[USRP-users] Newbie Issue

gsmandvoip gsmandvoip at gmail.com
Wed Jul 8 07:55:11 EDT 2015


Thanks Marcus, Tom for response.
It is happening when I am trying to monitor GSM spectrum, happens with all
frequency range.
Here is how it came in.
I am trying to use airprobe to monitor Sys Info of my surrounding BTS,
which I am able to do successfully  but when I am trying to analyze with
telit modem I see more BTS around, also I see those those frequencies in
Kaliberate but when I tune to it sometimes it is decodable and sometimes
not, which I blame to signal strength.
But here my point is if modem is able to get exact frequency and says it
has Beacon channel which I verified using airprobe, what level of
sensitivity do I need in order to keep USRP tuned??

Things get worse if I change gain of DBSRX, so I presumed it works best at
50 (±5) which is default.

My first guess for this random behavior was 52 MHz external clock installed
on it, but I get even more errors when I use onboard clock in another USRP
with same configuration.

So external clock is making things better but not as per my expectations

Also I want to learn more about PPM factor and its role in signal reception
or digitization. googled, but could not find from where should I start

Would you please guide me through my issue ref. signal reception using
USRP, would love to learn step by step









On Wed, Jul 8, 2015 at 12:37 AM, Tom Tsou <tom.tsou at ettus.com> wrote:

> On Tue, Jul 7, 2015 at 12:45 AM, gsmandvoip via USRP-users
> <usrp-users at lists.ettus.com> wrote:
> > I have used USRP1 + DBSRX1+LNA for a while with modified clock (52 MHz)
> in
> > order to work with GSM signals but I use to get lot of noise along with
> my
> > interested signals.
> > I want to know which factor is responsible to this noise??
> > Is it happening due to 20ppm calibration rate of USRP??
>
> If you are driving a USRP1 with a 52 MHz clock, the accuracy of the
> stock oscillator is not relevant. If it's phase noise you are seeing,
> then the modified clock is likely to blame. Other sources of
> noise/interference will be difficult to ascertain without additional
> information. Can you describe the noise you see in the frequency
> domain?
>
>   -TT
>
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