A similar problem occurs when a broadband signal source is used as the local oscillator for a noise figure measurement. Such devices typically have broadband amplifiers following their internal signal sources and modulators, providing isolation and much higher output power capability than could be obtained from the internal sources unbuffered. It is necessary to use a band pass filter on the output of these wideband sources unless they already have a YIG filter built in. As is true of the monolithic mixer, the magnitude of the problem depends on the isolation of the mixer, and the noise level at the amplifier output.
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