14. DETECTOR OR DEMODULATOR

So we deal within this course only with AM-receivers we will of course deal her only with AM-Demodulators. But we should bear in mind, that there are so called FM-detectors too, which are very different from AM-detectors.

2. Function of the detector

The detector has to filter the AF-signal from the RF-signal (in a “TRF-receiver”) or the IF-signal (in a superhetreceiver).

If we remember the IF-signal as shown in fig. 180 (upper part) we see easily that the detector should produce the signal which is represented by the intermitted line.

fig. 180

The way how a detector is working can easily be understood if we at first concentrate on a short interval of the IF-signal shown in fig. 180. (lower part).

Within the time interval represented here the intermitted line is almost horizontally, which means: the AF-signal desired at the detector output is equal to the peak voltage of the IF-signal. This could be produced by just rectifying the IF-signal and smoothing it by a reservoir-capacitor. But doing this would lead to an output signal as shown in fig. 181. (upper part).

fig. 181

Obviously the proposed solution is not enough yet satisfying. Beside rectification we have to make sure that the AF-signal will decrease again, if the IF-signal peaks decreases again. The desired relation between in- and output-signal is shown in fig. 181 (lower part).

This can be achieved by a resistor connected in parallel to the smoothing capacitor as shown in fig. 184.

fig. 184

3. ADVANCED DETECTOR CIRCUIT.

In the above introduced detector circuit it will be made sure that the capacitor is selected as small as possible in order not to interfere to the AF-signal.

But in reality this method will cause always still a RIPPLE on top of the AF-signal as represented in fig. 185

fig. 185

So always with this type of detector circuit the output will be a mixture of AF- and IF-signals.

In order to get only the AF-component of the input signal we find additional circuits necessary.

These additional circuits are the

RF-filter which removes the ripples.
DC-filter which removes the dc-component from the output of the RF-filter.

4. DIFFERENT KINDS OF DETECTOR CIRCUITS

A) Diode in series with R:

This is the type developed in the chapter before. It is the most often used one. It is loading the input-circuit which is normally a tuned circuit. In order to prevent that tuned circuit from distuning, the input signal is fed in via tapping transformer as shown in fig. 185.

B) Diode in parallel with R:

The advantage of this circuit is, that it is decoupled from the tuned circuit. Therefore we can connect this detector to any other potential.

fig. 186

Its disadvantage is: the resonant circuit is loaded even more than with the type shown above, because the resistor is always passed by a current while in the circuit above it is cutting off during one halfwave.

C) Demodulation by a voltage doubler

In order to produce a higher output voltage which will be less destorted, there is sometimes used a special circuit which is also used for special cases of rectification in order to reach higher output values. During the first halfwave with a positive potential at point A a diode 1 will be biased forward and C1 is charged. After this halfwave is over there will be found a certain voltage at C1 which is equal to the peak voltage of the first halfwave.

fig. 187

During the second halfwave diode 1 will be connected in reverse direction, but diode 2 will be biased in forward direction. And now capacitor C2 will be charged by the voltage originated from the tuned circuit and the voltage of C1.

At the end of this second halfwave C2 will be charged with a voltage two times as high as the peak voltage of the tuned circuit.