Vorrichtung zur Störfrequenzunterdrückung

Spurious frequencies are suppressed in a transmitter (7) by an RF pulse fall time controller circuit (10) having a quadrature hybrid matched attenuator (23) located between a predriver (8) which outputs substantially rectangular pulses of radio frequency signal, and the first stage of a series of class "C" common base transistor amplifiers (11). The controller circuit (10) linearly attenuates the RF pulses so as to slow the pulse fall time. Operation of the circuit (10) is initiated when the rising edge of a pretrigger input to a pulse shaper (12) starts a pretrigger generator (14) which then triggers the application of bias current (IBIAS) by the pulse shaper (12) to the attenuator circuit (23). After a controlled period of time determined by a monostable (16) in a pretrigger generator (14), the pulse shaper (12) decreases the forward current bias of PIN diodes (34,38) in the attenuator (23) thereby increasing the attenuation of the RF pulses. This creates a slow pulse fall time thereby preventing the occurrence of spurious emissions which would otherwise be caused by parasitic ringing of the cascaded Class C common base transistor amplifiers (11) in the transmitter (7). The RF pulses, after passing through the cascaded amplifiers (11), are emitted by the transmitter (7) as rectangular pulses.