We describe the design of a device intended to calibrate Multanova 6F Doppler speed radars. The Speed calibrator modulates the transmitted signal from the speed radar with the Doppler frequency, which is generated by a signal generator. This frequency corresponds to the speed of the target vehicle being measured by the speed radar. The range of "speeds" generated by the Speed calibrator is 25-250km/h and for both directions of movement (receding and approaching), which is identical to the range of speeds that can be measured by the speed radar. The modulated signal is transmitted back to the speed radar either as an Upper Side Band signal -for simulation of an approaching target vehicle, or as a Lower Side Band signal -for simulation of a receding target vehicle. The precision obtained by calibration of the speed radar using the Speed calibrator is ±1km/h within the range of speeds of 25-100km/h and ±1% within the range of 100-250km/h, for receding and approaching directions.