Engineering:Excess Noise Ratio

The term Excess Noise Ratio is primarily used in connection with noise diodes which are a common method of measuring the noise performance of an amplifier. The measurement technique is described in detail in application notes such as Keysight Application note 57-2 "Noise Figure Measurement Accuracy - The Y-Factor method."

By using a noise diode, the output noise of the amplifier is measured using two input noise levels (the Excess Noise Ratio) and by measuring the output noise factor (referred to as Y) the noise figure of the amplifier can be determined without having to measure the amplifier gain.

Background

Any amplifier generates noise. In a radio receiver the first stage dominates the overall noise of the receiver and in most cases thermal, or Johnson noise, determines the overall noise performance of a receiver. As radio signals decrease in size, the noise at the input of the receiver will determine a lower threshold of what can be received. The level of noise is determined by calculating the noise in a 50 ohm resistor at the input of the receiver as follows:

P_av = kTB

where:

k = Boltzmann's constant = 1.38 x 10-23 joule / k

T= Temperature (Kelvin)

B = Bandwidth (Hz)

Thus it can be seen that receivers with a narrow bandwidth have a higher sensitivity than receivers with a large bandwidth and that input noise can be decreased by putting the receiver input stage at a cold temperature.

A noise diode is a device which has a defined Excess Noise Ratio.

When the diode is off (unpowered) the noise from it will be thermal noise defined by the above formula. The bandwidth to be used is the bandwidth of the receiver.

When the diode is on (powered) the noise from it will be increased from the thermal noise by a figure called the "Excess Noise Ratio". This figure could be 6 dB for testing an amplifier with 40 dB gain and could be 16 dB for an amplifier with less gain or higher noise.

To determine the noise figure of an amplifier we use a noise diode at the input to the amplifier and determine the output noise "Y" with the diode switched on and off.

With knowledge of Y and a knowledge of ENR we can then determine the amount of noise contributed by the amplifier and hence we can calculate the noise figure of the amplifier.

Other techniques exist for making this measurement but either require accurate measurements of impedance or are inaccurate.

The following formula relates Y-factor to ENR:

[math]\displaystyle{ NF=10\log_{10}\frac{10^{(ENR/10)}}{10^{(Y/10)} -1} }[/math]

Measurements

Noise figure measurements can be made with a noise diode, a power supply for the noise diode, and a spectrum analyser. They can also be made with a specialist noise figure meter. The advantage of the noise figure meter is that it will automatically switch the noise diode on and off, giving a continuous reading of Y; it will also have the correct bandwidths in its receiver to average the received noise in an optimum fashion. However, accurate noise figure measurements are possible with the noise figure meter and a spectrum analyser.