Features less input noise than a 50 ohm resistor.
The phase noise of low noise oscillators and signal sources is usually determined by measuring the audio noise voltage at the output of the phase comparator in a phase-locked loop. The phase comparator is typically a low noise doubly-balanced mixer with a phase slope from a few tenths of a volt per radian to a few volts per radian. The low conversion gain of this type of phase detector yields a signal in the low nanovolt per root-hertz range for very low noise sources which is a level below the noise floor of most spectrum analyzers. Suitable low noise preamplifiers are readily constructed from discrete components or from modern low-noise op-amps.
Except for the noise voltage, the amplifier requirements are not particularly demanding. Since the output impedance of the phase detector is low, the input impedance of most amplifier circuits is adequate. A frequency response from a few hertz to 100kHz is usually adequate and the output load is usually a high impedance spectrum analyzer and oscilloscope. The phase detector output impedance is quite low so the noise current of ordinary bipolar transistors is sufficiently low. For example, an ordinary 2N4403 transistor exhibits a noise voltage below 1 nanovolt at 10 Hz when the source is a typical Schottky diode mixer. Several op-amps are available with noise voltages below 3 nanovolts and a few are available with noise voltage below 1 nanovolt. Simple amplifiers built from any of these parts will perform well in most applications. Bulk-metal, wirewound, and metal film resistors exhibit little excess noise and should be used instead of carbon film or carbon composition types. Most potentiometers should be avoided since film and cermet types are quite noisy.
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