Syllabus Sections:-

IF amplifier

4n.1 Understand the advantages and disadvantages of high and low intermediate frequencies and the rationale for the double superhet.

High IF gives good image rejection and poor selectivity.

Low IF gives good selectivity and poor image rejection.

You have to decided which to use. For instance if you had very low IF frequency, on a receiver on 28MHz, you may get good selectivity but you may suffer IMAGE that is receiving signals twice removed from the frequency you are interested in. On the other had if you had High IF you would get good image but poor selectivity.

HIGH IF followed by LOW IF  in the double superhet

In receiver we tend to have what is called a double superhet. We convert the signal frequency down to a high IF to give good image response and then further convert it to a second LOW IF to give good selectivity.  You might be able to remember this order by remember a usual greeting "Hi low everybody"!

So in the diagram above the IF1 is the high IF and IF2 is the low frequency.

Q Factor

The Q factor - if you look at a parallel tuned circuit and it has a high Q then the response curve will be very sharp. If the Q is poor we get a low Q circuit where the response is flat and broad.

4n.2 Understand the operation of an IF amplifier and the IF transformer.

Understand the concept of two LC tuned circuits utilising transformer coupling. Identify critical and over-coupled response curves.

Understand how the gain of an IF amplifier can be varied, how this may cause distortion and how the effects of the distortion are avoided. Note: the reason to vary the gain (AGC) is covered at item 4p.

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In a standard IF amplifier, whether it is a valve, transistor, or FET you would have an IF transformers on the input and output. These IF transformers would contain, most of the time, two parallel tuned circuits in the base circuit and two parallel tuned circuits in the collector circuit. There would therefore be four stages of selectivity four tuned circuits at IF.

When the IF transformers are made the spacing between the two coils of the parallel tuned circuits has to be adjusted critically. If the coils are physically too close together we get over coupling situation and gives what is called "double humping" which means you have a very wide response.

Over coupled

If you take the coils further apart then you get a wider coupling and this results in a much sharper response. There is limit to the distance between coils to produce "critical coupling".

As you move the coils apart the gain of the stage will reduce as the RF signal has to be coupled between the two. The manufacturer will do their best to provide a good transformer but it might not give you the selectivity you want and you may have to resort to an additional crystal filter.

To remind you the AGC is a controlling voltage to increase or reduce the gain of the IF amplifiers hopefully to keep the audio output in the loud speaker constant. Usually the control trigger for this is taken from the detector.


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