Receiver Circuits and Technology

by John of John’s Vintage Radio

Radio receivers of today are the result of engineering and technology developments that occurred shortly after World War I to produce 4 popular types of receivers. The most successful design, whose principles of operation are still in use today, gained widespread popularity in the early 1930s. Improvements in radio design and technology were driven by public interest and the commercial application of broadcast radio.

The Crystal Detector

In the early 1920s, radio manufacturers were producing radios built around the crystal detector circuit. With vacuum tubes at this time limited and very expensive, the crystal detector was a commercially viable option. It was easy to produce and worked well enough that the public could learn about and become interested in radio. As the popularity of radio grew, it wasn’t long before companies and individuals saw a commercial use for broadcasting to the masses. With popularity and demand increasing, radio stations began to appear everywhere across the country.

Crystal detector radios were very simple. Not requiring a power source, a crystal detector worked by using a variable capacitor or coil to tune the antenna to a radio station, and then "detecting" the voice signal using a crystal to create a small current that could be converted to sound. Its drawbacks were poor sensitivity, poor stability and no signal amplification limiting its use to headphones only. Only the most local and strongest stations would be received, and the radios required constant retuning.

Despite its limitations, crystal detector receivers were popular. Public interest in broadcast radio grew, and more stations came on the air. The need for a receiver capable of picking up the more distant, weaker stations grew. Being able to receive weaker stations made more stations accessible to the listening public.

The Regenerative Receiver

Engineers working with tube circuits found that by feeding back part of the received signal to the radio input, they could increase the sensitivity of the receiver. This was the basic idea behind the regenerative circuit. These receivers now required batteries making them a bit cumbersome, but the advantages of improved sensitivity, selectivity and audio power output made them superior to the crystal detector receivers that they replaced.

With vacuum tube production growing, tubes became more readily available and at lower cost. As the regenerative design was simple and easy to produce, the price of a regenerative tube receiver became less of an issue. But for as much of an improvement the regenerative circuit was over the crystal detector, it wasn’t without drawbacks.

As this design would feed back RF energy to the tube detector circuit to gain sensitivity, it would prove to be difficult to adjust and had poor stability. Changes in battery condition, tube operating characteristics and atmospheric conditions would make it almost impossible to consistently receive a given station at a certain dial setting. In addition, since RF energy was being fed back to the input stages, each receiver also became a low power transmitter capable of interfering with nearby radios tuned to the same frequency. The public tolerated these problems, but regenerative receivers were just about extinct by 1926.

The Tuned Radio Frequency (TRF) Receiver

Around 1923, engineers started to develop receiver circuits that tuned and amplified the received signal in stages that would be input to a non-regenerative detector and audio stage. This became the basis for the common 5-tube TRF receiver design. Typically, these receivers had 2 RF amplifier stages, followed by a detector stage feeding a 2 stage audio amplifier section.

This type of receiver performed so much better than the regenerative receiver that TRFs quickly became very popular and regenerative receivers all but disappeared except for a few low-cost models. With the number of radio stations continually growing, the separation between the stations became smaller requiring a receiver with improved selectivity as well as good sensitivity. It was here that the TRF design had to make some trade-offs.

The conventional TRF receiver used 2 tuned radio frequency stages and 1 tuned detector stage. This required 3 tuning dials that needed to be adjusted to tune in a station. Although the detector was non-regenerative, the high sensitivity RF stages preceding the detector were prone to oscillations causing the squeals and howls we associate with early radio. To overcome this, engineers had to reduce the sensitivity of these stages to prevent the oscillations. The end result was a circuit that wasn’t as sensitive as it could have been, but it didn’t interfere with other receivers nearby. A compromise the public seemed to accept.

From about 1924 to 1927, a modified version of the TRF receiver called a Neutrodyne became available. In this version, a small amount of signal was fed back to the RF stages to neutralize the oscillations that those stages were subject to. This change also improved tuning stability. It was now possible to log the dial settings used to receive a particular station so the listener can easily find it again in the future.

In about 1927, the Neutrodyne version was quickly replaced by the second generation TRF receiver. With a new generation of vacuum tubes with internal grids, engineers were able to design circuits that would not easily oscillate while regaining the sensitivity that had to previously be designed out. In addition, 1 and 2 dial models began to appear along with the common 3 tuning dial receivers.

The Superheterodyne (a.k.a. Superhet) Receiver

In 1932, RCA released its superheterodyne patents to its licensees. The superhet design differs from prior receivers as it changes the incoming RF signal to a lower intermediate frequency. This is done by mixing the incoming RF signal with an internal oscillator that changes frequency as the receiver is tuned to different frequencies. The internal oscillator is adjusted to have a specific offset from the incoming RF signal. The difference in frequency between the RF signal and the internal oscillator is always the same, and it is the intermediate frequency (IF) signal that is passed to the detector stage. Since the intermediate frequency doesn’t change, circuit design for amplification and selectivity in the IF stage can now be optimized.

By contrast, the TRF receiver tuned each individual RF stage to the incoming RF signal. Circuit design and tube parameters were a compromise of performance, as the circuit had to function across a broad range of frequencies. With the superhet, the IF stage will always work at a specified frequency. Engineers were able to design in the highest amount of amplification and sensitivity for that frequency. The end result was a superior performing receiver, and the TRF receivers quickly disappeared.

RCA obtained sole rights of the superheterodyne circuit just after Edward A. Armstrong received the patent in 1920. RCA refused to allow anyone to market superhet receivers for 10 years, forcing all other manufacturers to deal with regenerative and TRF designs. Although high priced compared to its competitors, RCA did offer the first superhet receivers for sale to the public in 1924. In order to prevent piracy of their circuit design, the chassis on these early models was often sealed. RCA released its superheterodyne patents in the early 1930s, and it wasn’t long before all radio makers abandoned TRF.

The superhet design also allowed radio manufacturers to offer receivers capable of short-wave and broadcast reception. This was done by switching in different tuning circuits in the RF stage for the different radio bands that were to be received. The conversion of the variable RF signal to a single intermediate frequency was ideally suited for multi-band capability, and allowed the radio to perform equally well across all bands.. The public was just as interested in hearing radio stations from other countries, as they were in hearing their local stations.

The basic operating principle of the superhet circuit can also be found in FM receivers. The first FM receivers were marketed in 1938, covering a frequency range of 41 to 44 Mhz. But it wasn’t until after World War II with improvements in FM detection and amplification, along with reduced production cost, before multi-band AM/FM radios would begin to appear.

John’s Vintage Radio provides quality antique radio repair and restoration of vacuum tube radios made during the 1930s through the 1950s. In addition to antique tube radios, John’s Vintage Radio is equipped to repair most vintage transistor radios, receivers and amplifiers made during the 1960s and 70s. For more information visit the web site: www.JohnsVintageRadio.com

>> Find Superheterodyne on this Site
>> Find Neutrodyne on this Site
>> Find Regenerative on this Site