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Re: [Discuss-gnuradio] Bluetooth Transmitter using GRC

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Bluetooth Transmitter using GRC
Date: Wed, 14 Jan 2015 21:42:38 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0

Hello Mostafa,

> However, there is another point needed to be noticed and that's the LO
> (local oscillator) capability of the daughterboard. I mean, does
> have the X-series enough ppm (lower than 3 ppm)? 
4€ bluetooth dongles don't have 3ppm accuracy, because real world
systems either are much more forgiving with respects to frequency
offsets and drifts, or because correction takes place all the time.
In the case of a half-duplex system, the transmitter can usually do
nothing but do its best to be stable (a 4€ price tag might make that
hard) in frequency, and let the receiver figure out the necessary offset
correction; often in RF communications, that's already necessary to
account for the doppler effect.

That being said, the X300 has a specification [1] that exceeds 3ppm
oscillator accuracy.
> The LO also shall have suitable switching time too. 
That would contradict what I wrote in my first mail: It doesn't have to
tune at all. If you have a device such as the X3x0 which can cover the
whole band with its physical sampling rate, you can just shift around
your band using the DSP chain, without any intervention of analog parts,
and especially without retuning the LO.

Best regards,

[1] http://www.ettus.com/content/files/X300_X310_Spec_Sheet.pdf

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