Hi Abhinav,
First off: since you want to have a sampling rate of 2kHz, it really
doesn't make sense to use the minimal N210 sampling rate 195312.5 Hz.
Use something that is a multiple of 2kHz, for example 200kHz or 1MHz.
Use a "low pass filter block" with the resulting decimation, the
sampling rate set to the rate coming into the filter, and the cutoff
frequency let's say to (15.0/16.0 * 2e3) which allows for a transition
width of (1.0/8.0*2e3).
The placement of the decimation depends a bit on your actual signal of
interest; generally, the earlier, the better, since that simply
eliminates a majority of noise power. Make sure, however, that the
passband width of the filter is "wide" enough so that the FLL can still
work.
Best regards,
Marcus
On 21.10.2015 22:45, abhinav narain wrote:
> 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] <mailto:[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] <mailto:[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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>>
>>
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