Reflex receiver
A single amplifier amplifies both RF and audio signals.
A reflex radio receiver, sometimes called a reflectional receiver, uses the same amplifier to boost both the incoming high-frequency radio signal and the lower-frequency audio signal that is extracted from it. In this design, the radio signal is first amplified, then demodulated to recover the audio. That audio is then sent back through the same amplifying device before reaching the earphone or speaker. This approach cuts down on the number of active components needed.
The concept was first described in 1914 by German inventors Wilhelm Schloemilch and Otto von Bronk. It was later rediscovered and expanded to multiple tubes in 1917 by Marius Latour and William H. Priess. During the 1920s, reflex receivers became popular because vacuum tubes were costly, and reducing the tube count was a major advantage. As tube prices dropped, receivers with more tubes became common. The technique saw a revival in the 1930s, when smaller pentode tubes made compact designs practical, and saving space became more important than saving money on tubes.
In a simple reflex receiver, the radio frequency signal from a tuned circuit is amplified, then passes through a high-pass filter to a demodulator. The demodulator extracts the audio signal from the carrier wave. That audio is fed back into the amplifier’s input and amplified again. At the output, a low-pass filter separates the audio from the radio frequency signal and sends it to the earphone. The amplifier may be a single stage or multiple stages. Because each active device amplifies the signal twice, a reflex circuit performs like an ordinary receiver with twice as many active devices.
The reflex receiver is not the same as a regenerative receiver. In a regenerative circuit, the same signal is fed back from the output to the input. In a reflex circuit, only the audio extracted by the demodulator is added to the amplifier input, so two separate signals at different frequencies pass through the amplifier simultaneously. Some receivers combined both techniques—for example, the Crosley Trirdyn sets used regenerative detection along with reflex amplification.
The two signals can pass through the amplifier without interfering because the amplifier is linear, following the superposition principle. Since they have different frequencies, they can be separated at the output using frequency-selective filters.
- First disclosed
- 1914
- Inventors
- Wilhelm Schloemilch and Otto von Bronk
- Rediscovered and extended
- 1917 by Marius Latour and William H. Priess
- Common application
- inexpensive single tube receivers in the 1920s
- Later use
- compact superheterodyne receivers from the 1930s into the 1950s, until at least 1959
Lore & Background
The reflex receiver was first disclosed in 1914 by German scientists Wilhelm Schloemilch and Otto von Bronk, and rediscovered and extended to multiple tubes in 1917 by Marius Latour and William H. Priess. It became popular in the 1920s because the high cost of vacuum tubes made reducing tube count attractive. As tube prices fell, receivers using more tubes became common. The technique was revisited in the 1930s when newer, smaller pentode tubes made compact receiver designs practical, and size became a primary consideration rather than tube cost alone.
The receiver functions as a tuned radio frequency (TRF) receiver. The radio frequency signal from the tuned circuit is amplified, then passes through a high pass filter to the demodulator, which extracts the audio frequency signal from the carrier wave. The audio signal is added back into the input of the amplifier and amplified again. At the output, the audio is separated from the RF signal by a low pass filter and applied to the earphone. Since each active device is used to amplify the signal twice, the reflex circuit is equivalent to an ordinary receiver with double the number of active devices.
The reflex receiver should not be confused with a regenerative receiver, in which the same signal is fed back from the output to the input. In the reflex circuit, only the audio extracted by the demodulator is added to the amplifier input, so two separate signals at different frequencies pass through the amplifier simultaneously. The reason they can pass without interfering is the superposition principle, because the amplifier is linear. If the amplifier is significantly nonlinear, intermodulation distortion occurs and the audio signal modulates the RF signal, resulting in audio feedback that can cause a shrieking in the earphone.
Reader's Guide
The reflex receiver's significance lies in its ability to reduce the number of active devices, which was especially valuable when vacuum tubes were costly. In the 1920s, the most common application was in inexpensive single tube receivers, as many consumers could not afford more than one tube; the reflex circuit got the most out of a single tube, making it equivalent to a two-tube set. During this period, the demodulator was usually a carborundum point contact diode, but sometimes a vacuum tube grid-leak detector. Multitube receivers like the TRF and superheterodyne were also made with some amplifier stages reflexed.
Low cost mains-powered radios using a reflex TRF design with only three tubes were still mass produced in the late 1940s. The reflex principle was used in compact superheterodyne receivers from the 1930s and continued into the 1950s, until at least 1959; the intermediate frequency amplifier stage was also the first audio frequency stage using a reflex arrangement, providing similar performance in a four-tube radio as one with five tubes. Often, such reflex receivers did not have Automatic Gain Control (AGC), and it was usually not possible to reduce the volume completely to zero, even at the minimum volume setting. At least one type of tube was specially designed for this kind of receiver design.
Frequently Asked Questions
What is a reflex receiver?
A reflex receiver is a radio architecture in which a single amplifier stage handles both the incoming radio-frequency signal and the recovered audio signal. The audio is routed back through that same stage before reaching the speaker or earphone, which is the defining trait of the design.
Who invented the reflex receiver?
The concept was first described in 1914 by German inventors Wilhelm Schloemilch and Otto von Bronk. It was later independently rediscovered and extended in 1917 by Marius Latour and William H. Priess.
How does a reflex receiver work?
The incoming radio signal is amplified, then demodulated to extract the audio, and that audio is fed back through the very same amplifier before reaching the output device. Because one active component serves both jobs, no separate audio amplifier stage is required.
Where was the reflex receiver most commonly used?
It became a popular architecture for inexpensive single-tube receivers throughout the 1920s. The design also found a second life in compact superheterodyne receivers from the 1930s through the 1950s, with examples still in use at least as late as 1959.
Why is the reflex receiver important in radio history?
By letting one amplifier handle both RF and audio, it dramatically reduced the number of active components a receiver needed. This made affordable, compact radio sets practical for everyday consumers during the early broadcast era.
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