In radar or radio receivers, sensitive low-noise amplifiers (LNAs) are bound to fail when subjected to large input signals. So, what are the solutions?
We can use receiver protection limiter (RPL) circuits to protect sensitive components. The "heart" of an RPL circuit is typically composed of PIN diodes, which protect components from large input signals without adversely affecting small signal operation.
RPL circuits operate without external control signals. Such circuits contain at least one PIN diode connected in parallel with the signal path, and one or more passive components, such as an RF choke inductor and a DC isolation capacitor. Below is a simple (but possibly complete) RPL circuit.
When there is no RF input signal or only a small RF signal, the impedance characteristic of the limiter PIN diode reaches its maximum value, typically several hundred ohms or more. Therefore, the diode produces a very small impedance mismatch, resulting in low insertion loss.
When a large input signal is received, the RF voltage forces charge carriers (holes in the P layer and electrons in the N layer) into the I layer of the PIN diode. Once in the I layer, the free charge carriers reduce the RF resistance, which, from the perspective of the RF port of the RPL circuit, results in impedance mismatch.
This mismatch causes energy from the input signal to be reflected back to the corresponding signal source. The reflected signal, in conjunction with the incident signal, generates a standing wave with minimum voltage in the PIN diode because the reflected signal temporarily presents the lowest impedance on the transmission line.
Each minimum voltage on a transmission line has a corresponding maximum current. The maximum current flows through the PIN diode, causing an increase in the number of free charge carriers in the diode's I layer, resulting in lower series resistance, greater impedance mismatch, and a "smaller" minimum voltage. Finally, the diode's resistance will reach a minimum value—a value that depends on the PIN diode design and the amplitude of the RF signal.
As the RF signal amplitude increases, it forces the diode to reach full conduction, thereby further reducing the diode's resistance until the diode saturates and produces the minimum possible resistance. The resulting curve comparing the output power and input power is shown below.
When the large RF signal ceases to appear, if the number of free charge carriers in the I layer is large, the diode resistance will remain at a low level (although the insertion loss is still relatively high). After the large RF signal is interrupted, the number of free charge carriers can be reduced through two mechanisms: charge conduction outside the I layer; and charge recombination within the I layer.
The magnitude of charge conduction is mainly determined by the DC resistance in the external current path of the diode.
The rate of charge recombination is determined by several factors, including the density of free charge carriers in the I-layer, the concentration of doped atoms in the I-layer, and other charge trapping sites, among others. Considering the necessary parameters of a diode, the larger the RF signal that a PIN diode can safely handle, the longer it takes to recover to a low insertion loss.
Therefore, the characteristics of the PIN diode's I-layer determine the performance of the RPL circuit. The thickness of the I-layer (sometimes called the width) determines the input power at which the diode reaches its limit: the thicker the I-layer, the higher the input reference 1dB compression level (also known as the threshold level). The thickness of the I-layer, the area of the diode junction, and the diode's manufacturing materials determine the diode's resistance, capacitance, and thermal resistance.
The simplest PIN RPL circuit can be implemented with just a PIN diode, an RF choke inductor, and a pair of DC isolation capacitors. The RF choke inductor is crucial to the performance of the RPL circuit; its main function is to complete the DC current path for the PIN diode. When a large signal forces charge carriers into the diode's inductor layer, a DC current is generated in the diode. Without providing a complete path for the DC current, the diode's resistance cannot be reduced, and the diode will not reach its limit. The DC current will flow in the direction of the rectified current, but it is not generated by rectification.
Installing a choke inductor in an RPL circuit is extremely challenging because inductors are the least desirable components in an RPL circuit. Due to inductance values and parasitic inter-winding capacitance, all inductors exhibit both series and parallel resonance. Therefore, extreme care must be taken to ensure that series resonance does not occur within the operating frequency band. Furthermore, the DC resistance of the choke must be minimized to shorten the recovery time of the RPL circuit.
Note: DC isolation capacitors are optional. They are only required when a DC voltage or current appears on the input or output transmission line that could bias the PIN diodes.
For example
Assuming the low-noise amplifier (LNA) can handle a maximum input power of 15 dBm, the required I-layer thickness of the PIN diode in the RPL circuit is approximately 2 micrometers. Designers can determine the acceptable capacitance of the PIN diode based on the RF signal frequency and the maximum acceptable small-signal insertion loss. If the designer assumes the RPL circuit operates in the X-band and the maximum acceptable insertion loss is 0.5 dB, the maximum capacitance of the diode can be calculated.
The insertion loss (IL) of the parallel capacitor (in decibels) can be calculated using the following formula:
We can calculate the value of C using the formula:
When f = 12 GHz, IL = 0.5 dB and Z0 = 50 Ω:
C=0.185pF
The obtained capacitance value, together with the I-layer thickness, determines the area of the diode junction.
If the I-layer is thin and the junction area is small, the diode will have relatively high thermal resistance. This necessitates exceeding its maximum rated junction temperature of 175°C to dissipate more energy. Typically, a 2-micron diode with a capacitance of 0.185pF can safely handle large CW input signals of approximately 30-33dBm. Because Joule heating occurs as current flows through the diode's resistance, large signals can damage or even instantly burn out the diode.
PIN diode RPL circuits provide reliable protection for sensitive components such as LNAs in radar or radio receivers, protecting them from large incident signals. When RPL applications require extremely low steady-state leakage output power and high input power handling capability, additional diode stages and other circuit enhancement components can be added to the input side of the RPL circuit.
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