On Feb 28, 2007, at 4:29 AM, seph 004 wrote:
Well, I have implemented a bit of a strange setup which seems to be working somewhat now. I'm not really trying to view only the first few samples, I'm currently trying to track where I might of made a mistake with trying to generate a defined pulse with a specific length. My aim is to generate a very narrow band chirp to transmit on a little ultrasonic tranducer. I figured out which numerical values to the gr.modulation block produce which frequencies. Using this, I made a vector of the exact number of samples I want with the values to produce the frequencies I want. In my case 39-41 kHz. For some reason these low frequencies still make it through the transformer on the basic TX db, so I'm using them. Initially I was just generating a sine wave, and trying to use gr.head to limit the number of samples, but I've since switched to this approach
I figured that if I wanted to produce a wave form lasting 10 msecs, then I would need at least 2500 samples. I got this number from the DAC rate of 128M and the highest interpolation factor of 512. When I test with the scope set to auto store, I spot the waveform, but it is only 1 msec long. So either I've lost a lot of samples somewhere, or my scaling is wrong.
How are you creating the flow graph?
I pretty much abandoned using gr.head, as it wasn't producing anything. At least now though I can see a waveform, though my scaling seems to have gone wrong somewhere. When you say you used a looback, do you mean you connected the TX db output and RX db input, and used the gr.oscope block to view your signals?
----- Original Message ----
From: Lee Patton <address@hidden
To: seph 004 <address@hidden
Sent: Tuesday, February 27, 2007 12:58:31 AM
Subject: Re: [Discuss-gnuradio] The shortest pulse length
On Mon, 2007-02-26 at 00:55 -0800, seph 004 wrote:
> I have a vector source producing a sine wave, and then I'm using
> gr.head to limit the number of samples sent.
I'm sure you checked this, but are you trying to capture the first few
samples of sin(x)? I so, sin(x) = x for small angles, and sin(0)=0.
So, you won't see anything in the first few samples anyway.
> From your explanation, I should be ok with even a low number of
> samples. When I tested my setup, I couldn't catch anything on the
> scope. There is probably some problem in how I made the app.
> I saw something mentioned elsewhere in the discussion archives that
> the usrp dumps the first few samples it receives from the host before
> transmitting. Is this still something to take note of?
I don't know whether or not the USRP dumps the first few samples. I
don't think I've ever experienced it though. I can say that there is an
unpredictable delay from the generation of the first sample in software
until the time it actually reaches the output port.
I haven't tried to do what you're doing -- i.e., capture the first few
output samples on a scope. How is the scope triggered? (What I did was
create a loopback whereby I transmit and receive by reading the
bleedover on the daugherboard.)
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