Mixers And Modulators

Mixers

Mixers (sometimes known as frequency converters), modulators, balanced modulators and other circuit blocks are considered below. Each works on the same basic principles.

How a Mixer Works 

A mixer circuit normally has two inputs - from two separate signal sources. In the diagram below, the sources are two oscillators. Each oscillator is a generator producing a sine-wave output, one at  frequency f₁ and the other at frequency f₂. We will use numerical examples later. 
The mixer multiplies the signals together. You don't need to know the details. Just remember that the output comprises a complex mixture of separate sine-waves at many different frequencies. The major output frequencies are shown on the diagram. 

The main point to note is that the output comprises the two separate input frequencies f ₁ and f ₂ and their sum, (f ₁ + f₂), and their difference, (f ₁ - f₂). In practice, there are other component signals too - but we can ignore those. 
A filter - which can be any one of various sorts - selects the required output from the mixer. In this diagram, a simple parallel tuned circuit is shown. The output will normally be tuned to the SUM, (f ₁ + f₂), or tuned to the DIFFERENCE, (f ₁ - f₂), signal as required. 
[For the mathematicians among us, refreshment of school trigonometry can illustrate what happens. Note this multiplication: 
2 sinA cosB = sin (A + B) + sin (A - B)]
Substituting numerical values and using typical examples for the two input frequencies in the diagram can illustrate the effect: 
Consider Oscillator 1 to generate a 9 MHz signal and Oscillator 2 to generate a 5 MHz signal. The output from a mixer will contain these two signals, plus their sum, 14 MHz, and the difference, 4 MHz. The mixer output tuned circuit could be tuned to 14 MHz if that output was required, or tuned to 4 MHz, should that output be required.
The output from a mixer contains many more combinations of frequencies - generated from the harmonics of the input signals mixing with the component signals. For purposes of this amateur radio examination these can be ignored. 
An alternative name for a mixer is frequency converter. 

What Makes a Mixer?

Almost any electronic device, diode, transistor, valve, can be used as a mixer. A square-law characteristic device is preferred - to minimize unwanted outputs. Refer to a radio text-book for circuits using a single diode, several diodes, transistors - of all kinds - and valves. You need to know the principles, not the details. 
The principle is: In a mixer stage, the output contains the SUM and the DIFFERENCE of the input signal frequencies. 

Modulators

A modulator to produce an amplitude modulated signal is generally nothing more than a mixer. In the following example, the radio frequency carrier signal ( shown as f_c ) forms one input, and a band of audio frequencies ( the incoming speech - shown as f_a ), is the other input. The audio signal f_a does not appear in the output because of the filter action of the modulator output circuits.

So the output from an amplitude modulator is a band of frequencies above and below the carrier frequency plus the carrier frequency itself. 
The signal  f_c is known as the carrier frequency. 
The signal at  (f _c + f_a)  is the upper side frequency. 
The signal at  (f _c - f_a)  is the lower side frequency.
To get the feel of the modulation principle, try this numerical example:
A signal at 3.60 MHz is amplitude-modulated with a 1 kHz tone. What are the output frequencies from this modulator?  (Answers at the bottom of this page.). 

The Balanced Modulator

Using clever circuitry, it is possible to arrange a modulator in which one of the input signals does not appear in the output. Sometimes both of the input signals may be balanced out (suppressed), so that only the products of the modulation process will appear in the output.
For example, in the modulator example given above, we saw that the output comprised the carrier frequency f_c,  the sum, (f_c + f_a), and the difference, (f_c - f_a).

With a balanced modulator, only the sum (f _c + f_a), and the difference (f_c - f_a), components appear at the output. The carrier signal f_c has been cleverly cancelled and does not appear at the output.
So the output from a balanced modulator comprises two side frequencies only - at  (f _c + f_a)  and at (f _c - f_a). The carrier at f _c has been removed
Please refer to your radio textbook to see examples of the symmetrical circuitry of balanced modulators. This diagram is one example: 
This modulator use a ring of diodes (a ring modulator). 
Note the symmetrical form of the circuit. The oscillator is fed to a centre-tap point across a tuned circuit. 
The pre-set controls C (a trimmer capacitor), and P (a potentiometer), are used to balance out the carrier (the oscillator signal) appearing at the output. 
The output signal is a double-sideband signal - i.e. upper sideband and lower sideband with no carrier. The carrier (oscillator signal) has been suppressed.