I depends on the design of the radar. The heart of any controlled radio transmitter, including radar, is an oscillator or timebase of some sort which either directly or indirectly determines the operating frequency. Other devices may be present which tune and filter the emissions to a desired spectral profile but they are normally designed around a target frequency determined by the timebase.
You can check the last 3 paragraphs of an article I posted on this website a while back, for a method I developed to change the nominal radar frequency for the purpose of avoiding interference between multiple radars operating in close proximity:
http://www.eeweb.com/blog/todd_hayden/micropower-impulse-radar-overviewThis was on a wideband transmission device, where the pulsed nature of the transmitter causes a spreading of the energy over a relatively large bandwidth. Because the regulated bandwidth where the device operates is wide enough, it allows small changes in operating frequency. This was achieved by directly shifting the oscillation frequency of the timebase of the entire unit, including the microcontroller which drove the transmitter and synchronous sampling receiver.
There was another way to change our radar operating frequency, covered by a seperate US patent, which also varied the transmitter frequency by dynamically changing the bias voltage to an RF transistor which oscillated at a harmonic of the radar timebase. The RF transistor oscillated nominally on the order of the 2,560th harmonic of the timebase. In the lab we mapped the sensitivity of the oscillation frequency versus bias voltage, allowing us to then shift the frequency between the 2,555th harmonic and the 2,565th harmonic, for example. In this case we only varied the tranmitter frequency but since the change was always a harmonic of the timebase, the synchronous sampling receiver would continue to function.
