|HIFI Local Oscillator Source Unit|
One of the essential components of an heterodyne system is the reference signal which is mixed with the signal from the astronomical source. The reference signal is called the Local Oscillator (LO). In many current astronomical receivers, the LO begins with a Gunn diode which resonates at a frequency around 90GHz. This is then multiplied by pumping further diodes with the 90GHz signal and extracting the harmonic at the desired frequency of 500GHz to 1200GHz. This method produces a reference signal, but the stability of such a system is insufficient to conduct observations with the long integration times required to detect faint astronomical sources.
In order to lock the reference system of these standard modern receivers to the desired frequency and with a constant phase, one generally begins by making a loop circuit which stabilizes a signal of around 5GHz. This is then used as the basis for locking the Local Oscillator up at the observed frequency. Since frequency multipliers are quite inefficient devices, the relative noise increases dramatically with each multiplication step. It is therefore essential that the reference oscillator for the lock loop is clean to a high degree, and that the loop itself be extremely efficient.
The recent development of Power Amplifiers at very high frequencies, beyond 100GHz, now makes it possible to use a different approach to building the Local Oscillator. Instead of building a lock loop to a Gunn diode at 90GHz, we can build a synthesiser at approximately 10GHz, the output of which will be amplified in power, and multiplied in frequency until we reach the desired output for HIFI at up to 2.7THz (the maximum frequency at which HIFI will operate). In order for this to work, the synthesiser output must be extremely clean in terms of phase noise and out-of-band noise. The task is made even more difficult by the requirement to avoid any contamination by signals within the HIFI Intermediate Frequency band of 4-8GHz. Nevertheless, the Design Study by MPB Technologies under CSA contract HIFI-MPB-9F007-9-11045 has demonstrated that the technology is feasible, and we are confident that the LOSU can be build to the required specifications.