RSPA1 : Sampling rate vs Bandwidth

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Hayfa
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Joined: Mon May 20, 2019 12:39 pm

RSPA1 : Sampling rate vs Bandwidth

Post by Hayfa » Mon May 20, 2019 1:02 pm

I would like to know if in the case of the RSPA1 receiver the used sample rate is different from the the bandwidth (When i used other types of receivers , the rtl-sdr for example, these two value are equal).
In the case that the sample rate and the bandwidth must be equal, am i limited to the values enumerated in the API (200k, 300k, 600k, 1536k, 5000k, 6000k, 7000k, 8000k)?
I'm asking this question because according to the RSPA1 gnuradio block , i can select the bandwidth from a limited list of values while i still can choose a different sample rate .
[img]
RSPA_gnuradio_sourceblock.PNG
RSPA1 Gnuradio source block
RSPA_gnuradio_sourceblock.PNG (63.9 KiB) Viewed 3038 times
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sdrplay
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Re: RSPA1 : Sampling rate vs Bandwidth

Post by sdrplay » Fri Jun 07, 2019 5:54 am

The sample rate and IF bandwidth are two different things but they have a relationship to each other.

All RSPs support an arbitrary native sample rate of anywhere between 2 and 10 MHz

The RSPs also have fixed IF bandwidth values of 200, 300, 600, 1536, 5000, 6000, 7000 and 8000 kHz

The general rule is that the sample rate shouldn't be less than the IF bandwidth. You will see some attenuation of the signal outside of the IF bandwidth so it makes sense to use some values such as...

sample rate of 2 MHz and IF BW of 1536 kHz
sample rate of 5 MHz and IF BW of 5000 kHz

what doesn't make sense is to select these values as you will see a large portion of the spectrum attenuated...

sample rate of 2 MHz and IF BW of 300 kHz
sample rate of 5 MHz and IF BW of 1536 kHz

If decimation is used then the same rule applies...

sample rate of 2 MHz with decimation of 2 to give a final sample rate of 1 MHz, so the sensible IF BW to use is 600 kHz

Most of the time applications will link the two parameters and pick a sensible IF BW depending on the final sample rate or visa versa.

Hope that makes sense.

Best regards,

SDRplay Support

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Hayfa
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Joined: Mon May 20, 2019 12:39 pm

Re: RSPA1 : Sampling rate vs Bandwidth

Post by Hayfa » Mon Jul 01, 2019 11:11 am

Hi,

Thank you for your answer. However, I still have some points I would like to understand in more depth.
sdrplay wrote:The RSPs also have fixed IF bandwidth values of 200, 300, 600, 1536, 5000, 6000, 7000 and 8000 kHz
Are these discrete values only applicable when using the IF mode? Or do we have the same restrictions even when using the Zero IF mode ?
sdrplay wrote:what doesn't make sense is to select these values as you will see a large portion of the spectrum attenuated...

sample rate of 2 MHz and IF BW of 300 kHz
I would like to understand why it does not make sense to use such a configuration. One use case I am thinking of is when we want to receive only 300 kHz of bandwidth, and since the minimal allowed sample rate is 2MHz, we will find ourselves using the case sample rate of 2 MHz and IF BW of 300 kHz. Am I right?
Or in this case it is better to use a sample rate of 2MHz and an IF bandwidth of 1536 kHz and then decimate by 4 in order to reach a sample rate of 500kHz and a bandwidth of approximately 300 kHz ?
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sdrplay
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Re: RSPA1 : Sampling rate vs Bandwidth

Post by sdrplay » Mon Jul 01, 2019 12:32 pm

The IF BW values are fixed to those that I listed for both Low IF and Zero IF mode.

From a processing load perspective (as well as visual) it would make more sense to specify 2 MHz sample rate, decimation of 4 and a IFBW of 300 kHz

this will give you a final sample rate of 500 kHz and only 100 kHz either side of the IFBW will be attenuated.

Hope that helps,

Best regards,

SDRplay Support

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Hayfa
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Joined: Mon May 20, 2019 12:39 pm

Re: RSPA1 : Sampling rate vs Bandwidth

Post by Hayfa » Mon Jul 01, 2019 1:07 pm

So in which use cases can using the 200, 300 and 600 kHz IF bandwidths be useful knowing that the minimum sample rate is 2 MSps?
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sdrplay
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Re: RSPA1 : Sampling rate vs Bandwidth

Post by sdrplay » Mon Jul 01, 2019 1:18 pm

If you use the rule to make sure the IFBW is equal to or lower the final sample rate (after decimation), this will avoid any aliasing issues.

SDRplay Support

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