Although single conversion superhet or superheterodyne radios are widespread, there are times when the additional performance advantages of double and sometimes triple conversion superhet radios might be needed.
This video looks at what double and triple conversion radios are, the advantages they provide and some of the formats used (there are very many different formats that are used dependent upon the design, so the aim is to provide the principles of the double and triple conversion superhets and detail their advantages).
One of the main issues with the superhet radio is that of the image response. If a signal on the image frequency is received at the same time as a signal on the wanted frequency, then this will cause interference and as a result it will impair the performance of the radio.
To overcome the image problem, tuning is placed before the mixer, but it will not provide complete elimination of the image signal and therefore the radio receiver needs to be optimised to reduce this, while still achieving a good level of adjacent chanel filtering, etc.
The image occurs at a frequency which is twice that of the intermediate frequency from the wanted one. Therefore increasing the IF will enable the RF tuning to reduce the image signal more. However the adjacent channel filtering is better at lower frequencies. Also a low frequency IF was often better where digital signal processing was used.
The solution is to have a high first IF to reduce the image, and a low frequency second IF for adjacent channel filtering.
Another reason for using a double conversion superhet occurred before the advent of frequency synthesizers. Up until then, radio receivers had suffered from drift as the local oscillator was a free running variable frequency oscillator. Often needing to run at high frequencies and also be switched for different bands, this was far from ideal. Changes in temperature, voltage and the like caused the circuit to drift in frequency and this, of course, made using these receivers difficult at times.
One solution to overcome this problem was to adopt a crystal controlled first conversion. This gave a much improved level of stability. Using this approach, signals were converted down to a wide band first IF, possibly covering 200 or 500kHz. A second local oscillator would operate at a much lower frequency. As a result it could be offer much better levels of stability. It did result in any receiver only being able to cover its frequency range in a large number of bands.
More information about the superhet radio can be found: [ Ссылка ]
More information about double conversion radios: [ Ссылка ]
More information about the image response: [ Ссылка ]
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