Early Radio Receivers

Coherer receivers

Early damped-wave receivers used a device called a coherer. This consisted of a glass tube containing metal filings. The coherer normally has a high resistance. When radio energy passes through the filings, the minute contact areas arc-weld together. This reduces the resistance dramatically. The simplest damped-wave receiver would place the coherer in a series circuit with a battery and an electric buzzer. When the coherer goes from a high resistance to a low resistance, it is like closing a switch. This completes the circuit to the buzzer. When the waves stop (when the transmitter operator lifts the code key) the coherer does not return to normal. The tiny welds need to be broken by lightly tapping the coherer. To facilitate this, a tapping device called a decoherer is attached to the buzzer's armature. Each time the buzzer is activated the decoherer lightly vibrates against the coherer, keeping the filings from remaining welded together.

The principle of an early simple damped wave receiver. The coherer (A) activates the buzzer (F) when radio waves are received. The decoherer (B) vibrates with the buzzer, tapping the coherer to return it to its high resistance state.

An actual coherer/decoherer

Diode detector receivers

The coherer could only detect the presence of radio waves. I could not follow the power variations that Fessenden hoped to use to carry voice over radio waves. Even before building a successful continuous wave transmitter, he developed radio wave detectors that could receive what would later be called AM transmissions. The most successful that Fessenden developed was the electrolytic detector. This consisted of a fine platinum wire in contact with a platinum electrode submerged in a weak acid solution. When electrical current passes in one direction, the detector has a higher resistance than when current passes in the opposite direction (therefore, the electrolytic detector was a diode). This could rectify radio waves and produce a dc voltage proportional to the instantaneous power of the detected waves.

Later, more practical detectors were developed using silicon and galena crystals or vacuum tube diodes. The following circuit is a modern version of these early receivers.

A modern version of a crystal receiver

Early AM receivers used a diode and a capacitor to rectify and filter the radio signal. The diode acts as a half-wave rectifier and the capacitor charges to the peak voltage of the signal. This is much like the rectifier and filter of a power supply. The difference is that the peak voltage of the signal is continuously changing as the amplitude of the transmitter signal varies. In the above example, if the transmitter is sending a 1 kHz tone, the voltage across the capacitor varies 1,000 times per second, recreating the original signal. A high-impedance earphone is used because it produces an audible output with the tiny currents extracted by the receiver.

The next generation of AM receivers used an audio amplifier to produce an output strong enough to drive a loudspeaker. Another improvement was to add a radio frequency amplifier before the rectifier/detector so that weaker signals could be received.

Heterodyning

To understand the next generations of receivers you need to understand heterodyning. This is where two signals can be mixed to produce new signals.

When two frequencies are mixed together the result is two new additional frequencies. You may occasionally hear a twin-engine propeller-driven aircraft with its engines out-of-synch (this is not common; it is bad form to let it happen). If you hear it, you will hear the characteristic beat as the sounds of the two engines interfere with each other.

The following diagram illustrates what happens. Each of the overlapping vertical lines represents a sound pulse from one engine. In the following case, one engine is causing pulses 100 times per second and the other is causing pulses 104 times per second. Notice that sometimes the pulses happen at the same time but other times they alternate. When they happen at the same time we get a doubly-strong pulse; when they are alternating, there is no definite end of one pulse before the beginning of the other, which weakens the overall strength of the pulses. With sine waves, when the waves are entirely out-of-synch (180° out-of-phase), one wave cancels the other.

Two frequencies of 100 Hz and 104 Hz interact with each other creating a periodic interference pattern (a beat frequency) at a frequency of 4 Hz. This illustration shows the interaction for ½ second so you can see two cycles of the beat frequency.

With one set of pulses occurring at 100 Hz and the other set occurring at 104 Hz, the pulses cycle from a state of being together to an alternating state four times every second. This causes a pulsing of energy at a frequency of 4 Hz. This third frequency is called a beat frequency. In the case of engines being out-of-sync, this pulsing four times each second would be very annoying to anyone on the aircraft, which is why running the engines out-of-synch is to be avoided. There would be yet another frequency produced, which would be 204Hz. Notice that the two new frequencies are at the sum of the two original signals (204Hz) and the difference (4 Hz). The difference frequency is much easier to illustrate, as shown in the illustration.

If you want to demonstrate this to yourself, hum together with a friend at the same pitch. Once you are clearly together, one of you slowly change your pitch while the other doesn't change. You will undoubtedly hear the beats when one of you drops out-of-synch.

This phenomenon is useful in two technologies for radio receivers. The first is the product detector receiver and the second is the Superheterodyne receiver.

The product detector receiver

The second generation of radio receivers used diode detectors instead of coherers (as shown above). Such receivers work well with AM because the rectified and filtered radio signal is a DC voltage that, at any instant in time, is directly proportional to the power of the radio signal. Therefore, as the power varies with amplitude modulation, the receiver recreates the "voice-shaped currents" that were used to modulate the signal.

Such a receiver doesn't work well with CW (Morse Code) transmissions. Since the transmitter is just turned on and off with a telegraph key, the output of the receiver is just pulses of DC voltage. All that is heard in the earphone is faint clicks. You may think that this on-and-off DC voltage could be used to operate a buzzer as the coherer receivers could. However, the coherer worked as a switch; there was a battery in the circuit to operate the bell or buzzer. The diode detector doesn't extract enough energy from the radio waves to operate a bell or buzzer. Before amplifiers were invented, the diode detector receivers could not work well with CW transmission.

One solution for CW reception was the Beat Frequency Oscillator (BFO). Instead of rectifying and filtering the received signal, an oscillator is run at nearly the same frequency as the transmitter. For example, to receive a CW signal at 1MHz, the receiver has an oscillator that operates at, for example, 1.001MHz. This is 1kHz above 1MHz (1MHz is 1,000,000Hz and 1.001MHz is 1,001,000Hz). Therefore, the difference between the two frequencies is 1,000Hz (1kHz). When you mix the 1MHz from the transmitter with the 1.001MHz at the receiver, you get a beat frequency of 1kHz. This is heard plainly in the earphone. Therefore, whenever the transmitter is on (when the transmitter's operator is pressing the telegraph key) the receiver's operator hears a 1 kHz tone in the earphone. The receiver's operator can vary the tone by varying the BFO frequency.