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The T.R.F. (tuned radio frequency) receiver was among the first designs available in the early days when means of amplification by valves became available. The basic principle was that all r.f. stages simultaneously tuned to the received frequency before detection and subsequent amplification of the audio signal.
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The principle disadvantages were (a) all r.f. stages had to track one another and this is quite difficult to achieve technically, also (b) because of design considerations, the received bandwidth increases with frequency. As an example - if the circuit design Q was 55 at 550 Khz the received bandwidth would be 550 / 55 or 10 Khz and that was largely
satisfactory. However at the other end of the a.m. band 1650 Khz, the received bandwidth was still 1650 / 55 or 30 Khz. Finally a further disadvantage (c) was the shape factor could only be quite poor. A common error of belief with r.f. filters of this type is that the filter receives one signal and one signal only.
Figure 1 - TRF three variable capacitors ganged to track together.
Let's consider this in some detail because it is critical to all receiver designs. When we discuss bandwidth we mostly speak in terms of the -3dB points i.e. where in voltage terms, the signal is reduced to .707 of the original.
If our signal sits in a channel in the a.m. radio band where the spacing is say 10 Khz e.g. 540 Khz, 550 Khz, 560 Khz.... etc and our signal, as transmitted, is plus / minus 4Khz then our 550 Khz channel signal extends from 546 Khz to 554 Khz. These figures are of course for illustrative purposes only. Clearly this signal falls well within the -3dB points of 10 Khz and suffers no attenuation (reduction in value). This is a bit like singling one tree out of among a lot of other trees in a pine tree plantation.
Sorry if this is going to be long but you MUST understand this basic principle.
Figure 2 - TRF shape factors against ideal
In an idealised receiver we would want our signal to have a shape factor of 1:1, i.e. at the adjacent channel spacings we would want an attenuation of say -30 dB where the signal is reduced to .0316 or 3.16% of the original. Consider a long rectangle placed vertically much like a page printed out on your printer. The r.f. filter of 10 Khz occupies the page width at the top of the page and the bottom of the page where the signal is only 3.16% of the original it is still the width of the page.
In the real world this never happens. A shape factor of 2:1 would be good for an L.C. filter. This means if the bottom of your page was 20 Khz wide then the middle half of the top of the page would be 10 Khz wide and this would be considered good!.
Back to T.R.F. Receivers - their shape factors were nothing like this. Instead of being shaped like a page they tended to look more like a flat sand hill. The reason for this is it is exceedingly difficult or near impossible to build LC Filters with impressive channel spacing and shape factors at frequencies as high as the broadcast band. And this was in the days when the short wave bands (much higher in frequencies) were almost unheard of. Certain embellishments such as the regenerative detector were developed but they were to some extent unsatisfactory.
In the 1930's Major Armstrong developed the superhetrodyne principle.
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Updated 13th July, 2000