# How can you recieve a 6 GHz signal with only 64MS/s ADC?

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

• The key is that the goal of sampling isn't to preserve a detailed time-domain picture of the radio signal. That is not necessary because the actual content which is carried in the signal doesn't rely on the amplitude details of every individual oscillation cycle of the carrier. – Kaz Nov 17 '13 at 21:39

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.

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.

Have a look at the updated block diagram of the chip on the B200.

A couple things:

• the max ADC rate is not 61.44, it's actually 640MSPS, but that assumes some decimation along the way.
• the max data output is 61.44 MSPS
• the max RF bandwidth is 56 MHz, and is set by the analog filters, not the ADC sample rate.