Re: [Discuss-gnuradio] [USRP-users] Can use WBX to receive dual channel?

[Scott Johnston]

I used the following tune commands.

self.uhd_usrp_source_0.set_center_freq(uhd.tune_request(tun_freq,
dsp_freq=-10e6, rf_freq=tun_freq+10e6, 
dsp_freq_policy=uhd.tune_request.POLICY_MANUAL,rf_freq_policy=uhd.tune_request.POLICY_MANUAL), 
0)
 
self.uhd_usrp_source_0.set_center_freq(uhd.tune_request(tun_freq-.1e6, 
dsp_freq=-10.1e6,rf_freq=tun_freq+10e6,dsp_freq_policy=uhd.tune_request.POLICY_MANUAL,rf_freq_policy=uhd.tune_request.POLICY_MANUAL), 
1)

My understanding is that these command will tune the RF front end to  
tun_freq+10 MHz, and then DSP 0 will shift by 10 MHz back to tun_freq 
and DSP 1 will shift by 10.1 MHz to be 100kHz less than tun_freq.

Do you see anything wrong?

Thanks

Scott


Josh Blum wrote:
> On 01/13/2012 11:59 AM, Scott Johnston wrote:
>   
> > Hi Josh,
> >
> > I am attempting to do what you described below, but I am seeing two
> > problems. First, I see a power difference of about 30 dB going from one
> > channel to two. Second, when I tune the second channel 100kHz away from
> > the first, the first channel moves 100kHz and the second moves by 200kHz.
> >
> > The one_channel picture is from the standard uhd_fft.grc in the examples
> > folder. I am generating the signal by using an FRS walky-talky.
> >
> > Thanks for any help
> >
> >     
>
> So, while you can receive two channels, you still only have one RX
> frontend to tune. You need to be careful when you tune. Suppose you want
> one channel at f0 and another at f1, and LO to be at the center:
>
> tr = uhd.tune_request(f0, rf_freq=(f0 + f1)/2,
> rf_freq_policy=uhd.tune_request.POLICY_MANUAL)
> usrp.set_center_freq(tr, 0)
>
> tr = uhd.tune_request(f1, rf_freq=(f0 + f1)/2,
> rf_freq_policy=uhd.tune_request.POLICY_MANUAL)
> usrp.set_center_freq(tr, 1)
>
> So, there is more than one way to tune everything, but this should give
> you a good idea (and you can do uhd.tune_request in grc BTW).
>
> It also dawned on me that it would be a lot less verbose if the whole
> POLICY_MANUAL thing was set automatically and not specified by the user
> since I'm sure thats what the user wants by specifying the rf_freq
> explicitly in the first place. Anyhow...
>
> -Josh
>