RE: [softrock40] Re: Interference from PC?
- I believe the gunk you see near the center frequency with or w/o hardware
attached is an artifact of the signal processing architecture, and noise
near DC on the audio input (ie 60 hz hum,. etc)
How the software tuning works is by having a software mixer fed with the
local oscillator to dowconvert the signal of interest. In the case of the
SoftRock, you have the hardware QSD downconverting and quadrature sampling
a 48 khz swath of RF centered on 7.056 Mhz. To listen to a signal at 7.060
MHz, the software mixer is set to run with a local oscillator at 4 khz to
mix down the 7.056 Mhz centered signal. As you tune closer to the center
frequency, the freq of the software local oscillator gets lower and
lower. Very near the center frequency, the local osc will be in the 100's
of hz, Mixed with 60 hz (and harmonics) hum this will tend to product
responses at 60Hz (and harmonics) +/- SoftwareLO etc -- thereby getting
you the gunk around the center frequency.
I will have to admit that my understanding of this phenomenon is not as
crisp as I would like it to be. As Tony said in a prior post, there's some
hope that some of this can be eliminated with DSP magic. One approach
would be to record and characterize the noise characteristics with no input
and then try and remove them with DSP magic when fed with a real
signal. One could do this in either the time domain or frequency domain,
although I'm not sure how one would maintain sync with the noise signal in
the time domain. My DSP skills are not strong enough (yet) to really know
how feasible such an approach would be.
Another (potentially naive) approach might be to try a different tuning
approach. If one is interested in a sig at 7.060, I'd think one could take
an FFT of the input signal, zero the FFT bins outside the passband of
interest, shift the spectrum over by 4 khz and then IFFT the result to
recover the signal of interest. I think this should work, but not having
tried it really can't say for sure. Also not too clear in my mind if this
would be better than the software mixing approach currently used as you get
closer to DC.
One thing to be aware of -- in a more complete SDR with a frequency agile
downconverter, you typically don't tune in software down near DC. For
example, on the SDR 1000, the tuning is always such that you're
approximately 11khz above DC for software tuning. The reason for this
being approximate is that the DDS tuning is limited to tuning to freqs
where the DDS generates a minimum of spurs.
I will be the first to admit I'm somewhat of a neophyte with DSP. SDR is
an area rich with opportunities for learning and experimenting, and we've
now got hardware and software accessible to the amateur community to
experiment with. Hoping folks get in there and do some experimenting and
At 01:25 PM 9/18/2005, Stan Rife wrote:
> Tony, one thing I noticed, when just playing around with the SDR
> software, was the 600 Hz tone that comes from the soundcard (?) at the
> 7.056 freq. after the Fixed HW oscilator is set up per the instructions.
> This is, of course, without the SoftRock hardware installed, as I do not
> even have my kit YET. (boo hoo).
> Is this oscilator tone, at that freq., something that will always be
> there? I saw a procedure in the manual that said something about going
> above 7.056, as much as the test oscilator is below 7.056 and adjusting
> for a null. I am not quite clear on all of this. I do have an XG-1, and I
> assume that is what this is refering to.
>K2 S/N 4216
When I operate on the low bands (40M thru 160M) I have to turn off my
PC's. In fact I sometimes have had to unplug them completely, as the
power supply isn't completely off when the On/Off switch is in the Off
I found the problem to be caused by the computer's switching power
supply. In fact I lost a power supply in one of my computers recenly,
and replaced it with a better grade supply, and then was happy to find
that the noise from that computer was gone.
Try unplugging your computer completely and see if that quiets things
--- In email@example.com, "observer35" <rein0zn@i...> wrote:
> Hi all,
> Just new to the group.
> Does anybody experience interference from the PC on 40 meters?
> What about noise and garbage on the 5 V USB line?
> 73 Rein W6/PA0ZN