Zero vs Low IF

Discussions about everything to do with SDRuno
KE2KB
Posts: 18
Joined: Fri Dec 02, 2016 1:11 am

Zero vs Low IF

Postby KE2KB » Mon Dec 05, 2016 11:52 pm

I had to learn the hard way when to use Zero-IF and when to use Low-IF, during which time I was baffled.
I had been listening to the 40m band using the default Zero-IF. At some point I switched to the AM broadcast band, and found that when I tuned to one of my local stations (880Khz), set the filter to AM + Soft, I found that when exactly tuned to the broadcast station's frequency, the audio was terribly distorted.
If I switched to either LSB or USB, then audio became much clearer, but at a lower volume.
The main spectrum display showed that I was right smack in the center of the signal.

Thing is, I had been listening to the same station earlier with no difficulty.
Long story short, I found that switching to Low-IF fixed the problem. I must have switched the IF before listening earlier, but had forgotten that I had done so.

I'm still very new to the world of SDR, having been active exclusively on V/UHF bands, I was playing with an SDR dongle (NooElec RTL-SDR) before buying the SDRPlay RSP1, when I started listening to the HF bands.
While SDRUno has quite a plethora of features to learn, I have found it much more to my liking than any other software.

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someYguy
Posts: 46
Joined: Sun Apr 10, 2016 7:12 pm

Re: Zero vs Low IF

Postby someYguy » Tue Dec 06, 2016 5:57 am

I use low IF on my RSP1 99% of the time. I really only use zero IF if I need to see more than about 1.5mhz of spectrum, which is rare for my needs. I personally have found very few situations in which zero IF yielded better listening.
Last edited by someYguy on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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13dka
Posts: 127
Joined: Sun Feb 14, 2016 8:40 am

Re: Zero vs Low IF

Postby 13dka » Tue Dec 06, 2016 11:58 am

In Zero IF mode you are getting a pronounced "DC spike" due to the design architecture. The RSP (and also the SDR software) are trying to suppress that spike in a way that's insufficient on VHF/UHF (there's still a spike) and over-sufficient on shortwave and below - the spike turns into a notch. If you're tuned right on that notch it will also suppress the carrier of the station you have tuned in, causing this distortion. The only way to avoid this is currently to make sure your LO frequency has some offset to the tuner frequency. Go to the SP1 settings window and turn on "Show F dial", this will give you an indication of the LO and tuner frequencies. By clicking somewhere on the spectrum you are changing only the tuner frequency, creating an offset, the offset will stay fixed when you retune to the wanted frequency and the DC annoyance stays out of the way.

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

Re: Zero vs Low IF

Postby KE2KB » Wed Dec 07, 2016 4:22 pm

Thanks guys;
I guess the SDRPlay defaults to Zero IF so that you can get the full 10Mhz bw, which seems to be a big selling point. I am finding that I use 2Mhz most of the time. After all, there isn't one HF Amateur band that spans even 2Mhz!

Interesting about the LO vs tuned frequency. When I first started playing with SDRUno and the SDRPlay, I was unfamiliar with all of that - having let all that ham knowledge go soft due to working exclusively in the V/UHF FM bands, and with stock equipment. Now that I'm getting back into HF, all of that becomes more important, which is good for those lazy neurons :)

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