Electronic – How to you recieve a 6 GHz signal with only 64MS/s ADC

adcsampling

The Ettus Research Software Defined Radio B210 has a range of 70 MHz – 6 GHz, a Bandwidth of 56 MHz and its maximum ADC rate is 61.44 MS/s. Doesn't Nyquist theorem state that the sampling rate should be 2*Fmax, where Fmax here is 6 GHz ?

EDIT: the board uses Analog Devices AD9361 RFIC

Best Answer

Please note the following:

  1. The device actually does not sample RF. It downconverts to 0 Hz (on frequency downconversion) and then samples it.

Internally it has various analog stages to control gain, bandwidth, as well as state machines to overcome limitations of direct conversion.

For details please check the following block diagram of the AD9361. https://github.com/analogdevicesinc/iio-oscilloscope/blob/master/block_diagrams/AD9361.svg

  1. The RF bandwidth can be 56 MHz and 64 MSPS will be sufficient. Why? Because the device is outputting 12 bits of I & 12 bits of Q samples at each MSPS rate specified. Therefore real sampling rate can be assumed to be 2X of specified MSPS.
  2. The device samples RF at much higher rate but the real reason for that is a little different than just satisfying Nyquist.
  • the type of ADC inside the device requires higher sampling to generate desired 12-bit resolution. It is an advanced Sigma-delta-modulator (3-bit SDM + sign i.e. -4 to 4). With some math, it converts these 3-bit values and generates 12-bit values. For details please check the AD9361 Reference manual section related to ADC overload detector.
  • Device needs to do some filtering so aliasing doesn't happen. In order to do that, it samples at a much higher rate, then digitally downsamples. During this process, it attenuates ADC images. There is also an additional FIR filter inside for correcting downconversion imperfections.
  • Note that the device still needs external filters for good performance.
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