External Reference Clock Source

You can program your NI RF signal generator to phase lock to a 10 MHz external Reference Clock signal connected to the REF IN connector on the PXI-5610 upconverter module front panel.

The pulling range of the upconverter module onboard OCXO is specified at ±10 Hz, as the external reference signal must be accurate to within ±10 Hz for the upconverter module onboard OCXO to phase lock to it. Reference signal accuracy is the frequency offset observed from a rubidium source at 25 °C room temperature. Most references available on the market do not meet the upconverter module OCXO pulling range requirement because of variability with temperature.

The phase noise of the system is also affected by external phase locking. Although the NI RF signal generator tries to minimize the effect of noisy references by using a very small phase-locked loop bandwidth, a considerable remnant may exist if the reference phase noise is 20 dB to 30 dB poorer than the upconverter module onboard OCXO. Some rubidium sources on the market have more phase noise than the upconverter module onboard reference but exhibit high long-term stability.

The external reference signal power level must be greater than -10 dBm or 0.2 Vpk-pk into 50 Ω.