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

[Josh Blum]

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.

-josh