Re: [Discuss-gnuradio] Why does a SINE wave have a real part?

[cswiger]

On Fri, 26 Aug 2005, sudhindra aithal kota wrote:

> Hi,
>  I am running the usrp_siggen.py program with the
> option -m 0x98. The signal transmitted is a SIN wave.
> My understanding of this option is, Channel 0's I part
> is connected to DAC0 and Channel 0's Q part is
> connected to DAC1.

That is correct.


> Since for a SIN wave the real part
> does not exist, there should not be anything
> transmitted via DAC0.

Whoa - 'SIN' is just the shape and our signal can have
real and imag components. Maybe confusing the real/cosine imag/sin
analysis with our real world signal ?
In usrp_siggen.py the signal source is complex
  self.siggen = gr.sig_source_c, and the signal sink is
complex   self.usrp = usrp_sink_c

Actually we were just discussing, due to the digital-upconverters
on the 9862 chips, sending a signal to either DAC will result
in output from both, even if one channel is all zeros.

>  Then I connect the TX and RX part of the USRP board
> and use the usrp_oscope.py function with the option -m
> 0x33333310 to view the signal I have transmitted. This
> would mean connect ADC0 to the I part of Interpolator0
> and ADC1 to Q part of Interpolator0.

I think you mean digital-downconverter, not Interpolator0,
those are in the Tx path - were in the Rx path now. Otherwise
yes, ADC0 to the I input of DDC0 and ADC1 to the Q input.
Do you have both two cables linking the boards or one? ADC1
gets it's input from J18.

> I see 2 signals
> with a phase difference of 90. Where is the second
> signal coming from, when I am transmitting only one
> signal?
>

the source in usrp_oscope.py is also complex from the usrp
u = usrp.source_c  and with 1 channel is getting the data
from digital-downconverter 0.   Interestingly, with these
arguments:

./usrp_oscope.py -d 64 -c 1e6
./usrp_siggen.py -c 1e6 -f 10e3 -i 64

with only one cable from J19 to J48 there's a big 450 unit
signal, with another cable from J18 to J49 connected it
becomes small with an amplitude of 25 ;)

I'm still exploring the possibilities myself.

--Chuck