A noise generator with a wideband output signal is always handy to have around when you’re adjusting receivers and other types of HF equipment.
The noise generator circuit described here uses the base-emitter junction of a transistor (in this case a BF199) operating under reverse bias. As a result, it acts as a Zener diode and generates a wideband noise signal. The noise signal passes through a 1-nF capacitor to the output connector (female BNC), which means that its low-frequency components do not appear at the output. The 47-Ω resistor gives the noise generator an output impedance of nearly 50 Ω.
You can easily fit the entire noise generator in a small metal enclosure equipped with a BNC connector. The supply voltage is not critical; anything in the range of 8 to 15 V will do.