Re: [Discuss-gnuradio] IF value of the RF front end

[Ryan Connelly]

I think I can clarify things a bit. The WBX and SBX default to what is
called direct conversion or homodyne mixing (see
http://en.wikipedia.org/wiki/Direct-conversion_receiver). I'm not sure
about this, but also if you offset the LO as per Nick's link, you
effectively make those boards do superheterodyne mixing or IF
conversion.

The TVRX2 is set to a fixed IF or superheterodyne mixing and you
cannot adjust the LO offset (need confirmation?). It also has AGC
always on.

The basic boards must use undersampling techniques with proper
bandpass filters to sample signals at frequencies higher than their
bandwidth (see page 5 of
http://www.pentek.com/deliver/TechDoc.cfm/DgtlRcvrHbk43.pdf?Filename=DgtlRcvrHbk43.pdf)

At the end these variations don't matter much because when the signals
pass through the DDC stage in the FPGA, they all end up being moved to
baseband and so look the same.

R

On Wed, Jul 11, 2012 at 8:36 AM, Jamie Wo <address@hidden> wrote:
> Hi Nick,
>
> Thanks for your reply.
>
> But I am not sure whether I understand what you said. My understanding is
> that the WBX converts RF signal into baseband signal directly, while for
> TVRX2, the RF signal is converted into low IF signal firstly by TVRX2 and
> then converted into basebad signal by USRP's FPGA. Am I right?
>
> Then, what is the sampling rate should I set to sample the signal of
> interest. For example, FM radio signal at 106.3MHz.
>
> Best regards,
> Jamie
>
> On Wed, Jul 11, 2012 at 12:34 AM, Nick Foster <address@hidden> wrote:
>>
>> Jamie,
>>
>> WBX is a direct-conversion receiver, meaning there is no IF: it is a
>> quadrature receiver where the LO frequency is equal to the RF frequency.
>>
>> TVRX2 is a low-IF receiver using a 12.5MHz IF.
>>
>> Both daughterboards can be treated as zero-IF direct conversion receivers
>> for tuning purposes, as the digital downconverter in the USRP's FPGA will
>> downconvert any residual IF to baseband before returning samples to the
>> host. In other words, when you tune the USRP to 2.2GHz, the resulting
>> samples will have a center frequency of 2.2GHz.
>>
>> If you have a need to tune the LO frequency separately from the DDC/DUC
>> frequency, you can do that using the advanced tuning parameters:
>> http://files.ettus.com/uhd_docs/manual/html/general.html#two-stage-tuning-process
>>
>> Best,
>> Nick
>>
>> On Mon, Jul 9, 2012 at 11:14 PM, Jamie Wo <address@hidden>
>> wrote:
>>>
>>>
>>> Hi all,
>>>
>>> As far as I know the maximum sampling rate for USRP N210 is 25M sps. To
>>> receive signal with high frequency, such as 300 MHz to 2.2 GH, the RF front
>>> end converts the RF into IF, so the ADC can sample the IF signal. My
>>> question is what  the IF value is for daughter-board WBX?  Where can I reset
>>> the immediate frequency of the daughter-board, such as WBX and TVRX2?
>>>
>>> It may be a simple question, but I cannot find the answer. Any
>>> explanation will be appreciated.
>>>
>>> Best regards,
>>> Jamie
>>>
>>> _______________________________________________
>>> Discuss-gnuradio mailing list
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>>>
>>
>
>
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