DETECTORS
What is a detector?
In radio, a detector is a device or circuit that obtains information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918).
Unlike present-day radio stations which usually transmit sound (an audio signal) using an uninterrupted carrier wave, the early transmitters communicated information using radio-telegraphy; to achieve this the transmitter carrier-signal is switched on and off to produce pulses of radio waves which spell-out text messages in Morse code. Morse code, often called CW — carrier wave even today because ONLY the carrier wave is used for communication.
CW#
Consequently, early radio receivers only had to detect the presence or absence of the radio signal. The device which did this was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector were used during the wireless telegraphy era until these methods were totally superseded by vacuum tube technology. (valves)
AM Modulation#
After sound (amplitude modulation, AM) transmission began around 1920, the term evolved to mean a demodulator, (usually a vacuum tube) which extracted the audio signal from the radio frequency carrier wave. This is its current meaning, although modern detectors usually consist of semiconductor diodes, transistors, or integrated circuits.
Superheterodyne#
In a superheterodyne receiver, the term detector is also used to refer to a mixer, (usually a non-linear mode device, where a fast-switch is the best case. However, mixers may be constructed from diodes, a valve, or transistor-based circuit) which converts the incoming radio frequency signal to the intermediate frequency. The mixer is sometimes called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector or product detector.
A Simulation Of A Simple Diode Amplitude Modulation Detector#
The above circuit#
Description of the signals#
- Green — the trace at the top in green shows the effect of the diode. Effectively, this has chopped the bottom half of the amplitude modulated wave off, leaving us with essentially positive going parts of the carrier but still amplitude modulated.
- Orange — this trace, in the middle, shows the filtered AM modulated signal recovered after filtering out the carrier using a simple RC low pass filter.
- Blue — here we see the original AM modulated carrier wave at 1 MHz.
What is happening?#
In the above circuit, we see the diode recovering the 50 kHz Amplitude modulated signal from the 1 MHz carrier wave. Essentially, as mentioned above, the diode chops one half off the wave, which, when broadcast, has amplitude modulation in both the positive and negative quadrants.
Trying to use the modulated 1 MHz signal without some treatment is useless because the AM signal within the carrier side occurs at both positive going, and negative going sides of the electromagnetic AM wave. (50 kHz is well above human hearing, but is used here such that it is clearly visible on the scope)
The product#
Because in the above diagram, there are signals overlaid atop of one another, and the amplitude of the product recovered from filtering the output of the diode is feeble, the following diagram is useful.
Here, we see the modulated signal imperfectly filtered from the product of the diode, using simple 4 element RC filters.
Simple RC filter#
In this simulation, after the diode, there is a simple 4 element RC filter circuit that lets us see the recovered signal.
The cutoff frequency (in hertz), is determined by the following formulae: