In the previous entry, I told that the acronym SIR stands for Superconducting Integrated Receiver. What I didn't explain, is what it actually means.
First the superconducting part: when you want to measure a very weak signal, at some point the electronic noise will interfere with your measurement. This noise is caused by small variations in temperature. The lower the temperature, the lower the electronic noise. Hence very sensitive instruments need to be supercooled.
Now the receiver part: the signal which is measured is between 500 to 750 GHz. This frequency sits between radio signals and light. You can use optics as well as antennas. The SIR chip uses a very tiny double dipole antenna which is etched onto the chip. A silicium lens is used along with mirrors to guide the signal onto the chip its antenna. Note that although the signal can be put on the wire, it will fade out very quickly. That's why right below the antenna, the SIR mixer is located.
Finally, the "integrated" part of the acronym. This chip contains a SIS mixer as well as beforementioned antenna. However, two signals are mixed, the one from the antenna and another one from the FFO, the flux flow oscillator. This is a voltage-controlled oscillator.
This FFO is also integrated on the SIR chip, and generates a set signal between 500 and 650 GHz, the frequency we want to measure on. The fourth and final part on the SIR is the Harmonic Mixer (HM), which receives the FFO-generated signal and mixes it along with a 20 GHz signal to get a signal which looks like the one black one below:
On 2006-02-23 Pictures from TELIS Project, I showed the boards, of which the most right one is the LSU board. This board generates a number of signals. It contains an ultra-stable 10 MHz oscillator which uses a crystal, sitting in a nice warm little oven to keep it happy.
One LSU signal has a frequency 20 to 22 GHz with a certain power, which is fed into the Harmonic Mixer. This is called the pumping of the mixer. It is mixed together with the FFO signal by the HM to get what you see above: a signal with a frequency of, say, 650 GHz together with one that's 4 GHz above it -- that's the red signal in the picture above.
The resulting signal from the HM is sent into the SIS mixer as the F1 signal shown in the sketch of the previous entry 2006-04-24 SIRs for dummies. It's called a harmonic mixer because whatever signal f you put in, out comes a signal 2f, 3f, 4f, et cetera. The LSU input signal is chosen to get the required frequency of around 650 GHz or whatever the spectrum is that we want to view. We use the 30th harmonic for this, so the LSU signal would be set to around 21.8 GHz.
Below is a bigger schematic picture of what's happening.
The upper part of the schema I've explained. On to the middle left. The FFO is happily radiating away and the frequency drifts. To keep the FFO from drifting, you want to lock it. An external (meaning outside of the cryogenic flask) piece of electronics creates a phase-locked loop (PLL). This keeps the FFO on its set frequency. It operates using a 400 MHz reference frequency generated by the LSU board, as well as the output of the HM, which is run through a mixer first. This frequency is in the MHz range for practical reasons.
Note that the FFO is extremely sensitive. You put a bias voltage on it and for every millivolt, it changes by 484 GHz. Since we want to work with steps of 0.5 MHz, this would mean that we would have to change in nanovolts.
Like the SIR, the Telis PLL is a Russian product. It has an analog output with which you can check the quality of the phase locking.
If you take a step back, you can see that the LSU, Harmonic Mixer, FFO and PLL work together to get a steady signal of, say, 656 GHz. It also is possible to control this signal in steps of 0.5 MHz. This is then used to feed into and read out the SIS mixer.