Mixers

Description: Mixers, also referred to as frequency converters, modulators, balanced modulators, and other circuit blocks, operate based on similar fundamental principles. A mixer circuit typically has two inputs from distinct signal sources. In the provided diagram, the sources are two oscillators, each functioning as a generator that produces a sinewave output: one at frequency f1 and the other at frequency f2. The mixer multiplies these signals together, resulting in an output that consists of a complex mixture of separate sinewaves at various frequencies. The primary output frequencies include the two input frequencies, f1 and f2, as well as their sum (f1 + f2) and their difference (f1 - f2). While there are additional component signals, they can be disregarded for this overview. A filter, which can take various forms, is employed to select the desired output from the mixer. In this instance, a simple parallel tuned circuit is illustrated. The output is typically tuned to either the sum (f1 + f2) or the difference (f1 - f2) signal as required. For example, if Oscillator 1 generates a 9 MHz signal and Oscillator 2 generates a 5 MHz signal, the mixer output will include these two signals, along with their sum of 14 MHz and their difference of 4 MHz. The output tuned circuit can be adjusted to 14 MHz or 4 MHz, depending on which output is needed. The mixer output contains numerous other frequency combinations, generated from the harmonics of the input signals mixing with component signals; however, these can be overlooked for the purposes of this discussion. Various electronic components, such as diodes, transistors, and valves, can serve as mixers, with devices exhibiting a square-law characteristic favored to minimize unwanted outputs. Reference materials on radio technology provide circuit examples using single diodes, multiple diodes, transistors of various types, and valves. Understanding the principles is essential, rather than focusing on intricate details. A modulator that produces an amplitude-modulated signal is fundamentally a mixer. In a typical scenario, the radio frequency carrier signal (denoted as fc) serves as one input, while a range of audio frequencies (the incoming speech, represented as fa) constitutes the other input. The audio signal fa does not appear in the output due to the filtering effect of the modulator's output circuits. For instance, a signal at 3.60 MHz is amplitude-modulated with a 1 kHz tone. The output frequencies from this modulator can be determined. Advanced circuitry can be designed to ensure that one of the input signals does not appear in the output. In some cases, both input signals may be balanced out (suppressed), allowing only the modulation products to be present in the output. In the earlier modulator example, the output includes the carrier frequency fc, the sum (fc + fa), and the difference (fc - fa). However, in a balanced modulator, only the sum (fc + fa) and the difference (fc - fa) components are present in the output, as the carrier signal fc has been effectively canceled. The output from a balanced modulator thus consists solely of the two side frequencies at (fc + fa) and (fc - fa), with the carrier signal suppressed. The preset controls C (a trimmer capacitor) and P (a potentiometer) are utilized to balance the carrier (oscillator signal) at the output. This output signal is a double-sideband signal, meaning it includes both the upper and lower sidebands without a carrier. The carrier (oscillator signal) has been eliminated. This device functions similarly to a mixer, serving to demodulate a signal within a receiver. The term "product" refers to the multiplication of the two input signals.

Mixers are critical components in various electronic applications, particularly in communication systems where signal processing is essential. They enable the combination of signals at different frequencies, facilitating the transmission and reception of information over various media. The ability to manipulate frequencies through mixing allows for the efficient use of bandwidth and enhances the clarity and fidelity of the transmitted signals. In practical implementations, mixers can be designed using a variety of architectures, including passive mixers that utilize resistive components and active mixers that incorporate amplifying elements to improve performance. The choice of mixer type often depends on the specific requirements of the application, such as linearity, dynamic range, and power consumption. Additionally, the design of the filtering stage following the mixer is crucial, as it determines which frequencies are passed to subsequent stages of the signal processing chain, thereby influencing the overall system performance. Understanding the operational principles of mixers and modulators is vital for engineers working in the fields of radio frequency design, telecommunications, and audio processing, as these components play a pivotal role in modern electronic systems.Mixers (sometimes known as frequency converters), modulators, balanced modulators and other circuit blocks are considered below. Each works on the same basic principles. 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 sinewave 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 sinewaves 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. 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. Almost any electronic device, diode, transistor, valve, can be used as a mixer. A square-law characteristic device is preferred - to minimise 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. 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 fc ) forms one input, and a band of audio frequencies ( the incoming speech - shown as fa ), is the other input.

(See Signals ). The audio signal fa does not appear in the output because of the filter action of the modulator output circuits. 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. ). 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 fc, the sum, (fc + fa), and the difference, (fc - fa).

With a balanced modulator, only the sum (f c + fa), and the difference (fc - fa), components appear at the output. The carrier signal fc 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 + fa) and at (f c - fa).

The carrier at f c has been removed. (See Signals ) 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. This device is just another mixer - used for demodulating a signal in a receiver. The term product refers to the multiplication of the two input signals - with s