Generating complex exponential stream

Hi,

I’am new to GNURadio. I started off writing a few signal processing
blocks
(since I’am more comfortable with C++ than Python).

So far, I have written a signal block for circular correlation (to get
an
idea of the entire process) and a signal source that gives off C/A Code
samples (to be used with acquisition) and both work as expected.

I’am now trying to use these block in Python and am kinda stuck.

The basic signal flow I’am trying to achieve is as follows:

===================== ============== ===============
| File Source | --------> | Multiplier |------> | Vector Sink |
| | ============== ===============
===================== ^
|
|
============== ==============
| CA Samples |------------> | Multiplier | <---- e^{-j2pifdt}
============== ==============

The CA samples block, is a source block that take the sampling
frequency,
and doppler frequency and samples the CA code, accordingly.

The questions I have are,

  1. How do i obtain the time t, to be able to generate e^{-j2pifdt}.
  2. How do I make sure the multiplier is multiplying the first value from
    CA
    samples with the first value of e^{-j2pifdt} and so on. Does
    GNURadio
    take care of this?

Any help/pointers will be appreciated.

Thanks,
Praveen

On Mon, Mar 21, 2011 at 6:53 PM, Praveen Vikram
[email protected]wrote:

Hi,

I’am new to GNURadio. I started off writing a few signal processing blocks
(since I’am more comfortable with C++ than Python).

Welcome

| File Source | --------> | Multiplier |------> | Vector Sink |
and doppler frequency and samples the CA code, accordingly.

The questions I have are,

  1. How do i obtain the time t, to be able to generate e^{-j2pifdt}.

It’s a software defined radio. The notion of time is irrelevant as all
you
are sending in and out of the system is samples that are produced by a
software module.

for example, if I want to generate 1000 samples of a sine wave with
frequency 1e3Hz and sampling frequency 1e6Hz, I do the following in
software.

int i;
double sine[1000];
double fc = 1000;
double fs = 1000000;
doubel ts = 1/fs; // <— This is my sampling width
for(i=0;i<1000;i++) {
sine[i] = sin(2PIfctsi);
}

This gives me 1000 samples of 1000Hz sine wave sampled at 1MHz but what
you
actually have is a bunch of numbers than represent the sine wave
described
earlier. So where is the time here? The time is the product of the
sample
number and the sampling width i.e. ( i * ts ).

  1. How do I make sure the multiplier is multiplying the first value from
    CA

samples with the first value of e^{-j2pifdt} and so on. Does GNURadio
take care of this?

In any GNU Radio block the behaviour of the block is defined by the work
function ( see gr_block.h ). If you look a gr_multiply_cc.cc file you
will
see that inside the work function the samples are multiplied
synchronously.
That means that you don’t have to worry about anything as GNU Radio does
what it is saying it is doing.

Any help/pointers will be appreciated.

Whenever you have a doubt about any gnuradio block your first stop
should be
the source code. It’s very well written and properly documented and it
saves
you time to actually do that rather than waiting for one of us on the
forum
to reply to your question. Nonetheless, this forum is a great place to
get
your questions answered.

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