5. Waveform Evaluation Methods and Waveform Measurements
6.3. Choice of Beat Frequency and Waveform
Since much of the work in the thesis had consisted of finding suitable waveform evalua- tion methods and eliminating the source of errors in the delivered circuit, the development of new waveforms were limited to studies of two fundamental FMCW waveforms that could be used further in the full implementation of the radar system. It were then de- cided to implement the up-chirp- and triangular continuous waveforms to study LFM properties with previously derived waveform analysis methods. Hence to look at a desirable region of the beat frequency at different chirp rates. Additionally should the waveforms be maximized to exploitation of the preassigned 1 GHz bandwidth at W-band in the future full radar implementation. Although the two waveforms were not suited for multiple Doppler target extraction, they would still be highly preferable for initial range and single Doppler target trails of the full system.
Prior to implementation of the new waveforms were some basic consideration made to the resulting beat frequency at different chirp rates, as described in section 2.7, to have a meaningful measurement set. Since the bandwidth were to be optimized to fit the 1 GHz W-band implementation, the only remaining variable were the chirp time Since the basic property of near clutter influence had not been studied this consideration were left out. Additionally were it also difficult to predict the degree of phase noise cancellation achieved in the full system implementation, although the Tx CW mid-band phase noise had been measured. However would both of these limiting factors desire as high minimum beat frequency as possible. Nevertheless were a general assumption made from previous experience with measurements of bistatic FMCW radars at W- band [35], that the minimum beat frequency should be avoided below 1 kHz for pure ranging. Additionally in respect to Doppler targets sensed with the two chosen waveforms, should the lower limit be set much higher. If a maximum relative velocity between two
6.3. Choice of Beat Frequency and Waveform 115
approaching cars, is considered to be in range of 200 km/h the resulting Doppler shift would be ±28 kHz. Thus indicating a minimum beat frequency at 29 kHz. However can the demand for minimum range detection be increased correspondingly, although this would then decrease the region of observation considerably for low chirp rates. In addition should also the maximum beat be considered in respect to the later implemented DAS, in respect to its capability to yield high sampling frequency (2 · fd,max), its intermediate
data storage capacity and data rate handling. Both these limitations have to be reviewed at a later stage of implementation to tune in on the best suited modulation and chirp rate that yield the best radar observability. Additionally could also the later finds in this thesis set limits to the future beat frequency setting and hence modulation in respect to chirp linear quality and effective bandwidth reduction caused by for instance excessive sweep recovery time (HRR-systems 2.7). However had these quality to be measured before any prehand considerations could be made on the subject.
Hence were this thesis investigation restricted to the study the modulation of waveforms in aspect of signal source agility to moreover examine the performance of the fundamental waveforms at different chirp rates. Thus trying to identify the region of chirp rates where the circuit produces good enough waveform quality for LFM-CW beat frequency generation. With this goal in mind were hence a reasonable test region of chirp rates derived according to table 6.1, with 1 GHz bandwidth and the assumption that a desired region of observation were in range of 3 m to 150 m. The chirp time values in table 6.1
Table 6.1.: Beat frequency region at different chirp times
t0 fb,min fb,max 0.1 ms 200 kHz 10 MHz 1 ms 20 kHz 1 MHz 10 ms 2 kHz 100 kHz 20 ms 1 kHz 50 kHz 40 ms 500 Hz 25 kHz
were hence also selected by the limitations given by the ESTI requirements [7, p.4] for 1GHz maximum frequency modulations all within the maximum chirp rates belove or equal to 10000 MHz/ms. Although the perivious testing had alluded problems with short chirps in respect to the PLL lock time, the limits of the ESTI were still used for full confirmation with new programmed waveforms. In the last row of the table were the previously intended 40 ms chirp modulation listed. However as the table show would this waveform produce a unacceptable low beat well in range of the most noisy PN region. Additionally did a possible use of this waveform not be perfered for Doppler sensing applications due to the low minimum beat frequency and possible masking of fast approaching targets. Thus were the 40 ms chirp modulation left out of the test set. In respect to the actual DDS programming for waveform generation, were there more variables that could be varied. The chirps of waveform could hence be made up of various combinations of DDS frequency steps and corresponding time resolution. However were it decided to limit the experiment further by the used of maximum time step resolution, hence setting RSRR/FSRR equal to 1 and time resolution at each frequency step to effectively 8 ns.
To achieve the desired continuous up-chirp modulation were hence the nodwell-enabled and the timing of the profile pin of figure 6.3, set to zero hence enabling a intermediate start-up of next frequency sweep. The actual modified programming for up-chirp modu- lation, are described in appendix C.1.1. To further set the DDS for triangular modulation as in figure 6.4 were the nodwell disabled. Additionally by setting RDW=FDW and RSRR=FSRR were a symmetrical triangular modulation achieved, where the profile pin period was matched to the total chirp modulation time, as described in 6.2. Hence were a pure triangular modulation with no excessive holding time created.