A simple Device for Decreasing Thermal Dependency of a Diode Detector Xavier Le Polozec, Stéphane Muffat-Joly Abstract—Power level detectors used in AGC loop applications or in Receive Strength Signal Indicator (RSSI) need to be, as much as possible, linearized and temperature compensated. Thermal dependent load is used in this paper to decrease the performance variation of a diode detector. Index Terms— Schottky compensated, linearized I.
diode,
detector,
modified such as for each temperature T, the response is the closest as possible to the detector transfer function at temperature T0.
temperature,
INTRODUCTION
T
he theoretical dependence of the transfer function on temperature and load values of a single series diode detector has been presented in [1]. The understanding of how this important detector characteristic varies with both temperature and load is used here to propose and test a very simple device for decreasing thermal dependency of a diode detector. II. DESCRIPTION OF THE DEVICE A. Design Fig. 1. shows the microwave AC-coupled series diode detector structure considered in this paper.
Rs C
i Cj
Rg
Fig. 2. Theoretical dependence of the detector transfer function on temperature (from [1]).
CL
Lt RL
vg
Fig. 1. Structure of the Series Diode Detector
When the temperature varies, detector output voltage varies according to what is shown in Fig. 2. When the detector load value decreases the detector output voltage decreases as well as shown in Fig. 3. A possible way to compensate the variations of the detector transfer function with temperature is to have a temperature dependent load. In other words the load value could be X. Le Polozec is with the Engagement Practice IP & Transport, Ericsson, 91348 Massy, France (e-mail:
[email protected]).
Fig. 3. Theoretical dependence of the detector transfer function on load value (from [1])
IV. CONCLUSION This can be done by using a simple thermistor having a negative temperature coefficient. This proposal is easier to design and has a lower cost than the device described in [2] which uses diode and resistors has variable load. III. RESULTS A. Detector Transfer function Measurement results obtained with a power level detector loaded with a unique 104 Ohms resistance load are compared to measurement made on the same detector loaded with a 104 Ohms thermistor in parallel with a 470.103 Ohms resistance. These two values were experimentally determined. 1.E+01 Vo (V) 1.E+00
1.E-01
1.E-02 70°C
1.E-03
25°C -30°C
1.E-04
Pin (dBm) 1.E-05 -5
0
5
10
15
20
25
Fig. 4. Transfer response evolution with temperature of a detector loaded with a 104 Ohms resistance. The detector (used in a AGC loop device), receives power through a 20 dB directive coupler. Incident Power is measured at the input port of the coupler. Source frequency is 7GHz and the Schottky diode used is a HSMS8101 from Avago. 1.E+01 Vo (V) 1.E+00
1.E-01
1.E-02
70°C 25°C
1.E-03
-30°C
1.E-04 Pin (dBm) 1.E-05 -5
0
5
10
15
20
25
Fig. 5 Transfer response of the same detector circuit as used for the measurement results shown on figure 1 except for the load which is a 104 Ohms thermistor in parallel with a 470.103 Ohms resistance. The error is less than 1 dB over a 20 dB range.
We demonstrate that a low cost device reduced to a unique thermistor can be used to compensate and linearize the transfer function of a diode detector. Note that with such solution the video bandwidth of the detector becomes temperature dependent as well, that can be seen as a drawback for some applications. REFERENCES [1]
[2]
R. G. Harrison and X. Le Polozec, “Nonsquarelaw behaviour of diode detectors analysed by the Ritz-Galerkin method,” IEEE Trans. Microwave Theory Techniques, MTT-42, pp. 840-846, May 1994. Hans Eriksson and Raymond W. Waugh “A Temperature Compensated Linear Diode Detector” RF Design June 2000 http://www.rfmicrowave.com/uploads/diodes/a_temperature_compensated_linear_dio de_detector.pdf
