Choosing A/D sample rate or Tx synthesis rate
The internal system clock, which samples the IF input signals and synthesizes the Tx output waveforms, can be configured to run at any frequency between 50 MHz ... 100 MHz.
The setup questions in the Mc menu select the sampling clock frequency and whether the clock is derived from a stable on-board crystal oscillator or from the external SMA reference. See Mc — top-level configuration.
The sample clock frequency affects many components of the radar and signal processing system.
| Consideration | Description |
|---|---|
| A/D quantization noise and dynamic range |
The inherent SNR of the A/D converter chip is spread over a Nyquist band, whose width is determined by the sampling frequency. As the sampling frequency increases, the A/D quantization noise that is contained within a given Rx bandwidth decreases, which means that RVP10 becomes more quiet. The dynamic range varies linearly with sampling frequency. RVP10 has 3 dB greater dynamic range at 100 MHz versus 50 MHz clock. |
| Quantization of trigger timing and range bin placement |
Triggers generated by RVP10 are specified by their start time in microseconds, width in microseconds, and polarity. However, there are triggers always produced that are deviant from these values by half of the sample clock for Tx timings, and by a sample clock for Rx timings. If you want the triggers to be precisely aligned down to the exact clock edge, the sample clock frequency should be chosen so that trigger edges fall on integer multiples of the clock period. Similarly, the range bin spacing is specified in meters and always within half a clock period of the ideal value. The bins can also be placed precisely in range, by choosing a clock period that is an integer multiple of the desired spacing. |
| Maximum length of FIR down-conversion filters |
The FIR filters that compute (I,Q) time series from raw IF samples must process those samples at the acquisition clock rate. A filter of a given length in microseconds must contain a greater number of taps (coefficients) as the sample rate increases. For very long filters (more than 40 μsec), it is sometimes necessary to limit the clock rate in order to achieve the desired impulse response length. The Mt<n> and Ps menus are helpful in determining the maximum length filter that can be achieved for a given RVP10 processing mode (affected by single/dual polarization, range bin spacing, and so on). |
Use the RVP10 setup menus to cross-check the above constraints. The system installer must choose a sample clock frequency that achieves the best set of trade-offs at each radar site.