It is often helpful to have a concise definition of signal to noise ratio as this can make it easier to check the overall specifications in radio receiver data sheets. Typically a certain input level required to give a 10 dB signal to noise ratio is specified. As the signal input level obviously has an effect on this ratio, the input signal level must be given. The difference is normally shown as a ratio between the signal and the noise, S/N, and it is normally expressed in decibels. Nevertheless the S/N ratio or SNR is an important specification, and is widely used as a measure of the performance of many RF circuit designs, particularly for radio receiver sensitivity White noise spectrum However it has a number of limitations, and although it is widely used, other methods including noise figure are often used as well. The signal to noise ratio of a signal in a system is easy to comprehend, and therefore it has been widely used in many areas. The concept of signal to noise ratio is also used in many other areas including audio systems, and many other areas of circuit design. Amateur radio transmitter receiver for which sensitivity is quoted as signal to noise ratio Concept of signal to noise S/N ratio SNRĪlthough there are many ways of measuring the sensitivity performance of a radio receiver, the S/N ratio or SNR is one of the most straightforward and it is used in a variety of applications. Unfortunately, as we will see in Chapter 3, the “faster” (low f /#) a lens is, the larger the aberrations.Video- radio receiver signal to noise ratio, SNRĪs the noise introduced by the first RF amplifier will be amplified the most, this RF amplifier becomes the most critical in terms of overall RF circuit design for the radio receiver sensitivity performance.Īccordingly, the focus of the RF circuit design for any radio receiver should focus on the initial stages of the radio as these have by far the greatest effect on the signal to noise performance. This means that an IR system with an f /1 objective performs four times better with regard to S/N than an f /2. The S/N is inversely proportional to the square of f /#, the relative aperture. This simple expression indicates the strong influence of the chosen optical system. D* is the signal-to- noise ratio when 1 W is incident on a detector having a sensitive area of 1 cm 2, and the noise is measured with an electrical bandwidth of 1 Hz. The noise equivalent power NEP is a function of the detector size d', the electrical bandwidth Δ f used in the measurement, and the detector figure of merit D*, which has the somewhat unusual dimension of cm Hz 1/2 W −1. With this and the substitution of f /# for f / D, Eq. The relationship between the radiant emittance and the radiance is N = W/π. The radiated power in watts per square centimeter from a flat diffuse source surface into a hemisphere is the radiant emittance W. Where d' is the linear dimension of the square detector. In this case, the area of the exit pupil is D 2π/4, and s' becomes f, which modifies the image-side expression of Eq. When the object is located at infinity, the image is formed in the focal plane. It provides a choice for determining the power transfer from either the object side (target side) or the image side ( detector side). The principal point to be made is that Eq. In this fundamental expression, N appears equal in both relations, indicating that no reduction of radiation has been accounted for due to loss in transmission or other factors. EP' and EW' are exit pupil and exit window and s' is the separation of the two. The primed symbols refer to the image side of the system. N is called the radiance of the source (W cm −2 ster −1). Where EP and EW are the entrance pupil and entrance window areas (cm 2) and s is the separation of the entrance window from the entrance pupil (cm). NEP represents the noise equivalent power, a measure of the minimum signal that yields a unity signal-to- noise ratio. Where P is the collected radiant power in watts that is received by the detector. In its simplest form, the signal- noise-ratio is stated by
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