NI 5650/5651/5652 Modulation Implementation

The main synthesizer has two modes: low and high, corresponding to 20 and 200 kHz loop bandwidths, respectively. During single tone (sine) generation, the low loop bandwidth is selected for the best possible phase noise performance. The specified phase noise is based on low loop bandwidth.

The modulation implemented on the NI 5650/5651/5652 requires that the high loop bandwidth must be used. Choosing high loop bandwidth increases phase noise from 20 kHz and outward; the phase noise close to the carrier does not change. Loop bandwidths do not affect far out noise density; that is, the far out noise of the system remains unchanged.

All frequency modulation occurs within the phase-locked loop (PLL) of the main synthesizer circuitry. Changing the direct digital synthesis (DDS) reference frequency directly modulates the generated signal. In effect, controlling the frequency of the DDS controls the frequency of the generated RF signal. The PLL bandwidth of the main synthesizer is set to high loop bandwidth when modulation is turned on in order to accommodate high modulation rates. During normal operation, with modulation turned off, the loop bandwidth setting returns to low loop bandwidth.