Chirped pulse Fourier transform microwave spectroscopy

The FTMW spectrometer monitors the composition of a gas pulse by irradiating it with a 26-40 GHz (Ka-band) chirped pulse of microwave radiation and recording the subsequent free induction decay (FID). A Fourier transform of the FID yields the spectrum of all polar molecules that have transitions in the Ka-band. A single measurement takes only around 10 μs, so the gas composition is monitored as a function of time with multiple chirp-FID cycles per gas pulse. 

In order to make a 26-40 GHz chirp, a 1.52-4.895 GHz chirp is generated by an Arbitrary Waveform Generator and then up-converted to 6.625-10GHz by subtracting the chirp from 11.52 GHz in a mixer. This chirp is then further up-converted to 26.5-40 GHz by a quadrupler. Before the chirp is blasted into the vacuum chamber, it goes through a Travelling Wave Tube amplifier that brings the power up to almost 200 W. 

When the Ka-band chirp is sent into the chamber, a switch is opened on the receiving side in order to block this powerful signal from hurting sensitive components. After about 100 ns, the switch closes in order to begin recording the FID. However, the oscilloscope that records the FID has a max frequency input of 16 GHz, so the Ka-band chirp is down-converted by subtracting it from 41 GHz in another mixer. The chirp that the oscilloscope receives is 1.00 to 14.5 GHz. This signal is then sent to the computer, where the program Blackchirp performs the Fourier transform.

Frequency spectrum of "chirp"