Bahn William L Civ USAFA/DFCS wrote:
>
> // SIGNED //
> William L. Bahn
> Instructor of Computer Science
> United States Air Force Academy
> e-mail: [EMAIL PROTECTED]
>
>
>
>>-----Original Message-----
>>From: Eric Blossom [mailto:[EMAIL PROTECTED]
>>Sent: Wednesday, January 17, 2007 4:45 PM
>>To: Bahn William L Civ USAFA/DFCS
>>Cc: gnuradio mailing list
>>Subject: Re: [Discuss-gnuradio] Interpretting results from
>>usrp_rx_cfile.py
>>
>>On Wed, Jan 17, 2007 at 04:37:58PM -0700, Bahn William L Civ
>
> USAFA/DFCS
>
>>wrote:
>>
>>>I have a function generator outputting a sine wave into the RX-B
>>>connector of the BasicRX board connected to the RX-B side of a USRP.
>
> I
>
>>>am trying to capture the waveform and store it to a file.
>>>
>>>Here are the commands I tried:
>>>
>>># ./usrp_rx_cfile.py -R B -d 256 -f 1000 sine_1k1.dat
>>># ./usrp_rx_cfile.py -R B -d 256 -f 10000 sine_1k10.dat
>>># ./usrp_rx_cfile.py -R B -d 256 -f 15000 sine_10k15.dat
>>>
>>>The function generator was producing a 1kHz sine wave for the first
>
> two
>
>>>and a 10kHz sine wave for the third.
>>>
>>>Each time I captured a few seconds worth of data. I then took the
>
> data
>
>>>files and extracted two streams of single precision floats for each
>
> of
>
>>>the first ten thousand 8-byte groups in the file and plotted them.
>
> The
>
>>>results were very similar regardless of the frequency specified on
>
> the
>
>>>command line (and I have no idea what that frequency is for) or the
>>>actual frequency of the signal that was applied to the inputs.
>>>
>>>What I got in all cases was basically a flat line (-3 to +3) except
>
> for
>
>>>in the vicinity of sample #530, which looks like an inverted sinc
>
> pulse
>
>>>with a peak amplitude of about -10000 (the first stream, which is
>
> made
>
>>>up of the first float value in each sample pair), and what looks
>
> like an
>
>>>extremely heavily damped sine wave (the second stream) with a peak
>>>amplitude of about 20,000 and roughly centered at the same place as
>
> the
>
>>>sinc pulse.
>>>
>>>This makes absolutely no sense to be whatsoever. Can anyone shed any
>>>light on this?
>>>
>>>Thanks
>>
>>The Basic RX only passes signals > 100kHz.
>>
>
>
> That helped quite a bit. When I raised the frequency to 200kHz and
> looked at the data, I now saw related data in the captured file after
> the large transient event. The data looked strange, but appeared to have
> a quadrature-like pattern to it. So that suggested to me that the
> frequency specified on the command line is the frequency for the
> oscillator of a sin/cos mixer. Changing the frequency to zero then
> produced a cosine waveform and a flatlined-waveform, which makes sense.
>
> As for the spike at about sample 530, I am now of the opinion that this
> is a switching transient as the board configures the hardware.
>
> I only see data when the signal is applied to the RX-A input, not the
> RX-B (here I am referring to the two inputs on the daughterboard, not
> the daughterboard slots on the USRP. I am still trying to figure out how
> to get data from the other input, but that is fairly low priority.
This is determined by the rx_mux setting uf the usrp.
In most examples this is determined automatically using the parameter you
specify for rx_subdevice_spec with the -R parameter of the example.
(Note that most/all examples only use one ADC if you use the basic_RX)
A gives you channel A of daughterboard A
A:1 gives you channel B of daughterboard A
B gives you channel A of daughterboard B
B:1 gives you channel B of daughterboard B
The magic happens in the line:
self.u.set_mux(usrp.determine_rx_mux_value(self.u, options.rx_subdev_spec))
It is also possible to use both ADCs as a complex (quadrature) input. For this
you have to determine the muxvalue yourself.
I use this peace of code to use the basic_RX as complex device (use both ADCs
as a quadrature input):
self.side = options.rx_subdev_spec[0] #if RX_A 0, if RX_B 1
if 0==self.side:
self.rx_mux_value=0x00000010
else:
self.rx_mux_value=0x00000032
self.u.set_mux(gru.hexint(self.rx_mux_value)) #set basix_RX as complex
device in stead of singlechannel
The documentation of usrp.set_mux(int muxvalue) (see
usrp/host/lib/usrp_standard.h and usrp/host/lib/usrp_basic.h ):
/*!
* \brief Set input mux configuration.
*
* This determines which ADC (or constant zero) is connected to
* each DDC input. There are 4 DDCs. Each has two inputs.
*
* <pre>
* Mux value:
*
* 3 2 1
* 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
* +-------+-------+-------+-------+-------+-------+-------+-------+
* | Q3 | I3 | Q2 | I2 | Q1 | I1 | Q0 | I0 |
* +-------+-------+-------+-------+-------+-------+-------+-------+
*
* Each 4-bit I field is either 0,1,2,3
* Each 4-bit Q field is either 0,1,2,3 or 0xf (input is const zero)
* All Q's must be 0xf or none of them may be 0xf
* </pre>
*/
bool set_mux (int mux);
I hope this helps,
Martin
>
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