Under-sampling

Under sampling

   When the signal frequency is high, and the tester frequency can’t catch the signal. We have to use the undersampling,

Undersampling is a technique in which the ADC sampling rate is lower than the analog input frequency. And this frequency can cause aliasing. Given the Nyquist criterion, it is natural to expect that undersampling would result in a loss of signal information. However, with proper filtering of the input signal and with proper selection of the analog input and sampling frequencies, the aliased components that contain the signal information can be shifted from a higher frequency to a lower frequency and then converted. This method effectively uses the ADC as a downconverter, shifting higher-bandwidth signals into the ADC's desired band of interest. For this technique to be successful, the bandwidth of the ADC's track-and-hold must be capable of handling the highest frequency signals anticipated.

 

Additional , following the Nyquist criterion, when Fs’ >2Ft, we can recover the original signal intact , otherwise , the signal will be aliasing.

Instrument undersampling mode

1.      signal path bandwidth

2.       track & hold circuit before the ADC

3.       phase noise of sampling clock