BRATS - Advanced Amateur Radio Tuition Course

Syllabus Sections:-

IF amplifier

3I4 26 Understand the operation of an IF amplifier and the IF transformer.

Understand the operation of an IF amplifier and the IF transformer.
In a standard IF amplifier, whether it is a valve, transistor, or FET, you would have an IF transformer 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 4 stages of selectivity and 4 tuned circuits at IF.
See also Fig 9.12 on page 61 of the Full Licence Manual from RSGB.

3I4 26 continued Understand the concept of two LC tuned circuits utilising transformer coupling.

In the top part of the diagram below you can see two parallel tuned circuits comprising of a coil and capacitor with an AC input signal (remember that a DC signal does work with transformers !!) and a load on the output representing the rest of a circuit. When the two coils are placed relatively close together there is induction between the coils just as in the primary and secondary of a transformer so this is utilising transformer coupling.

3I4 26 continued Identify critical and over-coupled response curves.

When the IF transformers are made the spacing between the two coils of the parallel tuned circuits must be adjusted critically. If the coils are physically too close together we get an over coupling situation which gives what is commonly known as "double humping" where there is a dip in the centre of the response cure as shown in our diagram, 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 curve. Just before the point of over coupling, where the dip becomes noticeable, is the limit to the distance between coils which produces the point of "critical coupling" where the response curve has the steepest sides without the dip in the top.

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.

3I4 26 continued 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.

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.
Distortion in the loud speaker or headphones is caused by over driving and this can be avoided by the use of or increase in the AGC control.

-------------------------------------------------------------------------------------------------------------------------------------------

3I5 26 Recall the source and effects of phase noise

Let's us first understand a few terms. A "frequency-domain" graph shows how much amplitude of a signal lies within each given frequency band over a range of frequencies.
The frequency-domain is displayed on a spectrum analyser where the X axis (the horizontal one) represents all the scanned frequencies displayed.
In the processing of a wanted signal, Phase Noise is represented in the frequency-domain of a waveform and consists of rapid, short-term, random fluctuations in the phase (or put it another way random fluctuations in the frequency) thus there are changes which detract from the perfect wanted wave form.
Therefore on a Spectrum analyser you would see the changes in the slight frequency also know as the random fluctuation which are short term.
In analogue amateur radio signals one considers phase noise of an oscillator (changes in frequency), whereas in digital-systems the phase noise is with the jitter (slight changes in the speed) of the control clock.

3I5 26 continued Recall the unit of measurement is dBc/Hz

Phase noise can be measured using a spectrum analyzer where if the phase noise of the device you are testing is large with respect to the spectrum analysers local oscillator. Care should be taken that observed values are due to the measured signal and not the shape factor of the spectrum analysers filters.

Phase noise is for our amateur radio purposes expressed in units of dBc/Hz, and it represents the noise power relative to the carrier contained in a 1 Hz bandwidth centred at a certain offsets from the carrier.