Frequency hopping spread spectrum (FHSS) fourier transform spectroscopy
Apparatus and techniques for broadband Fourier transform spectroscopy can include frequency hopping spread-spectrum spectroscopy approaches. For example, an excitation source power can be spread over a specified frequency bandwidth, such as by applying a sequence of short, transform-limited pulses to a sample. Each pulse can include a specified carrier frequency, and a corresponding bandwidth of the individual pulse can be determined by a frequency domain representation when Fourier transformed. A series of short excitation pulses can be used to create an excitation sequence, such as to deliver a specified or desired amount of power to the sample, such as by having the excitation source enabled for a time comparable to a free induction decay (FID) dephasing time.
1. An apparatus, including:
a pattern generator circuit configured to output a pulse sequence, the pulse sequence including durations during which frequencies selected from a first frequency range are output, including a count N of different frequencies output serially within the pulse sequence, the pulse sequence further comprising a count M of unique permutations of an order in which the count N of different frequencies are output, the count N being at least three;
a frequency multiplier circuit operably coupled to the pattern generator circuit and configured to upconvert the pulse sequence to an upconverted pulse sequence having a higher second range of frequencies beyond the first range of frequencies;
a sample cell in electromagnetic communication with the frequency multiplier circuit and configured to receive the upconverted pulse sequence as an excitation to a sample; and
an analog-to-digital converter configured to acquire a digital representation of an output from the sample cell elicited in response to the upconverted pulse sequence.
2. The apparatus of claim 1 , wherein the first frequency range comprises a microwave frequency range; and
wherein the second frequency range comprises a millimeter-wave frequency range.
3. The apparatus of claim 1 , wherein the frequency multiplier comprises an active multiplier chain (AMC).
4. The apparatus of claim 1 , wherein the pattern generation circuit comprises an integrated digitally-controlled frequency synthesizer circuit.
5. The apparatus of claim 1 , wherein the pattern generation circuit comprises a digital-to-analog converter (DAC).
6. The apparatus of claim 1 , wherein the analog-to-digital converter is operably coupled to a processor circuit, the processor circuit configured to obtain a frequency domain representation of a time-domain digital representation obtained from the analog-to-digital converter.
7. The apparatus of claim 6 , wherein the processor is configured to perform a discrete Fourier transform on the time-domain digital representation so as to yield the frequency domain representation.
8. The apparatus of claim 1 , wherein the count M of unique permutations contains a first permutation of the order in which the count N of different frequencies are output and a different second permutation of the order in which the count N of different frequencies are output.
9. The apparatus of claim 1 , wherein the count M is equal to N.
10. The apparatus of claim 8 , wherein the analog-to-digital converter is coupled to processor circuit, the processor circuit configured to:
obtain respective frequency domain representations of the time-domain outputs elicited by the first and second permutations; and
aggregate the respective frequency domain representations to provide an aggregated frequency domain representation.
11. The apparatus of claim 10 , wherein the processor circuit is configured to determine, based on the aggregated frequency domain representation, a central tendency of frequency domain information corresponding to each of the frequencies included in the upconverted pulse sequence.
12. The apparatus of claim 10 , wherein the processor circuit is configured to:
determine levels of each of the frequencies in the respective frequency domain representations; and
correct a level of a determined frequency in the aggregated frequency domain representation using information about a difference in determined levels of a respective one of the frequencies included in the first and second permutations.
13. The apparatus of claim 1 , wherein the pattern generator circuit comprises respective frequency outputs configured to provide respective ones of the frequencies amongst the count N of frequencies.
14. The apparatus of claim 13 , wherein the respective frequency outputs are fixed.
15. The apparatus of claim 13 , wherein the respective frequency outputs are adjustable.
16. The apparatus of claim 13 , wherein the respective frequency outputs are configured to provide a continuous output.
17. The apparatus of claim 16 , wherein the continuous output comprises a sinusoidal continuous wave (CW) output.
18. The apparatus of claim 16 , wherein the pattern generator circuit comprises a switching circuit configured to selectively couple a respective one of the respective frequency outputs to a main output of the pattern generator circuit.
19. The apparatus of claim 18 , wherein the switching circuit is configured to selectively couple a sequence of respective ones of the respective frequency outputs to the main output to provide the pulse sequence.