Principles of an FMCW Radar Philip Gabriel Philip

Principles of an FMCW Radar Philip Gabriel, Philip Lee

Purpose of Radar and Subject of Presentation n The CSU FM-CW radar is a prototype design intended to detect rain and to allow estimation of rainfall rate at very high spatial resolution (e. g. 15 meters ) over relatively short ranges (e. g. 60 km)…inexpensively. n To describe the physics and engineering of the radar, highlighting its advantages and disadvantages.

Background RADAR= “Radio Detection and Ranging” n Measures reflected energy sent out from transmitter. n Transmitter may send out continuous wave (CW), or frequency modulated (FM) signals or signals modulated in other ways. n

Review of Pulsed Radar Operation

Operation of FM-CW Radars: The Transmitter sends out signals of period Ts seconds whose duration >> return time of echo t. n Transmitter on continuously. n Transmitter produces a signal of constant amplitude modulated by a sawtooth waveform. n

Operation of FM-CW Radars: The Receiver Picks up not only the reflected signal, but also signal from the transmitter a short distance away (e. g. 0. 75 meters). n The two oscillations are multiplied together in a non-linear device (mixer) to extract a beat note which contains range information. n Range information is calculated using Fourier Analysis. n

Analysis of an FM-CW Signal Assume frequency of VCO increases linearly with time. n Assume Instantaneous flyback. n Assume the exact phase of the received signals is a delayed replica of the transmitted signal. n Assume the medium is non-dispersive. n

Analysis of an FM-CW Signal

Analysis of an FM-CW Signal The instantaneous frequency for a sweeprate of 2 K is: Relation between phase and frequency is:

Analysis of an FM-CW Signal

Analysis of an FM-CW Signal There are 2 voltages at the receiving antenna of the form: (1) Vg. Sin(fg(t)) from the transmitter and (2) Ve. Sin(fe(t)) from the echo. n The echo voltage is delayed by the roundtrip echo time, t=2 R/c. n These oscillations are mixed or heterodyned. n

Analysis of an FM-CW Signal n n n Heterodyning yields: Vg. Ve. Sin(fg(t))Sin(fe(t))=. 5 CVg. Ve[Cos(fg(t)-fe(t))- Cos(fg(t)+fe(t))] Sum term is extremely high frequency. It is removed with a band pass filter. Range information is contained in the low frequency component and is obtained via spectral analysis of:

Analysis of an FM-CW Signal

Analysis of an FM-CW Signal

FM-CW Simulations

Summary and Conclusion The advantages of the FM-CW Radar are: its portability, simplicity, low cost, high spatial resolution, good sensitivity. Range. Doppler ambiguity at close ranges can however be resolved in software. n The FM-CW radar is subject to microphonics, has limited range. n