Re: [Discuss-gnuradio] question about mux

[Martin Dvh]

Hi Chuck,
>I'm expecting
> to get a 1Khz signal in one channel and 2Khz in another channel,
> but end up with the two getting mixed etc.
>Now if I disconnect the wire connect J19 to J48 ch1 & ch2 indeed flatline as 
expected.
How do ch 3 & 4 look when you disconnect the wire connect J19 to J48.

You only set the tx_freq for channel 0
sink.set_tx_freq(0,5e6)

You should also do
sink.set_tx_freq(1,5e6)

This means you output frequencies of 5.001 Mhz and 5.002 MHz.

On the receive side you then also have to set the DDC frequency to 5Mhz to get 
back to 1 kHz and 2 kHz.

I have been playing with the usrp_oscope and found I got similar signals when I 
used high interpolation and DDC freq set to zero using just one channel.

I used an external source (signal generator) which I connected to J19.

source few hundred kHz, DDC freq set to 0, decimation 128

But of course you can't get a few hundred kHz if you use a decimation of 128.
This should give me a sinus at a mirror frequency, or nothing at all, but I got 
a signal similar to what you saw.
(Probably it is a sinus but I don't get the complete signal, so the next period 
starts before the previous period is finished or something like that)
I think the cordic doesn't handle this setup like I expected.

The signals started to look good when I used  a DDC freq which was close to the 
signal frequency
(and lower decimation factors and  higher frequencies.)
source 4.433619 Mhz, DDC freq set to 4.433618 MHz, decimation 32
I got a nice 1 kHz complex sinus output.

greetings,
Martin

cswiger wrote:
> Hi - this may be more involved that anyone wants to get into,
> just asking in case I'm missing something obvious.
>
> I'm trying to make two seperate circuits go thru the usrp - but
> they're are interacting somewhere.  Anyway here's a web page
> showing the test setup and unexpected results - I'm expecting
> to get a 1Khz signal in one channel and 2Khz in another channel,
> but end up with the two getting mixed etc.
>
> On the TX side, two complex sig sources are interleaved and
> the usrp is setup for 2 channels and mux set to send one signal
> to DAC0 and the other to DAC1. J48 and 49 are looped back to
> J19 and 18. The oscope is setup for 2 channels and mux set to
> send ADC0 to DDC0 and ADC1 to DDC1.
>
> http://webpages.charter.net/cswiger/usrp_diagrams/question.html
>
> Thanks
>
> --Chuck
>
>
>
>
>
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