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Amateur Radio Info & Exams - Circuits 2 - Digital circuits; amplifiers, oscillators, receivers & transmitters


Digital electronics uses signals which are switched between two values, in most cases a zero is any voltage close to zero volts, and a one is close to the positive voltage rail. This is called binary, as there are only two states.

The two popular classes: TTL, or transistor-transistor logic, with numbers starting in 74 for civilian versions, and 54 for military, such as 7401; and CMOS, being Complementary Metal Oxide Semiconductor with part numbers starting with 40, such as 4017, or 45; and various 74C, etc. TTL requires 5 volts, while CMOS, depending on the exact version, can handle a ranges, such as 3 to 15 volts, but check the datasheet, as things like 74HCT has a lower limit.

A specialised high speed logic family is ECL, Emitter Coupled Logic, using rails such as -5.2 volts, although other technologies may have overtaken it.

Consumer grade digital ICs live in black plastic DIL (dual-in-line) packages, or surface mount packages.

The simplest digital ICs are logic gates. An N before the name inverts the output.

There are a range of counters, and decoders. Digital counting uses a string of binary digits. If we have two bits, we can have 4 options. 3 bits is called octal, as there are 8 options; and 4 bits gives 16 options, called hexadecimal, often represented by 0 to 9, then A to F. But we humans like to count to 10, so there is also BCD, or Binary Coded Decimal, where a carry is generated after 9 (1001), with the count returning to zero. The carry can be used to trigger a 10s counter, and its carry a 100s counter, and so on.

A "clock" is an oscillator which use used to time the operation of some digital circuits, but the term also refers to making a chip load data from its input(s).

A register is the name for small amounts of memory is an IC, just a single byte each, typically. A shift register is a device with a number of memory cells, and a clock input where a level can be moved through the memory cells, or registers. Some are be bi-directional.

Off the exam, a shift register can be used for conversion between serial and parallel data. RS-232 is an example of serial data, while the Centronics printer interface used parallel data, moving 8 bits at a time, on 8 parallel wires. Load 8 bits from the parallel port, then clock them out to the serial port; or clock an 8 bit "byte" in from the serial line, then shoot them out the parallel port.

Processors and Microcontrollers

In days of old (the 1980s) if you wanted to build a simple computer to, say, control the bells for a school, you needed a Z80 microprocessor, and a bunch of memory (ROM and RAM) ICs, input/output ICs, and the like.

Nowadays, a microcontroller IC contains the memory we need, plus simplifies the input and output of the keypad, display, and bell relay. As smaller microcontollers are often inexpensive, they are an option to replace a handful of logic gates.


An amplifier can range from a sensitive low level device, to the output of a megawatt transmitter, and range from DC to gigahertz.

RF Power Amplifiers

There are several classes of RF amplifier, such as class C, which operates for a small percentage of the cycle, but uses a tuned circuit to generate a continuous wave. These are great for CW, as they are very efficient. A similar circuit is useful for FM. For SSB, and for AM generated at low level, we need a linear amplifier.

We can determine efficiency of an amplifier by dividing the RF power by the DC input power. Two example of amplifiers which generate 1500 watts: A Valve / tube one needs 1.1 amps at 1900 volts; a solid state might need 50 amps at 50 volts. We can work out the DC power and efficiency for each:

Eff = 1500 / (1.1 x 1900) = 1500 / 2090 = 71.77%
Eff = 1500 / (50 x 50) = 1500 / 2500 = 60%

An RF amplifier needs a circuit on the output connection, before the filter. This usually consists of a resistor and a small inductance. This helps to prevent a self oscillations (spurious emissions), and is called neutralisation. According my TAFE teacher winding the inductor over the resistor is not ideal.

Mixers and Modulators

Mixers are circuits which typically have 2 inputs, which accept, say a local oscillator, and the received signal, and outputs the sum or difference between these, as an intermediate frequency, or IF.

When we apply a carrier frequency, and audio (voice or processed data from a modem), and get an AM, DSB, or SSB signal out, we call this a modulator. A balanced modulator outputs DSB, or double-sideband without a carrier. This can be filtered to remove one of the sidebands, so we get SSB.


Filters pass some frequencies, and block others.

They consist of capacitors, inductors, and sometimes resistors. They use the increase in reactance with increasing frequency in inductors, and the decrease in reactance with increasing frequencies in capacitors.

Audio filters may include operational amplifiers.

These are low pass, high pass, band pass, and notch filters.

The elements used and the number of poles or stages determine things like how sharply the filter rolls off. The cut-off frequency is specified as the point at which a certain power reduction is reached, such as half power (3 dB).

Crystals, ceramic resonators, and mechanical elements can also be used in RF filters.

Relevant Questions

As always, these are actual questions from the General exam pool.

What is the reason for neutralizing the final amplifier stage of a transmitter?
A. To limit the modulation index
B. To eliminate self-oscillations
C. To cut off the final amplifier during standby periods
D. To keep the carrier on frequency

This eliminates self oscillations, also termed spurious emissions, answer B.

Which of these classes of amplifiers has the highest efficiency?
A. Class A
B. Class B
C. Class AB
D. Class C

A class C amplifier functions by generating a short pulse, and feeding it into a tuned circuit. They are typically used by modes such as CW. Class C is the most efficient amplifier mode, answer D.

Which of the following describes the function of a two input AND gate?
A. Output is high when either or both inputs are low
B. Output is high only when both inputs are high
C. Output is low when either or both inputs are high
D. Output is low only when both inputs are high

As implied by the term "AND", both input A and input B have to be high for the output to be high, answer B.

Which of the following describes the function of a two input NOR gate?
A. Output is high when either or both inputs are low
B. Output is high only when both inputs are high
C. Output is low when either or both inputs are high
D. Output is low only when both inputs are high

A NOR gate is an OR gate with an inverted output. Thus when activated it has a low output. This low output occurs when one or both inputs are high, answer C.

How many states does a 3-bit binary counter have?
A. 3
B. 6
C. 8
D. 16

Three bit systems have 2³ = 8 combinations, answer C.

This is called Octal.

What is a shift register?
A. A clocked array of circuits that passes data in steps along the array
B. An array of operational amplifiers used for tri-state arithmetic operations
C. A digital mixer
D. An analog mixer

It allows data to be stepped along the array, answer A.

What are the basic components of virtually all sine wave oscillators?
A. An amplifier and a divider
B. A frequency multiplier and a mixer
C. A circulator and a filter operating in a feed-forward loop
D. A filter and an amplifier operating in a feedback loop

These include filter components, and an amplifier with a feedback loop, answer D.

How is the efficiency of an RF power amplifier determined?
A. Divide the DC input power by the DC output power
B. Divide the RF output power by the DC input power
C. Multiply the RF input power by the reciprocal of the RF output power
D. Add the RF input power to the DC output power

RF power out over DC power in, answer B.

If an amplifier generates 150 watts, but needs 275 watts of DC power, we can work out the power as Eff = PRF / PDC = 150 / 275 = 0.54545 = 54.5%.

This in NOT a gain question, so RF input power does not come into it.

What determines the frequency of an LC oscillator?
A. The number of stages in the counter
B. The number of stages in the divider
C. The inductance and capacitance in the tank circuit
D. The time delay of the lag circuit

The inductance and capacitance in the tank circuit determines the (resonant) frequency, answer C.

Which of the following describes a linear amplifier?
A. Any RF power amplifier used in conjunction with an amateur transceiver
B. An amplifier in which the output preserves the input waveform
C. A Class C high efficiency amplifier
D. An amplifier used as a frequency multiplier

An linear amplifier preserves the input waveform, answer B.

It is this good for AM, and SSB.

For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?
D. All of these choices are correct

Class C amplifiers are used for CW, answer B.

Which of the following is used to process signals from the balanced modulator then send them to the mixer in some single sideband phone transmitters?
A. Carrier oscillator
B. Filter
C. IF amplifier
D. RF amplifier

A balanced modulator generates a double sideband signal. Thus the unwanted sideband must be filtered out, using, you guessed it, a filter, answer B.

Which circuit is used to combine signals from the carrier oscillator and speech amplifier then send the result to the filter in some single sideband phone transmitters?
A. Discriminator
B. Detector
C. IF amplifier
D. Balanced modulator

This is a balanced modulator, and generates a DSB signal, which can be filtered to generate an SSB signal, answer D.

What circuit is used to process signals from the RF amplifier and local oscillator then send the result to the IF filter in a superheterodyne receiver?
A. Balanced modulator
B. IF amplifier
C. Mixer
D. Detector

Combine the flour and eggs, in an electric mixer. Yes, the mixer combines the amplified incoming signal with the local oscillator, answer C.

What circuit is used to combine signals from the IF amplifier and BFO and send the result to the AF amplifier in some single sideband receivers?
A. RF oscillator
B. IF filter
C. Balanced modulator
D. Product detector

This uses the "product" of both signals, so is the Product detector, answer D.

Which of the following is an advantage of a transceiver controlled by a direct digital synthesizer (DDS)?
A. Wide tuning range and no need for band switching
B. Relatively high power output
C. Relatively low power consumption
D. Variable frequency with the stability of a crystal oscillator

DDS provides tuning in very fine steps, but with high stability, being based in a crystal oscillator, answer D.

What should be the impedance of a low-pass filter as compared to the impedance of the transmission line into which it is inserted?
A. Substantially higher
B. About the same
C. Substantially lower
D. Twice the transmission line impedance

The filter should have an impedance the same as the transmission like, answer B.

What is the simplest combination of stages that implement a superheterodyne receiver?
A. RF amplifier, detector, audio amplifier
B. RF amplifier, mixer, IF discriminator
C. HF oscillator, mixer, detector
D. HF oscillator, prescaler, audio amplifier

"Superhet" receivers use an oscillator, a mixer, and a detector, answer C.

What type of circuit is used in many FM receivers to convert signals coming from the IF amplifier to audio?
A. Product detector
B. Phase inverter
C. Mixer
D. Discriminator

This is the Discriminator, answer D.

What is the phase difference between the I and Q signals that software-defined radio (SDR) equipment uses for modulation and demodulation?
A. Zero
B. 90 degrees
C. 180 degrees
D. 45 degrees

These are "in quadrature", or 90 degrees apart, answer B.

What is an advantage of using I and Q signals in software-defined radios (SDRs)?
A. The need for high resolution analog-to-digital converters is eliminated
B. All types of modulation can be created with appropriate processing.
C. Minimum detectible signal level is reduced
D. Converting the signal from digital to analog creates mixing products

This allows all types of modulation to be created, with the correct software, answer B.

What is meant by the term "software defined radio" (SDR)?
A. A radio in which most major signal processing functions are performed by software
B. A radio that provides computer interface for automatic logging of band and frequency
C. A radio that uses crystal filters designed using software
D. A computer model that can simulate performance of a radio to aid in the design process

As the name implies, these perform most of the demodulation processes in software, answer A.

This can be done internal to the device, or by connection to a PC or Raspberry Pi.

What is the frequency above which a low-pass filter’s output power is less than half the input power?
A. Notch frequency
B. Neper frequency
C. Cutoff frequency
D. Rolloff frequency

The frequency at which the output is half the input power, or 3 dB below the input, is called the cutoff frequency, answer C.

What term specifies a filter’s maximum ability to reject signals outside its passband?
A. Notch depth
B. Rolloff
C. Insertion loss
D. Ultimate rejection

This is the ultimate rejection, answer D.

The bandwidth of a band-pass filter is measured between what two frequencies?
A. Upper and lower half-power
B. Cutoff and rolloff
C. Pole and zero
D. Image and harmonic

Bandwidth is the range between the upper and lower half-power frequencies, answer A.

What term specifies a filter's attenuation inside its passband?
A. Insertion loss
B. Return loss
C. Q
D. Ultimate

This is termed insertion loss, answer A.

Which of the following is a typical application for a Direct Digital Synthesizer?
A. A high-stability variable frequency oscillator in a transceiver
B. A digital voltmeter
C. A digital mode interface between a computer and a transceiver
D. A high-sensitivity radio direction finder

This provides a very stable oscillator, which can be commanded to change frequency rapidly, answer A.

Op-amps were designed to perform addition, subtraction, and multiplication on analogue computers, and while they are still be used in instruments, etc, they are now often used in signal level audio amplifiers.

Tri-state logic is logic where, in addition to low or high, there is a high-impedance mode, where neither output level is asserted, used on some data buses. Thus several devices can send data along a single line.

On to: Modes 1 - Modulation & Mixers

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, April 2022.

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