  The short-term stability of passive atomic frequency standards, especially in
pulsed operation, is often limited by local oscillator noise via
intermodulation effects. We present an experimental demonstration of the
intermodulation effect on the frequency stability of a continuous atomic
fountain clock where, under normal operating conditions, it is usually too
small to observe. To achieve this, we deliberately degrade the phase stability
of the microwave field interrogating the clock transition. We measure the
frequency stability of the locked, commercial-grade local oscillator, for two
modulation schemes of the microwave field: square-wave phase modulation and
square-wave frequency modulation. We observe a degradation of the stability
whose dependence with the modulation frequency reproduces the theoretical
predictions for the intermodulation effect. In particular no observable
degradation occurs when this frequency equals the Ramsey linewidth.
Additionally we show that, without added phase noise, the frequency instability
presently equal to 2x10-13 at 1s, is limited by atomic shot-noise and therefore
could be reduced were the atomic flux increased.
