IAN LANG ELECTRONICS
No 6. The Detector .
The task of the detector is simply to recover the audio information from the applied intermediate frequency and pass it to the audio stages. At this point we leave the superheterodyne system behind and carry on just as we would do for any other type of receiver.
The choice of detector depends upon the method of modulation you wish to receive. For AM the simplest and most commonly used method is the envelope detector, the active component of which is a germanium diode (it has a lower breakover voltage) and in theory we could use this for FM too, as a slope detector. In this method we lose some of the benefits of FM and so, if we were making a disposable ten component TRF we most certainly would, as we have gone to the trouble and expense of making a superhet we would be more likely to use a ratio detector at considerably more trouble and expense.
Back to our portable AM receiver then, and for this we find the ideal solution in the envelope detector. The following diagram shows one made with an 0A91 diode:
It really does not get much simpler than this, and it works by rectifying the incoming signal and developing a varying load across the resistor (known as the diode load) which in this case is a potentiometer. The reason for it being so rather than a fixed resistor is that we can alter the voltage and therefore power of the signal passed to the next stage, the audio amplifier, to increase or decrease impedance (impedance is the sum of reactance and resistance) for if there is too much power in the signal at the amplifier input we will get an impure and possibly distorted tone, or worse we may cause the amplifier to oscillate.
Input to the detector comes from the secondary coil of the last amplifier in the IF Amp chain, and the output is passed via the wiper of the potentiometer to the input of the amplifier.
Recent experiments have suggested that an optocoupler may do the work of the circuit above, but for now the envelope detector remains the better choice as a good quality optocoupler does not come too cheaply.