IV. Diode Detector Power Sensors and Instrumentation

Rectifying diodes have long been used as detectors and for relative measurements at microwave frequencies. For absolute power measurement, however, their use has been limited mainly to the radio and the low microwave frequencies and to power levels already covered by thermistors and thermocouples. The high frequency diodes have been of the point-contact variety and consequently fragile, not very repeatable, and subject to change with time. Now it is possible to construct metal-semiconductor junction diodes for microwave frequencies that are very rugged and consistent from diode to diode. These new diodes can measure power as low as -70 dBm (100 pW) to frequencies of at least 18 GHz. This chapter reviews the design of this detecting diode as a power sensor. 
The same power meters used with thermocouple sensors can be used for diode type power sensors because the electronic circuits for the two sensors have the same basic goals. First, voltages on the order of 100 nV are to be measured-so the same choppers, ac amplifiers, and synchronous detectors are needed. Second, power measurements with both kinds of sensors need a power-reference oscillator, with a precisely known power output, to adjust the calibration of the power meter to fit the particular sensor being used. 

Diode Detector Principles 
Diodes convert high frequency energy to dc because of their rectification properties, which arise from the non-linear current-voltage characteristic. It might seem that the ordinary silicon p-n junction diode would, when suitably packaged, be a sensitive detector. The basic problem with the silicon p-n diode is that, without bias, the diode has extremely high impedance and will supply little detected power to a load. An rf signal would have to be quite large to swing the junction voltage up to 0.7 volts where significant current begins to flow. One alternative is that the diode may be dc biased to 0.7 volts, then only a small rf signal causes significant rectified current. This effort turns out to be in vain, however, mainly because the forward bias gives rise to large amounts of noise and thermal drift. There is little, if any, improvement in the minimum power that can be metered. 
A detecting diode ideally obeys the diode equation 
	(4-1) 
where i is the diode current, v is the net voltage across the diode, Is is the saturation current and is constant at 
a given temperature. and  is a shortened form of writing q/nKT. K is Boltzmann's constant, T is absolute temperature, q is the charge of an electron and n is a correction constant to fit experimental data (its value turns out to be about 1.1 for the devices used here for sensing power). The value of  is typically a little under 40 (volts-1). Equation (4-1) is often written as the power series 

		(4-2)
It is the second and other even-order terms of this series which provide the rectification. For small signals only the second-order term is significant so the diode is said to be operating in the square-law region. When v is so high that the fourth and higher order terms become significant the diode is no longer square law. 

The simplified circuit of Figure 4-1 represents an unbiased diode detector for detecting low level. rf signals. Maximum power is transferred to the diode when the diode resistance for small rf voltages is matched to the source resistance. The diode resistance at the origin, found by differentiating (4-1). is 
Ro = 1/Is 
Resistance Ro is a strong function of temperature; this means the diode sensitivity and the reflection coefficient are also strong functions of temperature. To achieve less temperature dependence, Ro is much larger than the source resistance and a 50 ohm matching resistor is included to terminate the generator. If Ro. when substituted for the diode of Figure 4-1, were made too large, however, there would be poor power conversion from rf to dc; thus, larger Ro yields decreased sensitivity. A compromise between good sensitivity to small signals and good temperature performance results from making Is about 10 microamps and Ro about 2.5 K ohms. 
The desired value of leakage current Is can be achieved by constructing the diode of suitable materials that have a low potential barrier across the junction.