Re: [Discuss-gnuradio] Decimation questions

[Brian Padalino]

On Mon, Aug 11, 2008 at 6:28 AM, Sebastiaan Heunis <address@hidden> wrote:
> Now I would like to know if anyone could please help me with the following:
>
> 1.  When we use a tv_rx daughterboard, the FR_RX_FREQ_0 value is set
> to -20MHZ, right?  From what I know it seems as if the Microtune
> module mixes the signal down to 20MHz.

A slight clarification - FR_RX_FREQ_0 is a register address.  The FX2
writes to it using the SPI interface.  You can see the whole thing
here:

    
http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/fpga/sdr_lib/phase_acc.v

Other than that, I don't think all the daughterboards have the same
tuner in them, so the IF's might be different - but what you say
sounds good to me.

> 2.  Where does FR_RX_PHASE_0 come from?  Is this 0 because we start on
> the x axis with zero degrees phase when using the cordic algorithm?

It comes from the host.  Check the host code for more information
about what is set in this register, but I am pretty sure it's most
likely 0 as it's just a phase offset which can be compensated for
elsewhere in the host.

> 3.  sample_strobe gets set to 1 in master_control.v?  decimator_strobe
> is a pulse signal with 64MHz/(decimation_rate-1) pulses per second?  I
> assume that this has something to do with the rate changer in the CIC
> filter that drops the sampling rate from 64MHz.  I would just like to
> know why we use rate-1 in the strobe generator part in master_control?
>
> I'm just a little confused with the strobe signals.  Logic tells me
> that if we're decimating by 16, we have a decimation of 8 in the CIC
> and 2 in the HBF, so strobe_decim should be 64MHz/8 and hb_strobe
> should be 64MHz/(8*2).

The (rate-1) comes from zero-based indexing, I believe.  The downcount
in the strobe generator goes from (rate-1) down to 0 and pulses when
it hits 0, then resets itself.  Since it's comparing to 0 and not 1,
the starting value is (rate-1).

As for the strobe signals, I don't see anything wrong.  You are
correct - why do you think you're wrong?

Brian