Hi Marcus, I see your point, and I can add intepolation factor as 196K/2K =98 in my transmitter.
Can you please tell me about what should I do in my receiver now that I
have to do decimation.
I have the exact grc file attached.
I am doing :
USRP Source-> FLL Band Edge-> Polyphase Clock Sync with RRC RX filter [{
firdes.root_raised_cosine(ntaps,samp_rate,sps, rolloff, int(11*sps*ntaps)) }
corresponding to tx filter of {firdes.root_raised_cosine(ntaps, samp_rate,
sps, rolloff, int(11*sps*ntaps))} ]
-> Costas Loop -> MultiplyConst->Constellation Receiver ...
I do have to place a decimation filter I suppose with factor 196/2 (= same
as in interpolator filter), but the polyphase Sync block doesn't have a
Decimation factor in it.
In case I place Decimation block before FLL BandEdge block, I need to have
tap coefficient and I am not sure what should they be ?
I don't know how i can calculate square root of RRC taps and place those
taps in both - Decimation block and Polyphase block.
Please suggest.
Thanks,
Abhinav
On Wed, Oct 21, 2015 at 11:51 AM, Marcus Müller <[email protected]>
wrote:
> Hi Abhinav,
>
> I think most of your questions have been answered in my reply to your
> reply to Sylvain's mail; basically, yes, you need to use a sampling rate
> >=196kHz, but no one is stopping you from putting any signal into that
> sampling rate that has a smaller bandwidth. See my comments on oversampling.
>
> Best regards,
> Marcus
>
> On 10/21/2015 08:37 PM, abhinav narain wrote:
>
>
> Hi Marcus,
>
> On Wed, Oct 21, 2015 at 3:18 AM, Marcus Müller <[email protected]>
> wrote:
>
>> Hi Abhinav,
>>
>> which USRP are you using?
>>
> USRP N210
>
>
>> I'm pretty sure none of our devices supports a 2kS/s sampling rate --
>> it's just far too low to get directly interpolated to a "usable" DAC/ADC
>> rate. Especially those USRPs that can be equipped with RF frontends
>> (daughterboards) that can operate at near-baseband (400kHz) shouldn't be
>> able to work at such low rates. So: which daughterboard are you using?
>>
>> I see your point, I am using Basic TX/RX daughterboard.
> I didn't notice the shell output but it makes sense now :
>
> Terminal output:
> The hardware does not support the requested TX sample rate:
> Target sample rate: 0.001000 MSps
> Actual sample rate: 0.195312 MSps
> -- Tune Request: 0.400000 MHz
> -- The RF LO does not support the requested frequency:
> -- Requested LO Frequency: 0.400000 MHz
> -- RF LO Result: 0.000000 MHz
> -- Attempted to use the DSP to reach the requested frequency:
> -- Desired DSP Frequency: 0.400000 MHz
> -- DSP Result: 0.400000 MHz
> -- Successfully tuned to 0.400000 MHz
>
>
> Basically, the minimum sampling rate is 196 Ksamples/sec
>
> Hence, UHD should have told you that you can't use that sampling rate, and
>> a higher sampling rate was automatically selected, inherently
>> frequency-"stretching" the signal by the ratio of (actual rate/2kHz).
>>
>> You should heed Sylvain's advice. Just oversample your signal by using an
>> interpolating FIR filter to something that the USRP can happily work with.
>>
>> So, here is what I am not clear about- I should use 196 KHz as sampling
> rate ?
> So, the minimum BW of my signal will 196/2 =98KHz, and that is the best I
> can do with this equipment, right ?
>
>
>
>
>
>> Best regards,
>> Marcus
>>
>> On 21.10.2015 08:49, abhinav narain wrote:
>>
>> Hi,
>> I am transmitting using the flowgraph: vector src-> FIR interpolator
>> (with RRC filter as below) -> MultiplyConst -> USRP Block
>>
>> FIR filter - firdes.root_raised_cosine(32, samp_rate, sps, 0.55,
>> int(11*sps*32)), with
>> center freq= 400kHz
>> samp_rate= 1k
>>
>> I am listening using other USRP with
>> center freq= 400kHz
>> samp_rate= 2k
>>
>>
>> You can see the spectrogram at the receiver. I want to do narrowband(1
>> kHz) transmission hence I am keeping the sampling freq at transmitter at 1k.
>>
>> While I increase the signal amplitude(using MultiplyConst block with
>> slider), I see there is more and more spillage of energy in the neighboring
>> frequencies!
>>
>> Is there a way to remove that spillage from the transmitter and have a
>> cleaner transmitter ?
>> If not, how can I atleast mitigate it to the minimum ? is there something
>> I can do with RRC filter parameter or any other way ?
>>
>> In the figure: 1 - when amplitude is large; 2 - when amplitude is turned
>> down
>>
>> Thanks,
>> Abhinav
>>
>>
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>
gnuradio_question.grc
Description: application/gnuradio-grc
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