  We present a cryogenic microwave noise source with a characteristic impedance
of 50 $\Omega$, which can be installed in a coaxial line of a cryostat. The
bath temperature of the noise source is continuously variable between 0.1 K and
5 K without causing significant back-action heating on the sample space. As a
proof-of-concept experiment, we perform Y-factor measurements of an amplifier
cascade that includes a traveling wave parametric amplifier and a commercial
high electron mobility transistor amplifier. We observe system noise
temperatures as low as $680^{+20}_{-200}$ mK at 5.7 GHz corresponding to
$1.5^{+0.1}_{-0.7}$ excess photons. The system we present has immediate
applications in the validation of solid-state qubit readout lines.
