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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.
-josh