Superhet Design Evolution & Trends

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Superhet Radio Tutorial Includes:
Superhet radio     Superhet theory     Image response     Block diagram / overall receiver     Design evolution     Double & multi-conversion superhet     Specifications    
See also:   Radio types

The superheterodyne radio topology quickly became the most widely used form of radio receiver as a result of the superior performance it provided.

Naturally as time progressed the techniques used improved to provide much superior levels of performance whilst still retaining the basic concept and techniques of the superheterodyne technique.

As time progressed the complexity of the superhet radio increased as the radio evolved and the design trends over time enabled a variety of schemes to be used.

Mapping the design evolution and trends of the superhet radio technology provides a much clearer picture of the reasons why certain techniques were used.

Early superhet designs

The early superhet radios were basic. Certainly broadcast radios that were manufactured to a price would only have the basic minimum number of components.

Often the RF stage consisted simply of the RF tuning and the signal was then applied directly to the mixer.

Also the mixer and oscillator would typically be incorporated into one valve consisting of two sections. One example of this was a triode hexode such as the 6K8 used in many designs of the 1940s era. In later years the ECH81 triode heptode valve introduced in 1954 was a popular choice.

Very early superhet radios operated with low frequency intermediate frequency stages, often 100 kHz or less. However as the technology improved, the most widely used IF was between 455 kHz and 470 kHz. This was chosen because it was between the long and medium wave broadcast bands and relatively few high power stations operated in this region. Accordingly IF breakthrough where signals were picked up directly into the IF was less of a problem.

The choice of a 455 kHz IF enabled the simple LC IF transformers to give adequate selectivity for many applications as well as providing adequate image rejection for many applications, although as the input or received frequencies increased up to 30 MHz or so, the image rejection did suffer.

Also having an IF of 455 kHz enabled the use mechanical or crystal filters. Until the 1960s or so, manufacturing crystal filters much above this became expensive and the performance was not as good. Also mechanical filters did not operate much above this anyway.

Different intermediate frequency choices

As the level of image rejection required increased, the only option was to increase the intermediate frequency used. One design trend for communications receivers was to adopt a 1.6MHz IF to improve the image performance, especially as the frequencies rose above a few MHz.

As VHF FM using frequencies between 87.5 and 108 MHz became popular, a standard IF of 10.7 was adopted for most receivers and tuners. Some receivers used for AM broadcast and VHF FM used a dual IF that used both 455MHz transformers for the AM section, and within the same amplifier used 10.7 MHz transformers for the FM signals.

Quest for stability

One major issue that was encountered with many superhet receivers was that of frequency stability. Early radios had simply used an LC tuned oscillator to provide the local oscillator. Any drift on this was reflected as drift in the received frequency which could present issues when receiving Morse or SSB as any change in frequency was manifested as an equal change in audio frequency.

One design trend that appeared in the 1960s was to introduce a second conversion process, i.e. a double conversion superhet. The first conversion used a crystal oscillator which could be switched to provide the required band, typically 500 kHz wide. This was then converted to a lower IF where the main selectivity was provided.

This form of superhet had the advantage that the first oscillator was crystal controlled and this gave a very high degree of stability, even when it was switched.

Typically the wideband IF was located between 5.000 and 5.500 MHz, and then a non-switched, local oscillator operating over a relatively narrow band of 500 kHz from 5.455 to 5.955 MHz would convert the signals down to a 455 kHz IF.

As a separate crystal was needed for each 500 kHz band, this format was generally only used for amateur radio HF band receivers and transceivers.

Although the design trend to use superhet radios with a crystal controlled first conversion enabled much greater levels of stability to be obtained along with an equal tuning rate on all bands, it did open up the receiver to receiving strong signals within the bandpass filter stage. Even very high degrees of screening and isolation did not fully protect the receiver from receiving some signals in within the bandpass filter range.

Improved local oscillator performance

Although a double conversion superhet using a crystal controlled first conversion provided many advantages, there were other methods of achieving the same performance.

As technology developed the superhet design trends changed as it became easier to add additional stages and also utilise different techniques to provide more stable local oscillator signals.

• Crystal mixer VFO:   The first technique utilised a crystal mixer variable frequency oscillator. It had many similarities to the crystal controlled first conversion superhet design. It mixed a local oscillator running, at say, 5.00 to 5.5 MHz with the output from a switched crystal oscillator. In this way the variable frequency oscillator can run at a relatively low frequency and not have a switch in the resonant circuit which is always bad news.
  
  The downside of this approach is that it is difficult to remove the spurious signals generated. Accordingly it is not ideal unless particular attention is paid to the filtering of the signal from the overall oscillator.
• Frequency synthesizer:   In the late 1970s and early 1980s frequency synthesizer technology started to become a viable proposition for radio receivers. First using phase locked loop technology and then later direct digital synthesis became an option as well.

  Note on the Frequency Synthesizers:

  RF frequency synthesizers enable stable signals to be produced and controlled by a programmable input. There are several different types of synthesizer: some based on phase locked loop techniques, and others use digital technology to create a waveform directly. Often complete synthesizers may incorporate one or more types of technology

  Read more about Frequency Synthesizers.

  Although frequency synthesizers are widely used, one of the major issues for receiver use is that of phase noise. Careful design is required to ensure the optimum phase noise performance to ensure that issues like reciprocal mixing are reduced.

Multiple conversion superhet receivers

With the increasing performance requirements being placed on radio receivers, one superhet design trend was to utilise additional conversion stages. Many receivers have two and some high performance sets may have three conversions.

Using current filter technology, it is possible to create high performance crystal filters at much higher frequencies than was previously possible. Filters at 9 MHz are common and able to provide much higher degrees of performance than those of some years back even when they were using frequencies of 455 kHz.

This means that there might not be the need to have the final IF stages at a frequency of around 455 kHz or so, and this may enable one less conversion to be used.

Nevertheless, many high performance superhet radios often up-convert the incoming signal first, providing a level of filtering, before downconverting the signal again to a frequency where the main filtering can be provided.

The superheterodyne radio has evolved significantly over the many years the receiver topology has been available. Although software defined radio technology is making advances and in some instances it has made the superhet radio redundant. That said, there are still many instances when SDR technology can be used alongside the superhet to provide the best of both worlds. This is a useful trend and one which enables high performance receivers to be used over a wide range of frequencies.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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