Chirp modulation is used in radar technique to achieve pulse compression. Pulse compression increases the sensitivity and resolution of radar systems by modifying transmitted pulses to improve their auto-correlation properties. To chirp the radar signal is one way of accomplishing pulse compression. A chirp is a signal with increasing or decreasing frequency over time.
The R&S SMA always couples the chirp modulation with the pulse modulation. It generates the modulation signals for FM and pulse modulator, and synchronizes the signals internally. The internal pulse generator signal is the modulation source for the pulse modulator, and the internal LF generator signal is the signal source for the frequency modulation. Normal FM mode is used. Using external modulation signals is not possible for chirp modulation.
The following graph shows the FM demodulated signal of chirped pulses with a chirp bandwidth of 80 MHz and a pulse width of 80 µs. Chirp direction is up.

Chirp modulation is available for the basic unit (R&S SMA + frequency option R&S SMA-B10x) equipped with option R&S SMA-B20 or B22 and interface board with part number 1400.0530.02 or 1400.0552.02.
Chirp Modulation with extended ranges for resolution, pulse width and period require option R&S SMA-K23, Pulse Generator.
The PULSE EXT connector at the rear of the instrument can be used as external trigger or gate signal input.
The polarity and input impedance of the connector can be selected.
The pulse signal is output at the PULSE VIDEO connector at the rear of the instrument, the sync signal at the PULSE SYNC output.
The FM modulation signal can be output at the LF connector.
![]() | When pulse modulation is activated, the R&S SMA deactivates ALC automatically ("ALC OFF", i.e. switches to "Sample & Hold" state). The "Sample&Hold" state opens the ALC loop, and disables the automatic control of the output level. The level modulator is set directly. However, to correct the output level, the R&S SMA executes a "Sample & Hold" measurement after each change of frequency or level settings. The nominal level is ON for typically 3 ms to 5 ms after level or frequency setting, if:
The level is decreased by 30 dB during "Sample & Hold" measurement. |
![]() | The Sample&Hold measurement for the ALC is performed at the chirp center frequency. Therefore, the frequency response of the RF path can result in level errors for the FM modulated carrier especially with high chirp bandwidths. |
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