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Re: [Discuss-gnuradio] some questions related to DUC/DDC cordic, USRP tu

From: Josh Blum
Subject: Re: [Discuss-gnuradio] some questions related to DUC/DDC cordic, USRP tuning process
Date: Wed, 14 Nov 2012 12:38:52 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121028 Thunderbird/16.0.2

On 11/14/2012 12:15 PM, Zing Yu wrote:
> Hi Josh and others,
> I have some more questions related to DUC/DDC cordic, USRP tuning
> process:
> 1) When an eob tag is issued, the DUC phase accumulator on transmit
> DSP chain is reset. What about the DDC phase accumulator on receive
> DSP chain? Does it reset at well? I am asking because I have seen
> that the receive DSP chain and transmit DSP chain apply independent
> freq corrections in order to tune to the desired center RX and TX
> frequencies. In other words, is there any relation/binding between
> the DUC cordic and DDC cordic? I hope two separate/independent phase
> accumulators are used for DDC and DUC cordic's but I want to verify.

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.

> 2) What is the tuning granularity of RF front end? I can see from
> USRP FAQ on gnuradio.org that it is 4MHz for RFX boards. 
> http://gnuradio.org/redmine/projects/gnuradio/wiki/UsrpFAQDBoards#How-is-RF-tuning-accomplished-in-daughterboards
>  Is step size dependent on the daughterboard, mother board or both? I
> am asking because when I set my USRP N200 to transmit at say 892MHz,
> I observe the below values: Actual RF freq: 892857142.847963 Hz 
> Actual DUC DSP freq: - 857142.847963 Hz
> What does an RF frequency of 892857142.847963 tell about the step
> size of RF front-end (which is RFX900)?

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.


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