SDRplay Community Forum

I find there's always a fairly strong false line of "signal" showing center scale in the MAIN SP window. It exatly matches the "LO" frequency shown. I guess this is some kind of expected internal quirk or interference.

Trouble is, this is precisely where SDRuno is tuned on a cold start of the software. Any station actually tuned at that center position is distorted. Thus I find I always have to click somewhere else on the spectrum, THEN select a signal in order to get any kind of decent result.



My RSP1 works pretty well otherwise, so I wouldn't think this is a defect, but maybe it is - do you see this as well?

Assuming it's to be expected that the Local Oscillator frequency will interfere with tuned signals, wouldn't it be better to have the red tuning line start up where it was left last time, or at least off-center a little?

-Noel

It's normal. It's the famous 'DC spike' and is seen in all SDR's to a certain extent. I agree it would be preferable if SDRUno started with the tuned frequency slightly away from the spike, but it doesn't. Adjustment of the front-end gain can help reduce it, but it will always be there to a certain extent.
You can banish it completely by selecting 'LOW IF' (rather than the default zero IF), but this setting restricts bandwidths etc.

g1hbe wrote:It's normal. It's the famous 'DC spike' and is seen in all SDR's to a certain extent.

In all soundcard based SDRs. Makes it e. g. impossible to listen to wide FM with a Funcube pro+ which has only 192 kHz bandwidth. The DC spike is always in the middle of an FM wide signal and heavily distorts reception. With an RSP you can at least move away from the center.
No spike in direct samplers like Winradio G3xDDC, Perseus or similar.
73, Heinrich

Yes of course Danube, I was thinking of soundcard SDR's. Actually I've found that WFM is not unduly disturbed by the spike as long as the incoming signal is not too weak. AM modes are hit badly.

Thank you for the explanations, and for the "Low IF" vs. "Zero IF" workaround suggestion.

Just poking around, I see that configuring "Low IF" also completely changes the choices for Sampling Rate and Decimation.

Under what specific conditions does it limit the bandwidth?

Limited bandwidth would not be too much of a problem for me in that I find myself virtually always zoomed in and looking at 1 MHz or less of signals in the MAIN SP.

I have to say, so far overall the MAIN SP waterfall seems cleaner with the "Low IF" setting.

-Noel

Hi Noel
I'm not sure of the inner workings, but I'm guessing there is a hardware limitation of the Mirics chipset which limits the choices of sampling freqs in LOW IF mode. I generally use ZERO IF as it allows a greater choice, but like you I often use a span of no more than a MHz or two. I've also found that LOW IF can cause images which do not happen in ZERO IF. I was idly twizzing around civil airband today and in LOW IF mode there were images of digital signals all over the place. Switching to ZERO IF got rid of them.

Hi
In fact I found that the zero IF notch can be quite useful.
With a sound card it is hard to use because you pickup a lot of hum noises due to ground loops around it.
Two examples:
In Europe if you want to receive Boston on 1510 KHz you get a 2KHz tone due to European stations on 1512.
putting 1512 at zero IF does the trick and doesn't use any computing power or ADC dynamic range.
For DRM reception interference from AM stations can be a problem specially when both signals are affected with QSB. Here too zero IF is a solution

Maybe it's an SDRuno thing, but when tuned to the center scale frequency and Zero IF is selected I don't see a notch so much as a ghost signal that distorts the reception. The sound for USB isn't bad, because the low frequencies are excluded, but AM stations just sound distorted.

Examples: Center scale right now with Low IF:



...And with Zero IF. Note the nasty stuff on center frequency:



-Noel