Reflectional receiver
Encyclopedia
A reflectional radio receiver (also called a reflex radio) is a radio receiver design in which the same amplifier is used for both the high-frequency radio (RF) and low-frequency sound (AF) signals. The radio signal from the output of the amplifier passes detection and then re-enters the input of the amplifier. During the second pass, the sound frequency is amplified then passed to the earphone
. The German company Telefunken applied the German patent 293300 in the year 1913 for the reflex receiver.
The output of the amplifier is usually terminated by the radio frequency transformer. The amplified radio signal is taken from the secondary winding of the transformer k, passes detection (by the diode l in the given example) and then enters the same amplifier again as the low frequency signal through transformer o1. The amplified AF signal passes through transformer p1 to the earphone.
Reflectional radio receivers were used because fewer amplifier devices are needed. The 4 tubes General Electric superheterodyne receiver F40 used one tube for RF and AF amplification. They also consume less electricity than a receiver designed with two separate amplifiers
. However, this design is less stable, with the possibility of breaking into unwanted oscillation. A greater level of skill and experience is needed to debug the assembled device before it starts working. In the schematic to the right, the value of the resistor R1 must be carefully chosen by hand because of variations in current gain between triodes of the same part number.
For a reflex circuit to work properly, bypassing and filtering are major considerations; also, it is important to insure the signal is AT ALL TIMES entirely operating within the linear range of the amplifier, or else inter-modulation (IM) will occur. It is difficult to judge when minor amounts of non-linearity occur in an amplifier. A reflex circuit will immediately produce severe distortion and possible oscillation as soon as the signal exceeds the linear range of the active component's curves, as this is the circumstance where the active device begins to work as a detector, which means it is mixing signal with the AF being fed back to amplify. This adds considerable difficulty to the making of a circuit which is dependably free of problems for all signals available. Problems are most common with large signals. LAYOUT considerations are critical in order to avoid unwanted coupling of RF signal back to the input; chokes should be toroid, not linear in physical construction, to avoid magnetic coupling
for instance, and capacitive coupling
must be prevented with careful layout, possibly including interleaving ground leads on PC board layout, or by using shielded wiring for hand wired circuits. The experimenter may care to employ a double sided printed circuit board, with one side being a ground plane, and connections laid out on the other side. However, if done properly, a reflex receiver can astound one with its sensitivity and gain with minimal component count. One example on the web gives drives a speaker with enough volume level for comfortable listening with only one transistor. The experimenter does well to try such a circuit, which is very under represented in modern experimental electronics literature. The use of reverse feedback at the DC level with appropriate decoupling filtering in the loop will help insure stability by self adjusting the bias level automatically; adjustment to the correct point consists of adjusting the trim pots of the bias feedback loop to insure correct biasing for linear operation.
Crystal earpiece
A crystal earpiece, more properly called a piezoelectric earphone, is an earphone that produces sound by using a piezoelectric crystal, a material that changes its shape when electricity is applied to it...
. The German company Telefunken applied the German patent 293300 in the year 1913 for the reflex receiver.
The output of the amplifier is usually terminated by the radio frequency transformer. The amplified radio signal is taken from the secondary winding of the transformer k, passes detection (by the diode l in the given example) and then enters the same amplifier again as the low frequency signal through transformer o1. The amplified AF signal passes through transformer p1 to the earphone.
Reflectional radio receivers were used because fewer amplifier devices are needed. The 4 tubes General Electric superheterodyne receiver F40 used one tube for RF and AF amplification. They also consume less electricity than a receiver designed with two separate amplifiers
Tuned radio frequency receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
. However, this design is less stable, with the possibility of breaking into unwanted oscillation. A greater level of skill and experience is needed to debug the assembled device before it starts working. In the schematic to the right, the value of the resistor R1 must be carefully chosen by hand because of variations in current gain between triodes of the same part number.
For a reflex circuit to work properly, bypassing and filtering are major considerations; also, it is important to insure the signal is AT ALL TIMES entirely operating within the linear range of the amplifier, or else inter-modulation (IM) will occur. It is difficult to judge when minor amounts of non-linearity occur in an amplifier. A reflex circuit will immediately produce severe distortion and possible oscillation as soon as the signal exceeds the linear range of the active component's curves, as this is the circumstance where the active device begins to work as a detector, which means it is mixing signal with the AF being fed back to amplify. This adds considerable difficulty to the making of a circuit which is dependably free of problems for all signals available. Problems are most common with large signals. LAYOUT considerations are critical in order to avoid unwanted coupling of RF signal back to the input; chokes should be toroid, not linear in physical construction, to avoid magnetic coupling
Inductive coupling
In electrical engineering, two conductors are referred to as mutual-inductively coupled or magnetically coupled when they are configured such that change in current flow through one wire induces a voltage across the ends of the other wire through electromagnetic induction...
for instance, and capacitive coupling
Capacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
must be prevented with careful layout, possibly including interleaving ground leads on PC board layout, or by using shielded wiring for hand wired circuits. The experimenter may care to employ a double sided printed circuit board, with one side being a ground plane, and connections laid out on the other side. However, if done properly, a reflex receiver can astound one with its sensitivity and gain with minimal component count. One example on the web gives drives a speaker with enough volume level for comfortable listening with only one transistor. The experimenter does well to try such a circuit, which is very under represented in modern experimental electronics literature. The use of reverse feedback at the DC level with appropriate decoupling filtering in the loop will help insure stability by self adjusting the bias level automatically; adjustment to the correct point consists of adjusting the trim pots of the bias feedback loop to insure correct biasing for linear operation.
External links
- Schematic of FADAFadaFada is the capital of the Ennedi Ouest department of Chad, lying in the Ennedi Plateau. It has a population of 23,786 .It is known for the surrounding cave paintings and rock formations, while the Guelta d'Archei and a wood growing in a wadi are local attractions.It is the birthplace of the...
model 160 neutrodyneNeutrodyneThe Neutrodyne was a particular type of Tuned Radio Frequency radio receiver, in which the instability-causing inter-electrode capacitance of the triode RF tubes is cancelled out or "neutralized"...
radio, a reflectional receiver from the 1920s. - Schematic of General ElectricGeneral ElectricGeneral Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
model F40 radio, a reflectional receiver first manufactured in 1937.