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An RF probe is a useful tool for measuring RF signals in different parts of a circuit. It can be used to test oscillators, receivers, and transmitters. RF detectors can be used to measure the peak level of an RF signal and reproduce the original message. Different types of RF probes are used in electronic test equipment, such as the high impedance RF probe for oscilloscopes.
A radio frequency (RF) probe is a simple circuit that allows a voltmeter to indicate the relative amplitudes of RF signals on different parts of an RF circuit. It is a very useful test tool in a radio laboratory and usually has a capacitor direct current (DC) decoupling input which isolates any DC from the resulting measured voltage. The RF probe can be used to test oscillators, receivers and transmitters.
In active electronic circuitry, the RF signal can be an alternating current (AC) or an AC signal added to a DC level. A class A amplifier or linear amplifier with a silent or silent DC level at approximately half the DC supply voltage will generate an output that is a DC level with superimposed AC signal measured using an RF probe to isolate the component effect CC . For example, a linear amplifier with a 12 volt (V) supply may have a quiet DC output level of approximately 6 volts of direct current (VDC). When there is no RF input signal, the output is pure DC, but even if there is DC, the RF probe meter will indicate 0V. Once there is RF input and consequential output, the RF probe meter can indicate a DC voltage that correlates to the peak level of the RF signal.
Radio Frequency Oscillation (RFO) is a very important part of RF transmitters and receivers. RF oscillators are circuits that generate RF locally at receivers where they are referred to as RF local oscillators. In transmitters, these oscillators are referred to as RF carrier generators. The RF detector can be a simple capacitor input circuit which has a high frequency blocking diode which acts to short the negative loop while allowing the positive portion of the RF loop to reach the DC measuring circuit. This results in a DC voltage with an average level that is approximately equal to the peak level of the RF signal.
The RF detector may be an RF rectifier which simply produces the average peak level of the RF signal. The amplitude modulated (AM) RF carrier has an average peak level proportional to the message. By using the RF rectifier as an AM RF detector, you can reproduce the original message. There is a very small passive circuit that uses a wire antenna connected to a simple RF diode as an AM detector, which includes a resistor and capacitor to shape the result as low power audio. With an earphone, the result is a simple receiver with no active parts, and this passive receiver only works when there is a strong AM carrier in the area.
Electronic test equipment uses different types of RF probes. The oscilloscope displays a periodic waveform on a two-dimensional screen with amplitude on the vertical and time scale on the horizontal axis. There is the high impedance RF probe used for oscilloscopes which ensures that the test equipment does not alter the measured signal.
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