Tx path questions


#1

Hello,

I have a doubt about coarse modulation in AD9862 (tx path).
I saw in the faq (http://gnuradio.org/trac/wiki/UsrpFAQ/ADC) “…Block
B
only moves the signal +/- fs/4 or fs/8”…
OK, but when it moves fs/4 and when it moves fs/8? Is that controllable
by
software?

Another question:
Is the DAC frequency (fdac in the AD9862 datasheet) the same as
target_freq parameter in the tune function?

Thank you

Regards


Ronaldo A. Viera Nunez
Acadêmico/Engenharia Elétrica
Universidade Federal de Santa Maria

Santa Maria - RS - Brasil


#2

On Mon, Jan 05, 2009 at 09:48:47AM -0300, Ronaldo N. wrote:

Hello,

I have a doubt about coarse modulation in AD9862 (tx path).
I saw in the faq (http://gnuradio.org/trac/wiki/UsrpFAQ/ADC) “…Block B
only moves the signal +/- fs/4 or fs/8”…
OK, but when it moves fs/4 and when it moves fs/8? Is that controllable by
software?

It’s transparently controlled by software. Look at usrp_standard.{h,cc}

Another question:
Is the DAC frequency (fdac in the AD9862 datasheet) the same as
target_freq parameter in the tune function?

fdac is a constant, 128MHz. target_freq is the RF frequency that you
desire to have appear at 0 Hz in the complex baseband.

Eric


#3

Thank you for your answer Mr. Blossom,
So, the tx path is something like this:

PC --> USB --> CIC(FPGA) --> AD9862 --> Daughter Board --> Antenna

There is two frequencies involved in all transmissions:

  • A signal frequency (f_signal), generated by sig_source_c().
  • A RF frequency (f_tx), set with target_freq in tune().

The signal (which frequency is f_signal) is interpolated by a factor
between
4 and 512 on CIC interpolator,
after it is up converted to the RF frequency (by the NCO on fine
modulator),
so it is moved ± fdac/4 or fdac/8 on coarse modulator.
The signal will be converted to analog (DAC) and amplified and
transmitted.

Be free to correct me, I need to know exactly how USRP and GNUradio
works.

Thanks for all!

Best regards,

Ronaldo N.

2009/1/5 Eric B. removed_email_address@domain.invalid


Ronaldo A. Viera Nunez
Acadêmico/Engenharia Elétrica
Universidade Federal de Santa Maria

Santa Maria - RS - Brasil


#4

On Tue, Jan 06, 2009 at 10:41:22AM -0300, Ronaldo N. wrote:

4 and 512 on CIC interpolator,
after it is up converted to the RF frequency (by the NCO on fine modulator),
so it is moved ± fdac/4 or fdac/8 on coarse modulator.
The signal will be converted to analog (DAC) and amplified and transmitted.

Be free to correct me, I need to know exactly how USRP and GNUradio works.

Yes, you’ve got the basic path right.

Please see also the USRP FAQ, available here

http://gnuradio.org/trac/wiki/UsrpFAQ

or as a pdf

http://gnuradio.org/trac/attachment/wiki/UsrpFAQ/USRP_Documentation.pdf

Eric