RF Diodes are a passive semiconductor device used in radio frequency (RF) circuits. They have a variety of different rolls pertaining to their non-linear and directional characteristics. Applications that use RF diodes include: diode mixers, Tx/Rx switches, receiver tuning, voltage controlled oscillators and automatic gain control blocks (AGC).

Schottky diodes are hot carrier diodes with a low voltage drop and can operate as fast switches. They are metal-semiconductor devices as opposed to the normal diode semiconductor-semiconductor junctions. They have fast reverse recovery time – the time which the diode switches from and on to an off state. They are used in RF detectors and mixers and can operate at extremely high frequencies exceeding 50GHz. Historically, Schottky diodes were used as cat’s-whisker detectors in early radios.

Tunneling diodes are fast operating devices with heavily doped P and N-type junctions, so heavy that the bandgap characteristic of the diode is broken down, which facilitates  the quantum mechanical tunneling effect. Forward bias operation has reduced current with higher voltage. This is referred to as a negative resistance effect. Reverse bias tunneling diodes operate like backward diodes (a variation of a zener diode) but with extremely high linearity. This characteristic makes them excellent for detectors, rectifiers and high-speed switches. They are typically made from silicon, germanium or gallium arsenide. They are used in: frequency converters, RF detectors, oscillators and amplifiers. Their odd characteristic of conditionally having a negative differential resistance allows them to be used as RF amplifiers. Microstrip tunneling diode amplifiers are used in high frequency RF and microwave applications like satellite amplifiers and can operate over 15GHz. 

Pin diodes are used as non-ohmic contacts (non-rectifying junctions). They are characterized by having heavily doped P and N –type regions along with a wide intrinsic region. This improves their ability to be used as attenuators, fast (around 10ms) switches and photodetectors but makes them unsuitable for rectification. A PIN diode is turned into a low impedance series element when forward biased, but becomes a high impedance of several kW when not biased. This makes them suitable for use as Rx/Tx switches. A series pin diode in a pin switch circuit provides high impedance isolation to the transmitter, protecting it from damage. As a pin diode turns off it increases its impedance monotonically. This makes it suitable for voltage controlled attenuator applications as may be required in an RF AGC.

RF Mixers, also known as frequency mixers are nonlinear semiconductor devices that perform analog multiplication of two RF signals and outputs the resulting product. The main purpose of a RF mixer is to translate a signal from one frequency to another in receiver and transmitter circuits. An RF signal that is received or to be transmitted is multiplied by a local oscillator (LO) reference frequency. A receiver will input the received higher frequency input and convert this to a lower frequency referred to as an intermediate frequency (IF) or to 0Hz (baseband) in a direct conversion receiver. Transmitters use mixers in the opposite way multiplying a IF signal up to the frequency being transmitted.

Ideally two modulated carriers are multiplied (f₁ x f₂) by a mixer to produce a signal that has two superimposed modulated carriers with sum (f₁ + f₂) and difference (|f₁ - f₂|) frequencies. These are called heterodynes or upper and lower sideband image frequencies. In reality mixers have non-ideal transfer function characteristics and generate additional mixing products known as spurs. Spurs include various levels of all harmonic mixing products, called intermodulation products. Ideally only one product is required – the upper or lower sideband. Mixer topologies that are optimized to reduce spurs and provide a single sideband are known as single sideband (SSB) or image reject (IR) mixers. Suppressing one sideband reduces the filtering requirements of the receiver or transmitter circuit.

Mixers are most likely architected as balanced in reference to their symmetry. This provides the advantage of inherent isolation, intermodulation cancellation, common mode signal rejection and improved conversion efficiency. In order to isolate inputs from each other and the output, a 180^o hybrid is used to turn the 3-port mixer into a 4-port device. The 4-port device has equally split output signals that are 180^o out of phase. When two single balanced mixers are combined a double balanced mixer is created. This topology can be configured so that 75% of spurs cancel at the IF port. Other more complex combinations of balanced mixers is possible. An IQ mixer is a variation of an RF mixer that has two mixer circuits with LO Reference signals phase shifted by 90^o (quadrature). By also phase shifting the output of one mixer by 90^o and adding or subtracting the output to the other mixer, one of the images is cancelled/suppressed due to trigonometric relationships. This is called a Hartley image rejection mixer and has the advantage of reducing filtering requirements

RF mixers are available in a number of technologies, from simple diodes rings to gilbert cells that are comprised of bipolar or FETs arranged in a complex cascode topology. They can have integrated PLL and VCO elements and can incorporate amplifier circuits. Important characteristics of an RF mixer include conversion loss, isolation, 1dB compression point, intermodulation distortion (IMD) characteristics and noise figure. Conversion loss is the difference in power between the input RF power and output IF power levels. Isolation between ports reduces spurs caused by signals bleeding onto the wrong port. 1dB compression point is measure of the linearity of mixer and is the amount of input power required to increase conversion loss by 1dB. IMDs are mixing products created by mixing terms of harmonics caused by non-ideal mixer characteristics. These are referred to as single-tone IMD terms. Noise figure is a degradation of signal to noise ratio (SNR) caused by the mixer and is closer associated with the conversion loss of the mixer.