As described above, in some embodiments, receive-only interrogator 1404 may be configured to operates phase coherently with master interrogator 1402. For example, as shown in FIG. 14C, receive-only interrogator 1404 may be configured to generate RF signals that are phase coherent with the RF signals generated and transmitted by the master interrogator 1402. To this end, receive-only interrogator 1404 includes waveform generation circuitry 1454 for generating frequency-modulated waveforms using a phase locked loop in a “feedback loop perturbation” approach, which was described above with reference to FIG. 13B. However, unlike the waveform generation circuitry 1320 shown in FIG. 13B, the waveform generation circuitry 1454 replaces the fixed-frequency reference oscillator 1336 with shared fixed-frequency reference oscillator 1410 (which is shared among all the receive-only interrogators 1404, 1406, and 1408 shown in FIG. 14A). As shown in FIG. 14C, the shared fixed-frequency reference oscillator 1410 is coupled to frequency multiplier 1328 via line 1412.
