz, 1 MHz and 2 MHz) whereas in ZeroIF mode you can have any sample rate between 2-10 MHz and combined with that the decimation values enable you to have much more control over the final sample rate.

Framing the bands is currently only possible in Zero IF mode. If you press some of the band buttons and look at the combinations of SR and DEC that will be displayed in the Main Window, you will see how powerful that is to just show specific bands.

Low IF doesn't suffer with the "DC spike" as previously discussed, so having both methods available give the end user the ability to customise the display to what best suits their needs.

Setting the LO frequency to the centre of the 40m band (remember the LO is always at the centre of the spectrum you are using) and switching to Low IF is fine. You just won't be able to frame the band exactly. You will need to experiment with the best sample rate to get the entire band in without falling foul of the IF bandwidth filter roll off.

Hope that helps,

Best regards,

SDRplay Support

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WB5AGF
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Location: Garland, Texas (north side of Dallas)

Re: VFO & LO for dummies

Post by WB5AGF » Fri Feb 23, 2018 12:05 pm

(I apologize in advance if I'm 'missing' something.)


Hello SDRplay support .... I'm not sure that you answered this so I'll try again :
----------------------------------------------------------------------------------------------------------------
What if I'm in Zero IF Mode and (to listen to the 40M ham band) I manually set-and-lock
the LO to (for instance) 6.9 MHz ... then just tune the 40M ham band (7.0 MHz - 7.3 MHz) ?
In that situation is there any practical difference compared to running in Low IF Mode ?
----------------------------------------------------------------------------------------------------------------

My thinking is (in that situation) the LO is always out-of-the-band being tuned (I believe
that is one characteristic of Low-IF operation) and, other than the need to experiment
with the Sample Rate and Decimation to most effectively cover the 40M band, how would
operating in Zero IF differ from Low IF Mode ?

Sincerely;

Paul, WB5AGF

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sdrplay
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Re: VFO & LO for dummies

Post by sdrplay » Fri Feb 23, 2018 12:49 pm

I did try to answer your question. Whether you use ZeroIF or LowIF, the LO frequency displayed in SDRuno is ALWAYS in the middle of the spectrum being used. Due the different mixing processes, the spike in Low-IF is not in band, so you will specify 7.15 MHz as the LO frequency for the centre of the 40m band irrespective of the IF mode you are using.

The differences between ZeroIF and LowIF mode in terms of sample rate, etc. I covered in the previous post.

Best regards,

SDRplay Support

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WB5AGF
Posts: 19
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Location: Garland, Texas (north side of Dallas)

Re: VFO & LO for dummies

Post by WB5AGF » Fri Feb 23, 2018 9:03 pm

Hello SDRplay Support;

I understand that in Zero IF Mode, when I select a frequency, up pops the spectral display with the LO (and received frequency) at the middle of the screen (with a band of frequencies shown above-and-below the center frequency).

My question is .... are there any benefits in using the Low IF Mode ?

In Zero IF Mode .... (as an example) .... If I call up 6.9 MHz (such that the LO and received frequency show as being on that frequency), THEN I Lock the LO (so that it is held on 6.9 MHz) .... BUT then I chose to tune only between 7.0 MHz and 7.3 MHz ..... Is there any technical benefit (sensitivity, immunity to overload, less internally generated noise, etc) to (instead of the example given) running the receiver in Low IF Mode ?

When the receiver is in Zero IF Mode there is nothing which forces me to tune both above-and-below the center frequency; if I want to I can choose to limit my tuning to only one side of the center frequency or the other. If I am willing to do that (in Zero IF Mode) what benefit would operating in Low IF Mode have ?

Sincerely;

Paul, WB5AGF

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Mike2459
Posts: 318
Joined: Mon Jul 24, 2017 8:24 pm

Re: VFO & LO for dummies

Post by Mike2459 » Sat Feb 24, 2018 6:54 am

I always use low-if on HF and LF. Compare the two while tuned to around 300 khz at night when MW broadcasts are strongest. Images everywhere on zero-if.

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