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AM Detection is a basic requirement for radio reception and well as many other applications such as field strength and SWR meters. It is about the simplest thing you could want to do in terms of signal processing but it turns out to present a few complications. Often it is the weakest link in an otherwise great circuit. A variety of simple AM detector options are reviewed here.
Figure 1 shows a collection of circuits that may be used to detect radio frequency voltages. Due to the basic semiconductor physics involved all the circuits have a number of regions of operation. The first is where the input is too low to produce any output or the output is buried in the noise floor. The second is a square law region where doubling the input produces approximately four times the output. All except the transistor square law circuit and the FET drain bend circuit then move into a third region where the output becomes linearly related to the input.
Figure 1a shows the commonly used single diode circuit. At input signal levels above 600 or 700 mV the output level has a relatively linear correspondence with the input level. Between about 300mV and 600mV it operates as square law detector. Below 300 mV it becomes hopelessly inefficient.
Nevertheless this circuit can be used in AM radios where an Automatic Gain Control (AGC) circuit keeps the input to the diode detector at a reasonably high level. Obviously there is going to be considerable audio distortion from such a circuit even with AGC. The typical distortion level is about 10% which is acceptable for general listening.
A useful improvement in performance can be obtained by forward biasing the diode with a constant current source (Figure 1b). This extends the usable square law detection region down below 40 mV. In the square law region changes in the dynamic resistance of the diode rather than actual switching cause detection. The biased diode detector is useful because the input impedance can be adjusted to suit the application. A high value resistor connected to a relatively high voltage source can approximate a constant current source for this circuit.
Figure 1c is a FET infinite impedance detector. This has a high DC input impedance and an input capacitance that is typically around 30pf. On it’s own it is not much of an improvement over a biased diode circuit because the transconductance of a typical FET is somewhat low. It’s efficiency and linear region can be improved greatly improved by incorporating an additional transistor (Figure 1d). Such high input impedance circuits are useful because they can be directly connected to a tuned LC resonant circuit without causing excessive loading which would otherwise cause a loss of sensitivity and selectivity.
A similar circuit using a standard transistor is shown in figure 1e. This is a very efficient linear detector but in contrast to the FET circuit the input impedance at radio frequencies is very much lower. You can use the circuit successfully in applications where low impedance input signals are available such as in SWR meters.
The two remaining circuits are very sensitive square law only circuits.
Figures 2 and 3 show the relative efficiency of each of the detector circuits compared to what would be obtained with an ideal diode.
It is clear that all of the detectors will give a high level of distortion at low input levels. A detector operating in the square law region will give 25% audio distortion with a 100% modulated AM signal. Fortunately the amount of distortion decreases for more usual levels of AM modulation. The resulting audio quality is generally acceptable for a communications use. All the circuits may can be used for measuring radio frequency signal strength as well.