Maybe a newbie question .... oversampling

Projects LimeSDR
1

My Limesdr mini has Oversampling either in RX and TX.
I do not understand how to use that features … ie … if i set Sample Rate to 2Mbit/sec i expect to get 2 million samples for second … what happen if i set oversampling = 2 ?

Is there some document to read for understanding how it works ?
Tnx, Fabio,

2

The LMS7002M RFIC architecture is described in the datasheet:

https://wiki.myriadrf.org/LimeMicro:LMS7002M_Datasheet

3

If you change the oversampling, the chip will decimate the data on RX by that amount - so if you set the chip to deliver 100kSample/sec to you, and you set oversampling to 16, the chip will internally sample at 1.6MSample/sec, then use a digital filter to reduce the data by 16. On transmit, if you set the chip to take 100kSample/sec and you set 16x oversampling, the chip will take the 100kSample/sec from you, use a digital filter to increase the number of samples by 16, and run internally at 1.6MSample/sec.
This is done for a few cases:

  1. you want to frequency hop very quickly in a limited bandwidth - you can set the chip to run 65MSample/sec, and then digitally tune +/- 16MHz within that pretty much instantly, and by using the oversampling you can reduce the amount of data you get to something manageable.
  2. You want to operate below 30MHz, which is the lowest the analog section can tune. So you tune to 30MHz, run the internal sampling at 60Msample/sec, and then tune the digital NCO to the desired frequency and decimate.
1 Like
4

Tnx David for informations, now it seems more clear. I understand however that the sample rate i get out of the usb interface is what I’ve set. The oversampling is performed at the RF side. For quickly frequency hop i guess you refer to changing NCO value on the fly. I have the LimeSDRmini so may be that i cannot go too much under 30 Mhz, since the NCO in my case is unable from -7.5 to 7.5 mhz so at best i can go down to 22.5 mhz.
Just to recap, suppose i want to tune at 50 Mhz with 3 Mhz band coverage (from 48.5 Mhz to 51.5 Mhz) with a resolution span of 100 Khz. So i set che chip sample rate at 200 Ks/sec but i set the oversampling at 16 so the rf sampling is internally done at 3.2 Mbit/sec (200*16). Then i can move the frequency with the NCO tuned from --1.6 Mhz to 1.6 Mhz to cover the whole band i want to observe.
Let me know if i understand well.

Tnx, Fabio IZ0IBA

5

You have it. The only thing I would point out is that if you want 100kHz bandwidth, you don’t need 200kSample/sec since the samples are complex (I and Q) - 200kSample/sec will give you 200kHz bandwidth. Of course, if you want to have 100kHz of flat bandwidth, you might run a higher sample rate so that you can have an equalization filter to flatten the middle of the response out, and have a bit of “slop” at the edges.