Application Note 1328

Transcription

1 A Temperature Compensated Linear Power Detector Application Note 328 Introduction In many RF applications where an output power level from a source varies across frequency bands and time, it is critical that these variations are either kept to their minimum or allowed to occur in a controlled manner. For example, the power output from a portable two way radio needs to be strictly regulated across frequency bands to meet the specified output power (to minimize output power variation across frequency band). Another example would be in the cellular base station transmitter employing a TDMA scheme, such as GSM, where output power level has to be adjusted dynamically corresponding to different time slots (controlled variation of output power across time). In all these applications, a method to sample or to detect the output power has to be incorporated into the ALC loop to control the output power of the RF source. Figure shows a very simplified ALC loop. A small portion of the output power from the RF PA is being coupled out from the directional coupler for detection. The detector produces a DC level which is proportional to the incident power from the RF PA. The DC level from the detector can then be used to adjust the variable gain amplifier or the attenuation of a variable attenuator to hold the output power from the loop at constant level across frequency. In its simplest form, a power detector can be implemented using Schottky diodes shown in Figure 2. However, such a simple detector has limited applications due to poor linearity and its output dependence on temperature. This application note describes an approach to linearize the characteristics of the power detector and to stabilize the detector output across a wider temperature range. Comparisons between simulated and measured results of a simple detector and a more elaborate linearized detector with temperature compensation are also made. P IN (INCIDENT POWER) HMPS-2820 Figure 2. A simple diode detector for large signal detection. 68 pf V OUT VGA RF PA INPUT OUTPUT 50 W POWER DETECTOR Figure. A typical ALC loop with a detector.

2 A Simple Detector A Schottky diode can be configured as a simple detector as shown in Figure 2. Unlike a small signal detector where sensitivity is of primary concern, a power detector has relatively much larger input power and a shunt resistor can be placed at the anode side of the detector diode to provide low VSWR load to the driving source, which is usually a directional coupler to sample the incident output power from an RF power source. A simulated plot in Figure 3 shows a typical detector output with input power swept from -20 dbm to +30 dbm. In the simulation, the detector output was connected to a very high impedance load. It can be seen that at low temperature and low input drive level, the junction resistance of the diode becomes extremely high and much of the detected voltage will drop across the diode instead of the load presented by the measuring equipment. Equation () describes the relationship between the junction resistance and the saturation current of the diode. What is not obvious in equation () is that the diode saturation current I S is a sensitive function of temperature and thus makes R j very sensitive to temperature. Figure 4 shows the variation of R j with temperature, T, when there is no additional DC bias and RF input applied to the detector diode (I b = 0 and not rectified DC current). R j = nkt q (I s + I b ) Therefore, in most applications with practical load impedance levels, a simple diode detector will exhibit poor temperature stability as high R j at low temperature and low input power causes the large portion of detected voltage to drop across R j instead of across the load. This is shown in the measured result with MW load as shown in Figure 5. () VOUT (V) 0 SIMPLE DIODE DETECTOR AT 900 MHz V OUT vs. P IN -25 C +75 C Figure 3. Simulated simple detector output with very high impedance load. JUNCTION RESISTANCE CALCULATED JUNCTION RESISTANCE vs TEMPERATURE OF A HSMS-282x SCHOTTKY DIODE TEMPERATURE (K) Figure 4. Variation of junction resistance across temperature range. Io =.33 x -8 A Eg = ev n =.046 Rs = 8.839,000 SIMPLE DETECTOR OUTPUT VOLTAGE vs. INPUT POWER AT 900 MHz WITH MW LOAD, C +75 C Figure 5. Measured output of a simple detector with MW load. 2

3 The Temperature Compensated Linear Power Detector In order to stabilize the detector characteristics across temperature, one can lower the junction resistance of the diode. Clearly, the junction resistance of the detector diode can be drastically reduced with DC bias current. Alternatively, a variable load can be connected to the detector to maintain a constant voltage dividing ratio between the detector video impedance and the load impedance. This is illustrated in Figure 6 where a variable resistor varies its resistance with temperature variation to maintain a constant V out. V IN R2 V OUT R Rj Figure 6. Variable load for constant output. The detector shown in Figure 7 achieves this by connecting a shunt diode. The junction resistance of the shunt diode varies with temperature and rectified current. It acts as a variable resistor due to its temperature dependence characteristics of its junction resistance. The introduction of a series resistor at the output was to provide better linearity as discussed in Reference []. P IN (INCIDENT POWER) HMPS-2822 V OUT 68 pf HMPS-2822 Figure 7. A temperature compensated detector. 3

4 Over Temperature Measurement The compensated detector when measured across temperature gives the following output characteristics which exhibit better temperature stability than a simple diode detector with MW load. This is shown in Figure 8. A M3458A multimeter is connected across the load to measure the output voltage. With no load resistor connected to the detector, the input impedance of the M3458A multimeter is the only load to the detector. The result is shown in Figure 9 in the case of the simple detector. Figures 9 and give comparisons of temperature stability between the two detectors (simple detector and temperature compensated linear detector) when no load resistor is connected to the detector s output. It is obvious from the graphs that the compensated detector performs better than a simple diode detector.,000,000 0 ERIKSSON-WAUGH DETECTOR OUTPUT VOLTAGE vs. INPUT POWER AT MW LOAD ACROSS TEMPERATURE AT 900 MHz -25 C +75 C Figure 8. Detected voltage of a compensated detector with MW load at 900 MHz.,000,000 0 OVER TEMPERATURE AT 900 MHz SIMPLE DETECTOR OUTPUT VOLTAGE vs. INPUT POWER AT NO LOAD HOT TEMPERATURE +85 C COLD TEMPERATURE -35 C ROOM TEMPERATURE Figure 9. Measured temperature stability of a simple detector with no load.,000 OVER TEMPERATURE AT 900 MHz DETECTOR OUTPUT VOLTAGE vs. INPUT POWER AT NO LOAD, C +85 C Figure. Measured temperature stability of a compensated detector with no load. 4

5 Figure shows a photograph of a completed compensated linear power detector on a detector demo board. This board can be configured as a compensated or a simple detector as shown in Figure 2 and Figure 3 respectively. Figure. Photograph of a completed detector demonstration board configured as a temperature compensated linear power detector. Conclusion A simple diode detector exhibits very good temperature stability provided the detector output is driving an infinitely-high impedance load. Load impedance sufficiently high to maintain good temperature stability of a simple detector may not be easily realizable in practice. Over temperature measurement on the temperature compensated detector shows better temperature stability compared to a simple detector when a finite load (in the region of MW) is connected to the detector output. References. Hans Eriksson, Raymond W. Waugh, Design Tip: A Temperature Compensated Linear Diode Detector, Agilent Technologies. 2. Raymond W. Waugh, Design Tip: Large Signal Detector for Cellular Handsets and Base Stations, Agilent Technologies. 3. Paolo Antognetti, Giuseppe Massobrio, 988, Semiconductor Device Modeling with SPICE, McGraw-Hill. 4. Raymond Waugh, 999, Diode Seminar Materials, Agilent Technologies. 5

6 BIAS INPUT 0 W JUMPER RF INPUT DETECTOR OUTPUT LOAD HMPS-2822 Figure 2. Detector demonstration board configured as a temperature compensated linear power detector. 6

7 BIAS INPUT 0 W JUMPER 68 pf BYPASS CAPACITOR RF INPUT 0 W JUMPER DETECTOR OUTPUT LOAD HMPS-2820 Figure 3. Detector demonstration board configured as a simple diode detector. For product information and a complete list of distributors, please go to our web site: Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies in the United States and other countries. Data subject to change. Copyright Avago Technologies. All rights reserved. Obsoletes AV0-0344EN AV EN - August 4, 20