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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.
Which should you use to have a play around with logic, or build a project? In most cases CMOS is easier, including because the supply voltage can vary, and current draw is low - just use a 9 volt battery, or 12 volt bench supply.
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, such as AND and OR. An N before the name inverts the output, making NAND and NOR.
While 2 input devices are very common, but 3, 4, and 8 input devices exist. Hex buffers and inverters, quad 2-input gates, triple 3-input, and dual 4-input units are typical, along with single 8-input devices. 74LS51 combines ANDs and NORs. Tiny surface mount single gates are available, or you can make them from diodes and transistors.
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 is 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 "RTC" is a real-time clock, which may be a module with an IC and a lithium cell to keep the time and date ticking over.
A register is the name for small amounts of memory in an IC, just a single byte or word 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.
The remainder of this section is 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.
A latch is a another element which holds a value. A frequency counter can count pulses for exactly 1 second, then load than number into the display, say 7.106,014, indicating that the transmitter is 14 Hz (about 2 ppm) above 7.106 MHz, then repeat the count while the display holds for a second. A decoder can say take a 4 bit word and decode it to one of 16 lines, decode it to operate a 7 segment display. The CD4511B both stores and decodes BCD, to operate a 7 segment display. You can even parallel the inputs for the 7 or 8 chips / digits you are using, and load each one in turn by pulling its LE/STROBE low momentarily. The BL blanking pin can blank leading zeros.
The overbar indicates that an input is active when the pin is pulled low. This can also be indicated by a small ring at the input. Chips may have a Clock inputs cause the count to occur on the upwards going edge, or on the negative going edge. The 4017 increments on positive going pulses. Many have ENABLE, ENABLE, INHIBIT or INHIBIT pins. In many cases these should simply be tied to ground or positive. RESET pins can have several uses. In the case of a dice emulation using the 4017 connect the 7th output, "6" to the reset, and it will jump can to the first output, "0". While you won't care that Christmas lights you have on from the Sunday before Advent in late November to Candlemas on February 2, in some cases you need the count to start from the 0 position. In this case a resistor pulls the pin to ground, and a capacitor is placed between the pin and the supply. At turn-on the capacitor is low impedance, and pulls the pin high. A moment later this capacitor charges via the resistor, and becomes high impedance, and the pin is now low. 100 kΩ and 1 μF is probably fine.
A useful chip is the 40107, an 8-pin, 2 input open-drain NAND gate. This can be used to drive a LED, small relay, or small lamp when two inputs are high. Connect the load between a positive line (up to 20 volts) and the output pin.
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 microprocessor, such as the Z80; memory ICs (ROM and RAM); input/output ICs; and the like. The Z80 was also used in various home and school PCs, such as the MicroBee, VZ-200/300, early TRS-80 (pre-CoCo), and Sinclair ZX Spectrum.
Nowadays, a microcontroller IC contains the CPU, ROM, EEPROM, and RAM; and in the case above, simplifies the input from the keypad, and output to the display, and control of the bell relay. As smaller microcontollers are often inexpensive, they are an option to replace a handful of logic gates. In many cases some form of analog to digital and digital to analogue functionality is included, although perhaps in many cases at speeds suitable for monitoring battery voltages and temperatures than handling audio.
Microcontrollers are also available packaged onto boards such as the Arduino, and its clones, making many projects potentially even simpler to build, with many "shields" containing advanced input and output devices available.
An amplifier can range from a sensitive low level device, to the output of a megawatt transmitter, and range from DC to gigahertz.
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 small 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 its effect is called neutralisation. According my TAFE teacher, winding the inductor over the resistor is not good practice.
For audio there are 3 conventional classes of amplifier: Class A is used for small signals, and as power amplifiers by "audiophiles"; they are however inefficient, as there is a current flowing at all times (100%), OK in a signal amplifier where the quiescent is low, but not in a power amp, which might consume 100 watts whether they are putting out 1 watt or 25 watts at the time (quiet or loud passages). Class B conducts just under 180 degrees (50%), and generally two paired. They however suffer cross-over distortion near the zero point as they do not conduct when the wave falls below the forward voltage of the device, about a volt. There are little used now, and are really just a conceptual stepping-stone. The Class AB cleans up the cross-over area by operating in class A at low levels. Class AB typically uses positive and negative rails, and a complementary pair of NPN and PNP transistors. These all behave in a (fairly) linear manner.
Class C is discussed in the exam. These work by conducting for a short time period, triggering a resonant circuit to generate a full sine wave, just as brief push on a park swing results in a full arc of forwards and return motion. Conduction is 120 degrees (33.3% or less).
These, and further classes are discussed on: Wikipedia: Power amplifier classes. Class D, which uses PWM, can be used for audio, and 137 kHz (2200 metres). Class E is used for RF.
RF power anplifiers, generally are used within transmitters, and as external amplifier. Amateurs are permitted to use external amplifiers as long as power limits are observed. Thus includes that no one in Australia or Europe wants to hear you telling your buddy down the road the intimate details of your colonoscopy - only use the power necessary for your communication. It is only CB where they are banned, even if used only to amplify a low power unit to legal power. For AM, SSB, and related data modes linear amplifiers are use; for FM, and related packet modes they can operate in a mode closer to Class C.
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.
Thus, an HF oscillator, mixer, and detector forms a superheterodyne receiver, or "superhet". These are used in most receivers from a pocket transistor radio to high-end professional and amateur equipment, at least before the era of "software defined" radios.
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 suppressed carrier. This can be filtered to remove one of the sidebands, so we get SSB.
Traditionally AM was generated by amplifying the carrier frequency to high level, and the audio to half this power, and mixing the two using a transformer or series inductor.
Mixers / modulators are available in IC form, such as the NE/SA 602/612 families. These can be used with a 3.579545 MHz NTSC "colorburst" crystal or 3.58 MHz resonator to produce DSB; or to receive SSB in a direct conversion receiver.
Software Definede Radios can be a simple RF front-end feeding a PC soundcard or RaspberryPi, a black box controlled by a PC, or a conventional looking HF radio. It can be a receiver, or a full transceiver. New functions and modes can be added by updating software. I expect it may be possible to add something like CODEC2 to a radio which initially only does analogue modes. Or perhaps more simple things, such as varying the level of the carrier, sending one or two sidebands, etc.
You can also decode DRM - the Digital Radio Mondiale shortwave broadcasting system - by tapping a 12 kHz wide audio signal from a radio. For the FT-857D bypassing one of the optional filter ports with a capacitor, and making appropriate settings allows this to be done.
The use of "I and Q" signals which are in quadrature, or 90 degrees apart, is a common feature.
Filters pass some frequencies, and blocks, or attenuates, others*.
They consist of at least two of capacitors, inductors, or resistors. They use the increase in reactance with increasing frequency in inductors, and the decrease in reactance with increasing frequencies in capacitors.
The primary kinds are: Low pass filters, High pass filters, Notch filters to attenuate a specific frequency band, and Band-pass filters to pass a band of frequencies.
All passive filters have some loss in the pass band, called "insertion loss".
While things such as depth of attenuation varies in specifications, the examiner determines that the "Cutoff frequency" of a low pass filter is that at which the output power is reduced to half power. For a Band Pass filter they term the two frequencies of interest as the "Upper and lower half-power" points.
Audio filters may include operational amplifiers. Ging off the exam, elements in the feedback loop of the circuit can have their behaviour reversed: A gyrator is an op-amp with a capacitor in the feedback loop, which emulates an inductor, popular in graphic equalisers where you want a high "Q", or quality factor. While most people use them to adjust the sound of audio in home systems, the intended purpose is to compensate for the characteristics of an auditorium, or a school gymnasium being used for a performance. They can also be used to tune out frequencies where the other feedback, aka acoustic howl, can occur.
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).
Ultimate Rejection in the context of filters appears to be the maximum attenuation achieved by a filter for out-of band signals. Especially at RF, if you have a signal in a box there will be some leakage to the output, just as the attenuation possible in a single stage of an attenuator has its limits. Spurious capacitance perhaps also comes into play.
If a filter had an ultimate rejection of 20 dB, and a power amplifier produced a harmonic at 10 watts, there is still 100 mW of interfering signal being emitted, more than enough to upset the legitimate user of that frequency, or their customers.
You can read a paper which discusses RF filters, with some info on out-of-band rejection, here: https://sci-hub.se/https://doi.org/10.1002/cta.2895
Web searching this term gets few useful results, and some rather difficult ones. If you are not looking for the electronics (or optics) topic, please seek appropriate assistance.
Crystals, ceramic resonators, and mechanical elements can also be used in RF filters.
Not on the exam, all filters cause a phase shift. *As an aside, a Constant Amplitude Filter neither attenuates nor amplifies any frequency, but provides a phase shift. There is lots on filters on Facebook: Australian Electronics, including a downloadable PDF.
At audio a "cross-over" is a group of filters used in audio systems. Traditionally this is after the amplifier, and sends low frequencies to the bass / woofer, mids to the mid-range speakers or "drivers", and highs / treble portion to the tweeters. In modern systems the signals are spilt at low level, and sent to separate amplifiers. At RF diplexers / duplexers / triplexers are used to combine the signals from who radios, or from the (say) VHF and UHF outputs of a large radio to a single antenna, or from a single antenna connector on a radio to separate antennas; and to do the reverse for received signals.
Oscillators which generate sine waves consist of some form of amplifier with filter components in the feedback loop.
From memory the Extra goes into greater detail of the various topographies, often named for their designers. Interestingly, many designs work with valves / tubes, BJTs, or FETs. Op-Amps can also be used. At audio frequencies resistors and capacitors tend to be used, with inductors coming into play at RF. They can also use crystals or ceramic resonators.
Note that the 555 IC produces square waves (although the duty cycle is not 50:50).
The overall sensitivity of a receiver depends on factors including its input amplifier gain, the noise figure of this input amplifier, and the demodulator stage's bandwidth.
For SSB a product detector is used. Removed from the paper, a discriminator is commonly used for FM.
The computer portion of software defined radios often use I and Q signals as their input, which are 90 degrees apart. This can be to the stereo soundcard inputs on a PC or similar device, or on a USB "soundcard" module.
These are actual questions from the 2023-2027 General exam pool.
G7B01
What is the reason for neutralizing an amplifier?
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.
G7B02
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.
G7B03
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.
G7B04
In a Class A amplifier, what percentage of the time does the amplifying device conduct?
A. 100%
B. More than 50% but less than 100%
C. 50%
D. Less than 50%
Current flows for 100% of the time, answer A.
G7B05
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.
G7B06
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.
G7B07
Which of the following are basic components of a sine wave oscillator?
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.
G7B08
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.
G7B09
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.
G7B10
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 thus good for AM, and SSB.
G7B11
For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?
A. SSB
B. FM
C. AM
D. All of these choices are correct
Class C amplifiers can be used for FM, answer B.
G7C01
What circuit is used to select one of the sidebands from a balanced modulator?
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.
G7C02
What output is produced by a balanced modulator?
A. Frequency modulated RF
B. Audio with equalized frequency response
C. Audio extracted from the modulation signal
D. Double-sideband modulated RF
These produce a Double-sideband modulated RF signal, answer D.
It can be used used as is, or filtered to SSB.
G7C03
What is one reason to use an impedance matching transformer at a transmitter output?
A. To minimize transmitter power output
B. To present the desired impedance to the transmitter and feed line
C. To reduce power supply ripple
D. To minimize radiation resistance
Transistor amplifiers operating from 12 volts operate with very low impedance, and the output must be fed through a tansformer to match it to the 50 ohm coax, answer B.
G7C04
How is a product detector used?
A. Used in test gear to detect spurious mixing products
B. Used in a transmitter to perform frequency multiplication
C. Used in an FM receiver to filter out unwanted sidebands
D. Used in a single sideband receiver to extract the modulated signal
They are used in a receivers to demodulate single sideband signals, answer D.
G7C05
Which of the following is characteristic of a direct digital synthesizer (DDS)?
A. Extremely narrow tuning range
B. Relatively high-power output
C. Pure sine wave output
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.
G7C06
Which of the following is an advantage of a digital signal processing (DSP) filter compared to an analog filter?
A. A wide range of filter bandwidths and shapes can be created
B. Fewer digital components are required
C. Mixing products are greatly reduced
D. The DSP filter is much more effective at VHF frequencies
DSP allows custom filter bandwidths, and filter shapes to be created, answer A.
G7C07
What term specifies a filter's attenuation inside its passband?
A. Insertion loss
B. Return loss
C. Q
D. Ultimate rejection
This is termed insertion loss, answer A.
G7C08
Which parameter affects receiver sensitivity?
A. Input amplifier gain
B. Demodulator stage bandwidth
C. Input amplifier noise figure
D. All these choices are correct
All these factors are important, answer D.
G7C09
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.
G7C10
What is an advantage of using I-Q modulation with 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. Automatic conversion of the signal from digital to analog
This allows all types of modulation to be created, with the correct software, answer B.
G7C11
Which of these functions is performed by software in a software-defined radio (SDR)?
A. Filtering
B. Detection
C. Modulation
D. All these choices are correct
Software performs all these tasks, answer D.
This can be done within a stand-alone radio, or by connection of a simple receiver's IF output to a PC or Raspberry Pi. Many big-dollar radios now use SDR functions internally.
G7C12
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.
The rest are real things, the last is used regarding the point at which an amplifier's gain falls by 3 dB compared with its peak gain. There used to be lots of dodgy claims around "Hi-Fi" amplifier specs in the 1980s, and tricks like specifying the -6 dB points.
G7C13
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.
G7C14
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.
Some digital modes can be directly generated using DDS, such as WSPR, FSK, and PSK.
Op-amps were designed to perform addition, subtraction, and multiplication in 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. Many micro-controllers and single board computing modules do this to allow a pin to be either an input or output.
On to: Modes 1 - Modulation & Mixers
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, November 2024.
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