Re: Linrad Spur Removal

[VK2KU <clearmail.com.au; vk2ku@xxxxxxxxxxxxxxxx>]

Hi Leif,

> Elimination should be instant. If it is not, the S/N of the spur
> is not good enough due to modulation sidebands or otherwise.

OK, even with my SigGen output into a 50-ohm load and set to minimum
output, I can't reduce the level in Linrad below about 12dB above the
NF in the HiRes spectrum.
At this level removal of this clean and narrow spur is almost
immediate.
Actually I guess I could disconnect the antenna, but I haven't tried
this.
The other spurs are from my own computer, often around 7 to 8dB above
the NF. Some are narrow, about 5Hz, others wider.
Even the narrow ones are only sometimes removed, and then may reappear
and vanish periodically. Marginal S/N I guess.
I tried various Spur Time Constants in the range 1 to 5, without
seeing any reproduceable pattern.
One spur is about 15Hz wide on the WSJT waterfall, and I was never
able to do anything to this one with my 5Hz FFT2 bin size.
Lots of spurs are triplets, spaced about +/-55Hz, each component
fairly narrow (5Hz), at 8dB level above NF. Removal of these was
intermittent, with one or 2 components sometimes removed for a while
and then returning. I guess these are probably FM modulation, but I
haven't looked at them yet on my CRO.

> 111000/32768=3.387Hz. (0.3s) This would be the bandwidth of an
> un-windowed transform.
> With a sine squared window, the bandwidth would be 6.7 Hz
> while transforms (overlapping) would arrive at a rate of  6.7 Hz.

The FFT2 window is actually sine^1, which gives an FFT2 BW of 5.08Hz.
The update of the HiRes graph with 20 averages is almost exactly once
per second. I don't know what the overlap factor is

> The spur removal works on fft2 transforms, the minimum number
> is 4, so the minimum time over which the phase and amplitude
> must behave well is 0.6 seconds. The spur time constant is
> 0.6 seconds, setting a lower value like 0.1 does not change
> anything.

Right, that matches my observations above.

> ... but for abrupt changes of the phase or amplitude,
> the bandwidth is narrower than 1 Hz.

I think these changes are abrupt and quite rapid, say faster than
0.1s, in most of my spurs.

> > What about a JT65B sync tone? Is there any danger
> > that the very signals I am trying to see on the main waterfall will be
> > eliminated as supposedly unwanted spurs?
>
> I have no idea, I never tried it and nobody ever reported anything:-(

I will look at this in the coming days. It may be that the S/N is too
low, but it will be interesting to see the effect on big signals :-)

More anon,

Guy  VK2KU

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