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Re: [Discuss-gnuradio] some questions related to DUC/DDC cordic, USRP tu
Re: [Discuss-gnuradio] some questions related to DUC/DDC cordic, USRP tuning process
Thu, 15 Nov 2012 19:30:12 -0800
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On 11/14/2012 01:11 PM, Zing Yu wrote:
>> The DDC chain follows the same basic rules, but rather, it is
>> controlled by issuing stream commands. So when a receive burst is
>> ended, the CORDIC phase will also reset back to zero.
> Okay. If I understand you correctly, then if I have a GRC flow-graph
> which consists of both uhd usrp source and uhd usrp sink then both
> receive and transmit part of the flow-graph will be streaming samples
> indefinitely. That is, the uhd usrp source keeps on receiving samples
> from fpga and uhd usrp source keeps on sending samples to fpga until
> I kill the flow-graph. Now, if I manage to send bursts equipped with
> tags, the transmit side operates in "send N samples and done" mode
> but the receive side of the flow-graph should still be running in
> continuous mode. That is only the DUC cordic will reset but not the
> DDC cordic (in my case). Am I right?
Correct. The receiver chain and transmit chain are 100% independent.
Now, it is possible to have discontinuous reception. The API to issue
stream commands to the usrp source block is exposed:
>> The step size of the RF frontend depends on the synthesizer on the
>> daughterboard, the reference clock (from the motherboard), and
>> frequency requested. Its not possible to give a general rule. But
>> the driver can tell you what is possible.
>> If it helps, boards like RFX use integer N synthesizers, so the
>> step size is rather large, several MHz. Boards like SBX and WBX
>> use fractional N, the step size is on the order of kHz.
> Okay. I have done the following:
> set receive center RF frequency to 892MHz, actual RF frequency was
> 892857142.857. set receive center RF frequency to 896MHz, actual RF
> frequency was 896428571.429. I then took the difference 896428571.429
> - 892857142.857 which was 3.571428572 MHz. So, it means ~3.5 MHz is
> the step size for my SDR (RFX900 + USRP N200). Right? But that step
> size is not an integer value. Isn't it weird?
I dont think so. For the curious, if you turn on the logging you can see
what values the tune code is calculating.