Re: [Discuss-gnuradio] Basic USRP AM Transmission

[Martin Dvh]

Robert Roberts wrote:
> I added an interpolating FIR filter (used a lowpass filter) that stepped the 
> signal rate back up to 128mS/s, but it does not sounds any better. I have 
> been 
> playing with the interpolation value, as well as the filter values.

Since the bandwidth of the usb bus is limited, the maximum samplerate you can 
send to the usrp is 8Ms/s
Since the interpolation factor of the usrp is also limited (I think max 256) 
the min samplerate you can send to the usrp is 128/256 = 500 kS/s
Usrp interpolator must be in [4, 512] and a multiple of 4
So the min samplerate you can send to the usrp is 128/512 = 250 kS/s
The bandwidth of the signal of interest also has to fit in the samplerate.
And then you also need integer interpolation factors (unless you use a rational 
resampler)
usb bus rate:              min 0  max 8 MS/s
usrp interpolator factor : min 4, max 512
DAC         :              min 128 MS/s max 128 MS/s

So I would come to the following:
src                                                 32 kS/s
software interpolator:  interpolation factor 10 => 320 kS/s
usrp interpolation factor                   400 => 128 MS/s

Also pay attention to signal levels, and any filter parameters you use.

If this all still doesn't help.
Try what happens if you use a non DC baseband freq.
(That is, multiply your 320 kS/s signal with for example a 100 kHz complex sine 
(gr_sig_source_c))


Also make sure that the signal you use as input is not bigger then the const 
you add.
You add 1.0, which would mean that your input signal should be between -1.0 and 
1.0.
You can check with a scopesink.


Greetings,
Martin

> 
> I'll keep chugging away.  Thanks for your help!
> 
> ~Chris~
> 
> ** 
> 
> *----- Original Message -----*
> 
> *From*: Martin Dvh <address@hidden>
> 
> *Date*: Tuesday, April 18, 2006 1:01 am
> 
> *Subject*: Re: [Discuss-gnuradio] Basic USRP AM Transmission
> 
>  > Robert Roberts wrote:
>  > >
>  > > ----- Original Message -----
>  > > From: Martin Dvh <address@hidden>
>  > > Date: Monday, April 17, 2006 3:06 pm
>  > > Subject: Re: [Discuss-gnuradio] Basic USRP AM Transmission
>  > >
>  > >>Robert Roberts wrote:
>  > >>
>  > >>>Hello everyone,
>  > >>>
>  > >>>
>  > >>>I have been experimenting with the Flex400 board and have been
>  > >>
>  > >>trying to
>  > >>
>  > >>>implement a basic AM transmitter. I have a WFM and NFM transmitter
>  > >>>working, but I cannot get the AM one to transmit correctly.
>  > The
>  > >>
>  > >>code> below generates a much higher frequency tone. Any advice
>  > to
>  > >>what I am
>  > >>
>  > >>>doing wrong? Does my output need filtering befor! e connecting
>  > to
>  > >>
>  > >>the> sink?
>  > >>
>  > >>>
>  > >>>self.u = usrp.sink_c () # the USRP sink
>  > >>>
>  > >>># Code here for setting up the USRP, omitted
>  > >>>
>  > >>>src = gr.file_source (gr.sizeof_float, "audio-1.dat", True) #
>  > >>
>  > >>440Hz tone
>  > >>
>  > >>>file
>  > >>
>  > >>What is the sample_rate of the audio file
>  > >>What is you interpolation rate of the usrp
>  > >>What is the duc frequency of your usrp.
>  > >
>  > >
>  > > I have the file source sampled at 32kS/s
>  > >
>  > > I use the following code for setting up my usrp:
>  > >
>  > > self.dac_rate = self.u.dac_rate() #
>  > 128 MS/s
>  > > self.usrp_interp = 400
>  > > self.u.set_interp_rate(self.usrp_interp)
> &! gt; > self.usrp_rate = self.dac_rate / self.usrp_interp #
>  > 320 kS/s
>  > > self.sw_interp = 10
>  > > self.audio_rate = self.usrp_rate / self.sw_interp #
>  > 32 kS/s
>  > >
>  > > # determine the daughterboard subdevice we're using
>  > > if options.tx_subdev_spec is None:
>  > > options.tx_subdev_spec = usrp.pick_tx_subdevice(self.u)
>  > >
>  > > m = usrp.determine_tx_mux_value(self.u,
>  > options.tx_subdev_spec)> self.u.set_mux(m)
>  > > self.subdev = usrp.selected_subdev(self.u,
>  > options.tx_subdev_spec)> print "Using TX d'board %s" %
>  > (self.subdev.side_and_name(),)>
>  > > self.subdev.set_gain(self.subdev.gain_range()[1]) #
>  > set max
>  > > Tx gain
>  > > self.set_freq(options.freq)
>  > > self.subdev.set_enable(True) #
>  > enable> transmitter
>  > >
>  > >
>  > > This is the same code I used for both a! NFM and WFM transmitter,
>  > both of
>  > > which appeared to transmit without problems.
>  > But do you actually put a software interpolator in there somewhere.
>  > I didn't see any in your code:
>  > >>>self.connect (src, const)
>  > >>>self.connect (const, conv)
>  > >>>self.connect (conv, gain)
>  > >>>self.connect (gain, self.u)
>  > should be:
>  > self.connect (src, const)
>  > self.connect (const, conv)
>  > self.connect (conv, gain)
>  > self.connect (gain, softwareinterpolator)
>  > self.connect(softwareinterpolator,self.u)
>  >
>  > greetings,
>  > Martin
>  > >
>  > > Sincerely,
>  > > ~Chris~
>  > >
>  > >>If the sample rate of your audio file is for example 48000 Herz
>  > >>Then the usrp interpolation rate should be 128000000/48000 = 2666
>  > >>Which is I think way out of range for the usrp
> &! gt; >>You first have to interpolate in software to something in the
>  > range
>  > >>of the usrp.
>  > >>
>  > >>Greetings,
>  > >>Martin
>  > >>
>  > >>>const = gr.add_const_ff (1.0)
>  > >>>conv = gr.float_to_complex()
>  > >>>
>  > >>>
>  > >>>gain = gr.multiply_const_cc (4000.0) # transmitter gain
>  > >>>
>  > >>># connect it all
>  > >>>
>  > >>>self.connect (src, const)
>  > >>>self.connect (const, conv)
>  > >>>self.connect (conv, gain)
>  > >>>self.connect (gain, self.u)
>  > >>>
>  > >>>
>  > >>>
>  > >>>
>  > >>>Thanks,
>  > >>>~Chris~
>  > >>>
>  > >>>
>  > >>>
>  > >>>
>  > >>>__________________________________! _____________
>  > >>>Discuss-gnuradio mailing list
>  > >>>address@hidden
>  > >>>http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>  > >>>
>  > >>
>  > >>
>  > >
>  >
>  >