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PIN Diode

The ordinary diode is composed of PN junction. A thin layer of low-doped Intrinsic semiconductor layer is added between the P and N semiconductor materials, this P-I-N structure is a PIN diode. Because of the existence of an intrinsic layer, PIN diodes are widely used, from low frequency to high frequency applications, mainly used in the RF field, used as RF switches and RF protection circuits, and also used as Photodiodes. PIN diodes include PIN photodiodes and PIN switch diodes.

Introduction

The microwave switch uses the PIN tube to exhibit an impedance characteristic that is approximately on or off under DC forward-reverse bias voltage, and realizes the conversion effect of controlling the microwave signal channel. The DC volt-ampere characteristic of the PIN diode is the same as the PN junction diode, but in there are fundamental differences in microwave frequency bands. Because the total charge of the PIN diode I layer is mainly generated by the bias current, instead of being generated by the instantaneous value of the microwave current, it only exhibits a linear resistance to the microwave signal.

This resistance value is determined by the DC bias. The resistance value is small when forward biased, close to short circuit, and large when reverse biased, close to open circuit. Therefore, the PIN diode has no nonlinear rectification effect on the microwave signal, which is the fundamental difference from the general diode, so it is very suitable for microwave control devices.

Therefore, the PIN diode can be used as a variable impedance element. It is often used in high-frequency switches (ie microwave switches), phase-shifting, modulation, limiting and other circuits. 

Consider parameters

1. Insertion loss: when the switch is turned on, the attenuation is not zero, which is called the insertion loss.

2. Isolation: When the switch is off, its attenuation is not infinite, which is called isolation.

3. Switching time: Due to the charge storage effect, it takes a process to turn on and off the PIN tube, the time required for this process.

4. Withstand power: Under the given working conditions, the maximum input power that the microwave switch can withstand.

5. Voltage standing wave coefficient: only reflects port input and output matching.

6. Video leak.

7. Harmonics: PIN diodes are also non-linear, which will produce harmonics. In broadband applications of PIN switches, harmonics may fall on the frequency band of use and cause interference.

Working principle

Because the RF resistance of the PIN diode is related to the DC bias current, it can be used as an RF switch and attenuator. Series RF switch circuit: when the diode is forward biased, it is turned on (short circuit); when the diode is zero biased or reverse biased, not only the maximum operating frequency of the switch will be limited, but also the minimum operating frequency will be limited, such as the PIN diode cannot control the on and off of DC or low frequency signals. Affected by the cutoff frequency of the diode, the switch also has an upper limit operating frequency. The frequency band of the switch is required to be as wide as possible, because the frequency band of the signal source is getting wider and wider.

Characteristic
When negative voltage (or zero bias) is applied, the PIN diode is equivalent to capacitance + resistance; when positive voltage is applied, the PIN diode is equivalent to small resistance. By changing the size of the structure and selecting the parameters of the PIN diode, the reflection phase (reference phase) of the short-circuited step ridge waveguide is the same as the reflection phase of the short-circuit waveguide controlled by the positive voltage PIN diode. It is also required that the reflection phase of the short-circuit waveguide controlled by the PIN diode with a negative voltage (or 0 offset) is opposite to  the standard phase (between -164° and +164°).

Other

Control method: adopt TTL signal control. '1' on '0' off.

The selection of PIN diode model is mainly determined according to the measurement range of the optical power meter. The commonly used PIN diodes (such as FU-15PD) are small-signal working devices. The photosensitive surface is not suitable and the range of optical power that can be received is very limited.

The PIN diode can also be adjusted to a high frequency range. To improve the isolation characteristics, we can connect two or more diodes in series, but at the same time it will cause an increase in insertion loss. PIN diodes are essentially current controlled resistors. To reduce insertion loss, they need to use a large amount of DC power to reduce the resistivity in the I (intrinsic) region. This obviously affects battery life. This feature, coupled with the PIN diode solution requires a large number of devices, makes this technology difficult to apply to portable handheld products.