Double superheterodyne radio receiver
- the double superheterodyne radio receiver concept and topology and how the double superhet enables high levels of receiver performance to be achieved.
Although the basic idea for the superhet or superheterodyne radio receiver works very well, to ensure the optimum performance under a number of situations, an extension of the principle, known as the double superhet, or double superheteroyne radio receiver may be used.
The double superheterodyne radio receiver improves the performance in a number of areas including stability (although synthesizers have largely overcome this problem), image rejection and adjacent channel filter performance.
The double superheterodyne radio receiver is still widely used, especially at high frequencies where factors such as image rejection and filter performance are important.
Reason for using double superheterodyne radio receiver
When choosing the intermediate frequency for a superheterodyne radio receiver there is a trade-off to be made between the advantages of using a low frequency IF or a high frequency one:
- High frequency IF: The use of a high frequency IF means that the difference between the wanted frequency and the unwanted image is much greater and it is easier to achieve high levels of performance because the front end filtering is able to provide high levels of rejection.
- Low frequency IF: The advantage of choosing a lower frequency IF is that the filters that provide the adjacent channel rejection are lower in frequency. The use of a low frequency IF enables the performance to be high, while keeping the cost low.
Accordingly there are two conflicting requirements which cannot be easily satisfied using a single intermediate frequency. The solution is to use a double conversion superheterodyne topology to provide a means of satisfying both requirements
Basic double superheterodyne receiver concept
The basic concept behind the double superheterodyne radio receiver is the use of a high intermediate frequency to achieve the high levels of image rejection that are required, and a further low intermediate frequency to provide the levels of performance required for the adjacent channel selectivity.
Typically the receiver will convert the incoming signal down to a relatively high first intermediate frequency (IF) stage. This enables the high levels of image rejection to be achieved. As the image frequency lies at a frequency twice that of the IF away from the main or wanted signal, the higher the IF, the further away the image is and the easier it is to reject at the front end.
Once the signal has passed through the first IF at the higher frequency, it is then passed through a second mixer to convert it down to a lower intermediate frequency where the narrow band filtering is accomplished so that the adjacent channel signals can be removed. As the lower frequency, filters are cheaper and the performance is often higher. (Although it must be said that filter technology now allows effective filters to be made at much higher frequencies than was previously possible.)
Double superheterodyne topologies
While the basic concept for the double superheterodyne radio receiver involving two stages of frequency conversion may remain the same, there are a number of different "styles" that can be adopted:
- Fixed frequency first oscillator: This style of double conversion superheterodyne receiver was popular before the days of frequency synthesizers and other very stable local oscillators. To ensure the frequency stability, a crystal oscillator was used to provide the local oscillator for the first conversion. A bandpass filter would be used to provide selectivity and allow a band of frequencies to be passed. The second local oscillator would allow tuning over the range allowed by the bandpass filter. When further coverage was required, the first, crystal controlled oscillator, would need to be switched to the next crystal. In this way continuous coverage could be obtained, albeit with a large number of crystals.
Double conversion superheterodyne receiver with fixed frequency first LO
Apart from providing high levels of image rejection, this concept gave considerably improved levels of frequency stability for receivers of the time. Nowadays frequency synthesizers mean that this topology is rarely needed or used.
- Tuned first oscillator: This is the most usual form of double conversion superheterodyne receiver. The first conversion uses a variable frequency oscillator which converts the signal to the first IF.
Double conversion superheterodyne receiver with variable frequency first LO
Although little selectivity is generally provided in the first IF, often a filter known as a roofing filter may be used to provide some adjacent channel filtering. This prevents very strong adjacent channel signals from overloading the later stages of the IF. However the main selectivity is still provided in the lower frequency IF stages.
Whatever the style of double conversion superheterodyne receiver, the basic concept of providing two frequency conversion stages applies.
Triple conversion superheterodyne receiver
For some advanced designs, especially those using microwave frequencies, there may be advantages in using three conversions. While these receivers are les common, they are still found on some occasions. Their design provides for high levels of close in or adjacent channel selectivity, while still achieving high levels of image rejection. For applications where both specifications are particularly important, this may only be achieved by using a number of conversion processes. In cases like these, triple conversion superheterodyne receivers may be needed, although costs are naturally high in view of the additional circuitry required.
The double superheterodyne radio receiver is normally found where high levels of performance are required. The additional circuitry needed adds cost, and to minimise the occurrence of spurious signals within the receiver, the design must be undertaken with care. However, it is possible to achieve very high levels of performance with a double superheterodyne or where particularly needed a triple superheterodyne radio receiver.
By Ian Poole
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