In the radar or radio receiver, the sensitive low noise amplifier (LNA) must be corrupted when a large input signal is subjected to a large input signal. So, what solution?
We can protect the sensitive components using the receiver protection device limiter (RPL) circuit. The "heart" of the RPL circuit is usually composed of a PIN diode, which protects the component from a large input signal, and does not adversely affect small signal operations.
The operation of the RPL circuit does not require an external control signal. Such circuitry contains at least one PIN diode in parallel with the signal path, and one or more passive components, such as RF choke inductance, and DC isolation capacitance. The following is a simple (but may be a complete) RPL circuit.
When there is no RF input signal or there is only RF small signal, the impedance characteristics of the limiter PIN diode reaches the maximum, typically a few hundred ohms or more. Therefore, the diode produces very small impedance mismatch, accordingly, low insertion loss can be made.
When a large input signal occurs, the RF voltage forces the charge carrier (the hole of the p-layer and the N-layer electron) into the I layer of the PIN diode. After entering the I layer, the free charge carrier reduces its RF resistance, from the RF port angle of the RPL circuit, which produces impedance mismatch.
This mismatch causes energy from the input signal to be reflected to the corresponding signal source. The reflected signal is coordinated with the incident signal, generating a small voltage minimum in the PIN diode because the reflected signal temporarily presents a minimum impedance on the transmission line. Each minimum voltage on the transmission line has a corresponding maximum current. The maximum current flows through the PIN diode, resulting in an increase in the amount of free charge carriers in the diode I layer, thereby producing a minimum voltage of lower series resistance, larger impedance mismatch, and "smaller". Finally, the resistance of the diode will reach the minimum - the value depends on the design of the PIN diode and the magnitude of the RF signal. When the RF signal is increased, the diode forces the diode to achieve a sufficiently turning state, thereby further reducing the resistance of the diode, until the diode is saturated and produces as minimum resistance as possible. This results in the contrast curve of the output power and the input power, as shown below.
When the RF large signal does not appear, if the free charge carrier amount in the I layer is large, the resistance of the diode remains at a lower level (at this time the insertion loss is still larger). After the RF large signal is interrupted, the amount of free charge carriers can be reduced by two mechanisms: (1) The charge conduction (2) is performed in the I layer in the I layer.
The magnitude of the charge conduction is mainly determined by the DC resistance of the diode external current passage.
The rate of charge recombination is determined by a plurality of factors, including the density of the free charge carrier in the I layer, the concentration of the inclusion in the I layer and other charge capture points, and the like. Considering the necessary parameters of the diode, the larger the RF signal of the PIN diode can be safely treated, the longer the time required to return to the low insertion loss.
Therefore, the characteristics of the PIN diode I determine the performance of the RPL circuit. The thickness of the I layer (sometimes referred to as the width) determines the input power when the diode reaches the limit: The thicker the I layer, the higher the input reference 1DB compression level (also referred to as a threshold level). The thickness of the I layer, the area of the diode and the manufacturing material of the diode determine the resistance, capacitance, and thermal resistance of the diode.
Just a pin diode, an RF choke inductor and a pair of DC isolation capacitors to achieve the simplest PIN RPL circuit. The RF choke inductor is critical to the performance of the RPL circuit, and its main function is to make the DC current path of the PIN diode become complete. When the large signal is forcing the charge carrier to enter the diode I layer, a DC current is generated in the diode. If the full path is not provided for the DC current, the resistance of the diode cannot be reduced, and the diode does not reach the limit. The DC current will flow in the direction of the rectifier current, but this is not generated by rectification.
Installing choke in an inductance in the RPL circuit is a very challenging thing because inductance is the least ideal component in the RPL circuit. All inductors have series and parallel resonance based on inductive value and parasitic winding between capacitance. Therefore, it must be very careful to ensure that tandem resonance occurs in the working frequency band. In addition, the DC resistance of the choke is required to minimize the recovery time of the RPL circuit.
Note: DC isolation capacitors are optional. A DC isolation capacitance is only required when there is a DC voltage or current that may cause the PIN diode biased on the input or output transmission line.
Example
It is assumed that the low noise amplifier (LNA) can withstand the maximum input power of 15 dBm, requiring the I layer thickness of the Pin diode in the RPL circuit to about 2 microns. Designers can determine the acceptable capacitance of the PIN diode based on the acceptable maximum of the RF signal frequency and the small signal insertion loss. If the designer assumes that the RPL circuit operates in the X band, and acceptable maximum insertion loss is 0.5 dB, the maximum capacitance of the diode can be calculated.
Insertion loss (IL) (IL) (in decibels) can be derived according to the following formula:
We can solve the C value according to the formula:
When f = 12 GHz, IL = 0.5 dB and Z0 = 50Ω, C = 0.185pf.
The resulting capacitance value and the thickness of the I layer determine the area of the diode junction.
If the I layer is thin and the junction area is small, the diode will have a relatively high thermal resistance, so that the junction temperature exceeds its maximum rated value 175 ° C to dissipate more energy. Typically, 2 micron diodes having a capacitor are 0.185 pF can be safely treated with a large CW input signal of approximately 30-33 dBm. Since the current flows through the diode resistance, the jeopathy is generated, the large signal may be damaged or immediately burn the diode.
The PIN diode RPL circuit provides reliable protection for sensitive components such as LNA in the radar or radio receiver to protect it from a larger incident signal. When the RPL application requires extremely low steady-state leak output power and higher input power processing capability, additional diode grade and other circuit enhancements can be added on the input side of the RPL circuit.
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