2550Re: Spur in the middle of the spectrum...
- Dec 2, 2010Hello Henk,
> At the moment I am using a M-Audio Delta44 as my soundcard, but(from your first posting on this thread)
> also this soundcard has some kind of noise in the middle of the spectrum. I
> tried also yesterday my onboard soundcard. The result was poor. Much worse
> than the M-audio Delta44.
> In my case I have some spur in the middle of the bandpass of my SDR. Simon(from your second posting on this thread)
> has got an option to reduce this. It is a little better, but it is still
> present and it is anoying me a little.
> Does someone know why this is present there and what a solution is to reduce
> this? Is it something about the adjustment of my SDR on the hardware side?
It is non-trivial to get rid of the center spur in a direct conversion
radio that uses a PC soundcard.
The spur is caused by several different mechanisms in the hardware.
The way to remove the spur is to first reduce it to a modest
level and then apply front end RF gain that rises the antenna noise
level well above the spur level.
I can not judge from your image what the dominating problem
might be because the resolution is too low and I can not read
the frequency scales. As a first step I suggest that you
replace the antenna with a dummy load. Then look at the
spectrum and you should see the spur but no other signals.
In cas this simple test shows an improvement you suffer
from ground loop problems. Try to connect the antenna
ground to the dummy load ground. If that affets the spur
level you can eliminate that problem with an RF transformer
at the input.
The Delta 44 suffers from design errors so it has a rather
strong enter spur even if nothing is connected to it.
This link shows how you can modify the card to eliminate
Figure 1 shows the card in its original shape with
nothing connected to the input. The center spur is symmetric
and consists of two peaks separated by 10 to 20 Hz.
(Different in the two hannels)
The spur is 1/F noise that would cause a very high level
at frequency zero, but there is a digital high pass filter
in the Delta 44 that starts to attenuate at 20 Hz or
so (I do not remember the exact response) The high
pass filter attenuates 0Hz infinitely an that causes
the double peak.
The dominating source of 1/F noise is poor decoupling
of the 5V DC supply lines to the A/D converters. See
The modifications lower the center spur by about 20 dB
as you can see here:
Figures 1 and 2 have the center spur at 48 kHz.
The modifications clean up the spectrum in several other
I suggest that you disconnect the cable from the Delta 44
and look at the spectrum. Then connect only the ground
pin(s) from your radio hardware. In case you see a difference
you suffer from the ground loop error. Step 1 here:
Your mixer is likely to produce 1/F noise and there might
be pick up of magnetic fields and many other things. They
can all be mastered and give you nice spectra like these:
There is an infinitely deep notch at the enter but it is so
narrow that it is unlikely to cause any problems. If the
carrier of an AM station would fall exactly on it a normal
AM detector would fail however.
Leif / SM5BSZ
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