Diode detectors are categorized into two types based on signal power levels:
- For small signal (<-20dBm @ 50) power measurements, they use square law region of diode transfer characteristic,
- for large signal (>-10dBm @ 50) power measurements, they use linear region of diode transfer characteristic
Principle of Operation
The schematic of a simple single diode power detector is shown in Figure 1. The diode detector either use square law or linear I-V characteristic of the diode for RF power measurements.
: reverse saturation current,
: thermal voltage; 26mV @ room temperature,
: Boltzman’s constant,
: absoulte temperature in degrees Kelvin,
: voltage across the diode
By series expansion of Eq-(1),
Let us consider a voltage signal applied at the input of diode detector. To simplify the analyis, let us assume that for small signal levels, then .
If , diode current is approximated by the first two terms in Eq-(2).
The fundamental and second harmonic components of the diode current are bypassed by a capacitor at the output. From the above equation we can see that the DC components of current (I_b) is directly following the square law behavior. The deviation to square law behavior starts when the signal level approaches thermal voltage( = 26mV).
|Table 1. SPICE parameters of Schottky diode HSMS 282x series|
Under these small signal condictions, junction resistance() of the diode is
Therefore junction resistance() of the diode is a function of the total current flowing through it and absolute temperature of the junction (dependence through ).
At DC, the Norton equivalent circuit contains DC current source () in parallel with junction resistance (). The open circuit voltageis given by . The Thevinin equivalent circuit contails in series with junction resistance ().
is parasitic junction capacitance of the diode, controlled by the thick-ness of the epitaxial layer and the diameter of the Schottky contact.
Bondwire connecting die to package, leadframe, bulk layer of silicon, etc add some parasitic resistance to the diode. They can be modeled as a series parasitic resistance of the diode. RF energy coupled into is lost as heat—it does not contribute to the rectified output of the diode.
The linear equivalent model of the diode, by taking , and into account, is shown in Figure 2.
The DC output voltage under these small signal conditions is given by
Avago’s HSMS 282X series Schottky barrier diode is picked for the following analysis, and the SPICE parameters shown in Table 1. The diode forward current versus voltage at different temperatures is illustrated in Figure 3. The simulation results indicate the strong dependence of diode transfer characteristics on temperature, particularly at low temperatures.
……………. square law and linear regions ……………
Figure 4 illustrates the variation of junction resistance or dynamic resistance, derived from Figure 3 at different temperatures, with bias current in accordance with Eq.(5). At large signal levels there is large variation in junction resistance, where it will go below the series resistance(). In Figure 3 we can notice droop in the current at high forward voltages( say beyond …) due to . Beyond this point the forward current is mainly determined by series resistance and varies almost linearly with input voltage.
Temperature Compensated Diode Detector
The effect of temperature on diode I-V characteristics is shown in Figure 3. The effect is mainly due to and dependence on temperature. This dependence results in variation of junction resistance with temperature. The effect of junction resistance variation on output voltage is given by Eq-(6). At low temperatures variation in is very high , so we can predict larger deviation in detector dc output voltage from the ideal transfer characteristic.
If a variable load is connected which can track the detector output impedance and maintain a constant voltage ratio between detector output impedance and load impedance, then temperature effect on the output can be suppressed. Figure 5 illustrates an approach where an identical diode () in series with the load and acts a variable resistor tracking the detector’s diode junction resistance.
Small Signal Diode Detector
The small-signal detector operation is dependent I-V characteristic of the diode in the neighborhood of the bias point. As indicated by Eq-(3) the output current(voltage) is proportional to the square of the input voltage or directly proportional to input power, hence they are also called as “square law” detector.
Figure 8 illustrates the transfer characteristic of small signal diode power detector shown in Figure 1 for different temperatures. The temperature dependency of transfer characteristic is due to variation of junction resistance.
For the temperature compensated diode detector circuit shown in Figure 3 the values of and are set 2.35k, and simulation is performed in circuit simulator for input power range from -70dBm to -10dBm, over different temperatures. The simulation results shown in Figure 9 indicate good linearity and almost no variation with temperature for input power levels upto nearly -20 dBm.
For small signal detectors sensitivity is of primary concern.
Minimum low level sensivity specification for Zero Bias Schottky Detectors is 0.5 millivolts per microwatt (0.5mV/mW)
Dynamic range :
The square law dynamic range may be defined as the difference between the power input for 1 dB deviation from the ideal square law response (compression point) and the power input corresponding to the tangential signal sensitivity (TSS).
Large Signal Diode Detector
Diode detectors with input power levels greater than -20dBm falls under this class. The large-signal detector operation is dependent on the slope of the IV characteristic in the linear portion, consequently the diode functions essentially as a switch. In large-signal detection, the diode conducts over a portion of the input cycle and the output current of the diode follows the peaks of the input signal waveform with a linear relationship between the output current and the input voltage.
They used resistive impedance matching at the input due to availability of large signal swings at the input. This results in input broadband matching and improve flatness over frequency. Generally they are self biased or zero current biased, and mainly used for power monitoring and control in power amplifiers.
Figure 10 illustrates the transfer characteristic of large signal diode power detector shown in Figure 1 for different temperatures. The strong temperature dependency of transfer characteristic at lower input power levels is due to large variation of junction resistance. At high power levels the junction resistance is very small and sometimes dominated by .
For the temperature compensated diode detector circuit shown in Figure 3 the values of and are set 2.35k, and simulation is performed in circuit simulator for input power range from -15dBm to 25dBm, over different temperatures. The simulation results shown in Figure 9 indicate good linearity and almost no variation with temperature for input power levels upto nearly -20 dBm.
- transmit power monitoring and control through AGC in power amplififers
- As RSSI detectors in receivers