Active hydrogen masers
Metsähovi Radio observatory provided two CH1 - 75 Active hydrogen masers in 2001 delivered by Institute of Electronic Measurements, "Kvarz". The two masers (see Figure 1) are used in Metsähovi as a frequency reference. All main measurement devices are usually locked to the frequency value of 5 MHz. The phase-lock circuits (oscillators) of the recievers, signal generators and spectrum analyzers etc. are locked to 5 MHz. The 5 MHz signal of some applications has to be multiplied to a higher frequency which is used for the local oscillator as a stabile reference. In Figure 2 is the illustrated spectrum of the 5 MHz signal. As we can see the signal is pure 5 MHz without any disturbing harmonic components (span 100 kHz).
Figure 1. Two Active hydrogen masers in cellar room.
Figure 2. 5 MHz signal from Active hydrogen maser.
PPS (pulse per second) signal together with the 5 MHz signal are used for VLBI (Very Long Baseline Interference) measurements as the time stamp for the later data handling.
Behaviour of hydrogen masers can be monitored with a measurement system called Clodi. Clodi compares delays between GPS-clocks and hydrogen masers. Accuracy of this system is 10 ns. The basic value of the hydrogen masers drifts with time, generating a delay. Therefore, the masers have to be adjusted few times in a year.
Phase noise of the Active hydrogen masers (5 MHz reference signal) and HP-GPS (10 MHz reference signal) in the figure 3 has been illustrated. The level of noise is less than -100 dBc even at 10 Hz offsets from the main signal.
Figure 3. Phase noise from Active hydrogen masers (69 and 70) and HP-GPS.