A frequency ruler for phase noise measurements

In commercially available phase noise measurement instruments, the necessary reference signal is generated internally by an oscillator that can be tuned over a large frequency range. The measurement is thus limited by the frequency range of the reference oscillator (typically only a few GHz) and its noise. In contrast, the new system uses a low-phase-noise component of the frequency ruler to externally convert the high GHz range measurement signals to MHz range signals prior to the actual phase noise measurement. This allows the system to minimize inherent measurement system noise during phase noise measurements.

This technique exploits the fact that mode coupling in femtosecond pulsed lasers allows the extremely low phase noise of optical continuous-wave lasers to be transferred to the pulsed laser’s repetition rate of a few hundred MHz. When this repetition rate is detected by a photo diode, the short optical pulses generate a comb of repetition rate harmonics. Each frequency component of this comb exhibits nearly the same stability as the reference signal. The width of the electrical frequency comb thus generated is limited by the bandwidth of the photo diode. Commercially available photo diodes have bandwidths of up to 100 GHz. For the purposes of phase noise measurements, the frequency component which is closest to the frequency of the signal being measured is filtered out from the electrical comb. A signal in the MHz range is generated by mixing the two frequencies. It is then specified using a low-noise, low-frequency phase noise measurement station.

A first demonstration experiment was performed in which the reference of the frequency comb in the optical frequency range was replaced by particularly low-noise reference signals at 100 MHz and 9.6 GHz that are available at PTB. This experiment showed that the low noise of these reference signals was transferred to all components of the microwave comb.

In the future, such a system can be used to provide calibration services for RF oscillators up to the mm wave range.