[USRP-users] B200 Broadcast FM and other signals on airband

Marcus Müller marcus.mueller at ettus.com
Thu Apr 23 14:27:15 EDT 2015


Hi Paul,

sorry to hear you're still in trouble; I'll try my best to help you,
though HDSDR and Windows are really not my forte.
Balint is currently not in office, so I can only give you my piece of
insight:

first of all, I'm a bit confused; your settings say you've got a
sampling rate of 16MS/s, and your center frequency is 120.8MHz, so your
observable spectrum should be 112.8MHz - 128.8MHz. However, HDSDR's
waterfall shows different frequency limits -- maybe there's some
sweeping or caching, or is maybe my definition of tuning and LO
frequency different from that of HDSDR?

I can, however, explain the incompatibility warning: HDSDR is linked
against a specific version of UHD, and can only work with exactly that
version of the UHD.dll, which is why Balint packages it with the whole
Extio framework. Installing a more modern version of UHD system-wide
doesn't help you, here, since HDSDR continues to use the packaged
UHD.dll. Thus, after installation of UHD and flashing of the images,
you've got "modern" FPGA and firmware on your B200, and "old" UHD
talking to the device -- which is what the error complains about.

Regarding your ghost signals: Those are interesting, though, if I read
that correctly, they are at -82dB, which is relatively little,
considering your 27dB gain. Can you try to disable AGC? If that's not
enough, my first guess would be to play around with gain: Too much gain
leads to spectral images[1], too little gain together with AGC might
over-emphasize spurs.

Greetings,
Marcus

[1] If you have a strong signal, and high gain, your amplifier, being
composed of nothing more than transistors, goes into saturation; the
transmission function then is no longer practically output = input *
gain ($y(t) = a x(t)$), but gets a quadratic term ($y(t) = ax(t) -
bx^2(t)$); now using the some properties of the trigonometric functions,
in this case $\sin \alpha \; \sin \beta = \frac{1}{2}\left(\cos
(\alpha-\beta) - \cos (\alpha+\beta)\right)$ applied to an input signal
$x(t) = \sin(f t)$, you'll notice that for $x^2 = \sin^2(ft)=
\sin(ft)\sin(ft) = \frac12 \left(\cos(ft-ft)-\cos(ft+ft)\right)$ you see
a signal with twice the input frequency. In fact, semiconductor mixers
are usually based on that principle, and thus are basically amplifiers
driven out of spec :)


On 04/23/2015 01:07 AM, Paul B via USRP-users wrote:
> Hi guys,
>
> My troubles seem to be at no end at the moment!
>
> After reinstalling my Windows 8.1 due to a hard drive change, i've
> been having trouble with my B200 on HDSDR using
> Balints ExtIO_USRP+FCD+RTL2832U + BorIP-1.6 BETA 3. 
>
> Before re-installing Windows 8.1, i had no issues at all, but since
> re-installing, i've been getting  what seem like ghost signals on
> airband. They appear from about 114MHz through to 128MHz. They appear
> to be broadcast FM, Trunking and MotoTRBO signals, though when i move
> the frequency slider, they vanish, only to reappear when i move
> further along the band.
>
> I've included a picture of the problem, and my settings in ExtIO:
>
> https://www.dropbox.com/s/o6u3oe83ch4gm9o/Signals.jpg?dl=0
>
> and my settings:
>
> https://www.dropbox.com/s/082d17pi0osnw36/Settings.jpg?dl=0
>
> I'm running the B200 over USB 3, and it's using 003.007.001, which
> unfortunately, i can't seem to upgrade due to getting an
> incompatability warning when trying to use HDSDR after upgrading. 
> I've installed the B200 USB Driver, and in Device Manager, my device
> shows up as USRP B200. 
>
> Does anyone have any idea what could be causing this problem, and any
> possible solution?
>
> Thanks for any help,
>
> Paul
>
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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