A method for measurement of rotational spectra of molecules in a gaseous sample in response to excitation using a specified range of excitation frequencies selected from the microwave to terahertz range, the method comprising: generating a specified chirped waveform and a specified downconversion local oscillator (LO) frequency using a digital-to-analog converter (DAC) ; upconverting the chirped waveform via mixing the chirped waveform with a specified upconversion LO frequency; frequency multiplying the upconverted chirped waveform to provide a chirped excitation signal for exciting the gaseous sample ; receiving an emission from the gaseous sample , the emission indicative of a free induction decay (FID) elicited at least in part by the chirped excitation signal; and downconverting the received emission via mixing the received emission with a signal based on the specified downconversion LO signal to provide a downconverted emission signal to an analog-to-digital converter (ADC) , the down-converted emission signal being within the bandwidth of the ADC ; wherein the generating the specified chirped waveform includes generating a first chirped waveform during a first duration, and a second chirped waveform during a second duration, the first and second chirped waveforms including respective different frequency ranges, the different frequency ranges having respective bandwidths each specified to be within the bandwidth of the ADC and resulting in received emissions including a total bandwidth corresponding to the specified range of excitation frequencies when the received emissions are received sequentially and aggregated to encompass a bandwidth wider than the bandwidth of the ADC . generating a specified chirped waveform and a specified downconversion local oscillator (LO) frequency using a digital-to-analog converter (DAC) ; upconverting the chirped waveform via mixing the chirped waveform with a specified upconversion LO frequency; frequency multiplying the upconverted chirped waveform to provide a chirped excitation signal for exciting the gaseous sample ; receiving an emission from the gaseous sample , the emission indicative of a free induction decay (FID) elicited at least in part by the chirped excitation signal; and downconverting the received emission via mixing the received emission with a signal based on the specified downconversion LO signal to provide a downconverted emission signal to an analog-to-digital converter (ADC) , the down-converted emission signal being within the bandwidth of the ADC ; wherein the generating the specified chirped waveform includes generating a first chirped waveform during a first duration, and a second chirped waveform during a second duration, the first and second chirped waveforms including respective different frequency ranges, the different frequency ranges having respective bandwidths each specified to be within the bandwidth of the ADC and resulting in received emissions including a total bandwidth corresponding to the specified range of excitation frequencies when the received emissions are received sequentially and aggregated to encompass a bandwidth wider than the bandwidth of the ADC . <EOS>