Diode Detector 2 comments

Diode detectors are categorized into two types based on signal power levels:

  • For small signal (<-20dBm @ 50\Omega) power measurements, they use square law region of diode transfer characteristic,
  • for large signal (>-10dBm @ 50\Omega) 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.

Figure 1. Simple diode power detector

Figure 1. Schematic of simple diode power detector

For small signals over a limited signal range, the I-V characteristic of a diode is described by the equation:

(1)   \begin{equation*} i_d = I_s \left(e^{v_d/\eta V_t} -1 \right) \end{equation*}

where,
I_s : reverse saturation current,
V_t = {kT\over q} : thermal voltage; 26mV @ room temperature,
K : Boltzman’s constant,
T : absoulte temperature in degrees Kelvin,
v_d: voltage across the diode

By series expansion of Eq-(1),

(2)   \begin{equation*} i_d = I_s \left[ \left({v_d \over \eta V_t}\right) + \left({1\over 2}{v_d \over \eta V_t}\right)^2 + \ldots \right] \end{equation*}

Let us consider a voltage signal v_{RF}(t) = V_{RF} \cos(\omega_{RF}t) applied at the input of diode detector. To simplify the analyis, let us assume that V_{RF} \gg V_o for small signal levels, then v_d \approx v_{RF}.

If V_{RF} \ll V_t, diode current is approximated by the first two terms in Eq-(2).

(3)   \begin{eqnarray*} i_d &=& I_s \left[ \left({V_{RF} \cos(\omega_{RF} t) \over \eta V_t}\right) + {1\over 2!}\left({V_{RF} \cos(\omega_{RF} t) \over \eta V_t}\right)^2 \right] \\ &=& \underbrace{{I_s \over 4}\left({V_{RF}\over \eta V_t}\right)^2}_{I_{b}} + I_s \left({V_{RF}\over \eta V_t}\right)\cos(\omega_{RF} t) + {I_s \over 4}\left({V_{RF}\over \eta V_t}\right)^2 \cos(2\omega_{RF} t) \end{eqnarray*}

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(V_t = 26mV).

Parameter Value Units
Bv 15 V
Cjo 0.7 pf
Eg 0.69 eV
IBV 1E-4 A
IS 2.2E-8 A
N 1.08
RS 6.0 Ω
PB 0.65 V
PT 2
M 0.5
Table 1. SPICE parameters of Schottky diode HSMS 282x series

Under these small signal condictions, junction resistance(R_j) of the diode is

(4)   \begin{equation*} R_j = \left({\partial i_d \over \partial v_d}\right)^{-1} \Rightarrow \underbrace{\quad R_{j,0}={\eta V_t \over I_s}\quad}_{\mbox{at zero bias current}} \end{equation*}

At bias current I_b, junction resistance of the diode is

(5)   \begin{equation*} R_{j}\left|_{I_b}\right. = {\eta V_t \over I_s + I_b} \end{equation*}

Therefore junction resistance(R_j) of the diode is a function of the total current flowing through it and absolute temperature of the junction (dependence through V_t).

At DC, the Norton equivalent circuit contains DC current source (I_b) in parallel with junction resistance (R_j). The open circuit voltageis given by V_{d,DC}= {V_{RF}^2 / 4 \eta V_t}. The Thevinin equivalent circuit contails V_{d,DC} in series with junction resistance (R_j).

C_j 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 R_s of the diode. RF energy coupled into R_s is lost as heat—it does not contribute to the rectified output of the diode.

The linear equivalent model of the diode, by taking R_j, R_s and C_j into account,  is shown in Figure 2.

Figure 2. Linear equivalent circuit model of diode

Figure 2. Linear equivalent circuit model of diode

The DC output voltage under these small signal conditions is given by

(6)   \begin{equation*} V_{out} = {R_1 \over R_1 + R_j} V_{d,DC} \end{equation*}

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 ……………

Foward I-V characteristics of a Schottky diode

Figure 3. Foward I-V characteristics of a Schottky diode

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(R_s). In Figure 3 we can notice droop in the current at high forward voltages( say beyond …) due to R_s. 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 V_t and I_s 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 R_j 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 (D_2) in series with the load R_1 and R_2 acts a variable resistor tracking the detector’s diode junction resistance.

Figure 5. A temperature compensated diode detector

Figure 5. A temperature compensated diode detector

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.

Simulation Results

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.

Figure 6. Small signal diode detector transfer characteristic (output voltage versus input power)

Figure 6. Small signal diode detector transfer characteristic (output voltage versus input power)

For the temperature compensated diode detector circuit shown in Figure 3 the values of R_1 and R_2 are set 2.35k\Omega, 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.

Figure 7. Temperature compensated small signal diode detector transfer characteristic at different temperatures

Figure 7. Temperature compensated small signal diode detector transfer characteristic at different temperatures

Specifications

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.

Simulation Results

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 R_s.

Figure 8. Large signal diode detector transfer characteristic (output voltage - input power ) at different temperatures

Figure 8. Large signal diode detector transfer characteristic (output voltage – input power ) at different temperatures

For the temperature compensated diode detector circuit shown in Figure 3 the values of R_1 and R_2 are set 2.35k\Omega, 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.

Figure 9. Temperature compensated diode detector transfer characteristic at different temperatures

Figure 9. Temperature compensated large signal diode detector transfer characteristic at different temperatures

Applications

  1. transmit power monitoring and control through AGC in power amplififers
  2. As RSSI detectors in receivers

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2 thoughts on “Diode Detector”

  • Reply ↓
    Mark

    Thanks for the post – but can you add the option of a printer-friendly format too, as many blogs do. Have you tried printing this page out? The web heading (and other screen objects) overlay the text on every page. It’s rather ugly.
    Why am I printing it? – Because I need to make notes as I go through it. Just something for you to consider.
    Thanks.

    • Reply ↓
      Radio Geek Post author

      Hi Mark, Thank you for the feedback. I had tried earlier once and even today to enable print option. But due to theme related issues I’m not able to do at present. I hope future upgradation of theme may allow me. If you require let me know, I can email you a softcopy.