2.1 Thermal RF Power Sensors
Intro
There are a several different technologies available for the measurement of RF power. These generally fall into four categories:
Thermal The heating effect of RF power upon a sensing element is measured.
Detector The RF signal is rectified or “detected” to yield a DC voltage proportional to the signal’s amplitude.
Receiver A “tuner” type circuit is used to receive the signal, then measure its amplitude component.
RF Sampling The RF signal is treated as a baseband AC signal, and is directly digitized.
Both thermal and detector type measurements are typically “direct sensing,” in which the amplitude of the RF signal applied to a load element is measured by converting the RF to an easily-measured DC quantity. This RF-to-DC conversion is typically performed close to the signal source by connecting a small converter probe known as an RF power sensor to the device under test.
Power Meter System
Direct Power Measurement Block Diagram
The receiver and RF sampling methods are usually indirect – the signal is brought into an instrument via a cable connection, processed through a multi-stage circuit to yield amplitude information, then scaled to power.
Following is a discussion of each of these technologies.
2.1 Thermal RF Power Sensors
Thermal sensors use the incoming RF energy to produce a temperature rise in a terminating load. The temperature rise of the load is measured either directly or indirectly, and the corresponding input power is computed. The simplest is the early “light bulb” power detector used by Ernst Lecher in the late 1800s.
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Figure 2.1.1 Thermistor Sensor Diagram
Thermistor (Bolometer) sensors use a thermal element, known as a thermistor, as both the RF load and the temperature measurement device. The thermistor’s resistance changes with temperature, making it simple to measure its temperature by detecting in-circuit resistance.
The most common implementation places the thermistor element in one corner of a wheat-stone bridge, and uses a DC substitution technique, in which a controlled DC bias current is applied to the bridge to heat the thermistor until its resistance equals that of the other bridge resistors and the bridge is in balance. An auto-balancing circuit is used to amplify the bridge output and drive the entire bridge with this bias signal, heating the thermistor until balance is achieved. The net effect is that the thermistor will be operated at a constant temperature point where its resistance remains at the correct value to properly terminate the incoming RF – typically 50 or 100 ohms.
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Figure 2.1.2 Bolometer Diagram
The total power dissipated by the thermistor is the sum of the incoming RF power and the power due to the DC bias. The power dissipated due to the RF heating can be computed by subtracting the thermistor’s reference “DC-only” power (measured and stored when no RF is applied) from its total (DC+RF) power. When the bridge is balanced, the thermistor’s dissipation due to the DC bias is easily computed as one-quarter of the total bridge power (bridge current multiplied by bridge voltage). The other three resistors in the bridge are designed to have a negligible temperature coefficient of resistance.
In practical implementations, there are two identical thermistor bridges, but only one is exposed to the RF. The second bridge is used to compensate for ambient temperature changes.
An RF signal is applied to the terminated load of a thermocouple sensor and the rise in temperature is measured. The rise in temperature is due to the Thermocouple Principle. A thermocouple is formed by a metallurgical junction between two dissimilar metals which produces a small voltage in response to a temperature gradient across each metal segment – typically just a few tens of microvolts per degree C.
In a practical thermocouple power sensor, a number of thermocouples may be electrically connected in series to form a thermopile. This increases the output voltage so it can be more easily amplified and measured by the meter. The thermopile often forms the RF load as well, and is connected in such a way that the RF energy flows through and heats only one end (the “hot junction”) of each thermocouple. This is done by capacitively coupling the RF while maintaining DC coupling for the output signal.
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Figure 2.1.3 Diagram illustrating Thermocouple Principle
The output voltage of a thermocouple type power sensor is very linear with input power and has a relatively long time constant due to heat flow delays. This means that they will tend to produce a reading which is proportional to the average of the applied RF power. Because of this, thermocouple sensors are commonly used for measuring the average power of a modulated signal. Their relatively low sensitivity, however, limits their usefulness when the RF power level is less than several microwatts.