KB6NU's Ham Radio Blog

///////////////////////////////////////////
2019 No Nonsense General Class Study Guide: Practical Circuits

Posted: 11 Feb 2019 11:48 AM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


As always, comments are appreciated. I especially had some trouble
distilling the explanation of questions on software-defined radio. There
were three questions added on this topic to the question pool..Dan

Power supplies; schematic symbols

Power supplies are devices that convert AC power to the DC voltages needed
to power amateur radio equipment. There are two main types of power
supplies available: linear power supplies and switching power supplies.

Linear supplies use a transformer to transform the voltage up or down, a
rectifier to convert the AC voltage to a DC voltage, and capacitors and
inductors to smooth the output voltage. The rectifier in a linear supply
may be a half-wave rectifier, a full-wave rectifier, or a bridge rectifier.

As the name implies a half-wave rectifier converts half, or 180 degrees, of
an AC cycle to DC, while a full-wave rectifier converts all 360 degrees of
the cycle to AC. The bridge rectifier also converts the complete cycle to
DC.
QUESTION: What portion of the AC cycle is converted to DC by a half-wave
rectifier? (G7A05)

ANSWER: 180 degrees
QUESTION: What portion of the AC cycle is converted to DC by a full-wave
rectifier? (G7A06)

ANSWER: 360 degrees

Each type of rectifier has its advantages and disadvantages. A half-wave
rectifier uses only a single diode, but only converts half the AC power
available into DC. A full-wave rectifier converts the entire AC power
available into DC, but requires a center-tapped transformer. A bridge
rectifier does not require the use of a more expensive center-tapped
transformer, but needs four diodes.
QUESTION: What is an advantage of a half-wave rectifier in a power supply?
(G7A04)

ANSWER: Only one diode is required
QUESTION: Which type of rectifier circuit uses two diodes and a
center-tapped transformer? (G7A03)

ANSWER: Full-wave

The output of an unfiltered rectifier circuit is a series of DC pulses. For
the half-wave rectifier, the frequency of the pulses is the same as the AC
input, as there is an output only when the AC input goes positive. For the
full-wave rectifier, the output frequency is twice that, as it outputs a
pulse when the AC goes positive and negative.
QUESTION: What is the output waveform of an unfiltered full-wave rectifier
connected to a resistive load? (G7A07)

ANSWER: A series of DC pulses at twice the frequency of the AC input

The output of a rectifier connects to a filter made up of capacitors and
inductors. The purpose of this filter is to filter out the AC component of
the pulsed DC and output a steady DC voltage.
QUESTION: Which of the following components are used in a power supply
filter network? (G7A02)

ANSWER: Capacitors and inductors

A component often found across the output of a power supply is a
power-supply bleeder resistor. The purpose of this resistor is to
discharge, or bleed off, the charge on the filter capacitors when the power
supply is turned off. The value of this resistor is normally very high
value so that very little current flow through it during normal operation.
QUESTION: What useful feature does a power supply bleeder resistor provide?
(G7A01)

ANSWER: It ensures that the filter capacitors are discharged when power is
removed

Switching, or switched-mode power supplies are now being sold by many
vendors. Switching supplies are smaller and lighter than linear supplies,
but the circuits are much more complex than linear power supply circuits.
QUESTION: Which of the following is an advantage of a switchmode power
supply as compared to a linear power supply? (G7A08)

ANSWER: High frequency operation allows the use of smaller components

When designing or troubleshooting radios, amateur radio operators use
schematic diagrams to describe circuits. Various symbols represent the
different types of components. A typical schematic is shown in Figure G7-1.


QUESTION: Which symbol in figure G7-1 represents a field effect transistor?
(G7A09)

ANSWER: Symbol 1
QUESTION: Which symbol in figure G7-1 represents a Zener diode? (G7A10)

ANSWER: Symbol 5
QUESTION: Which symbol in figure G7-1 represents an NPN junction
transistor? (G7A11)

ANSWER: Symbol 2
QUESTION: Which symbol in Figure G7-1 represents a solid core transformer?
(G7A12)

ANSWER: Symbol 6
QUESTION: Which symbol in Figure G7-1 represents a tapped inductor? (G7A13)

ANSWER: Symbol 7

Digital circuits

Digital circuits are circuits whose output are one of two voltages—either
“on� or “off,� “high� or “low,� “one� or “zero.� Digital circuits use the
binary system to represent numbers because each of the digits in a binary
number is either a 1 or a 0.

We use digital circuits to implement logic functions, and we call acircuit
that implements a logic function a “gate.� Two typical logic functions are
AND and NOR. The output of an AND gate is 1 when all of the inputs are 1.
The output of a NOR gate is 1 only when both inputs are 0.
QUESTION: Which of the following describes the function of a two-input AND
gate? (G7B03)

ANSWER: Output is high only when both inputs are high
QUESTION: Which of the following describes the function of a two input NOR
gate? (G7B04)

ANSWER: Output is low when either or both inputs are high

Integrated circuits that provide more complex logic functions, such as
flip-flops, counters and shift registers, are also available. As the name
implies, a counter counts input pulses and outs the count in binary. The
figure below shows a three-bit counter made with 3 D flip-flops. A D
flip-flop is a circuit whose output, Q, changes when it receives a clock
pulse. After the clock pulse, Q is equal to D. The other output, Q[bar] is
the inverse of Q.

A three-bit counter has eight output states.
QUESTION: How many states does a 3-bit binary counter have? (G7B05)

ANSWER: 8

A shift register consists of an array of flip flops whose outputs are
connected to the input of the next flip-flop in the chain. When the shift
register receives a clock pulse, data is shifted down the array.
QUESTION: What is a shift register? (G7B06)

ANSWER: A clocked array of circuits that passes data in steps along the
array

Oscillators and amplifiers

An oscillator is a circuit that generates an AC output signal. It normally
consists of an amplifier, part of whose output is fed back to the input
through a filter. This feedback keeps the circuit oscillating, while the
filter determines the frequency of the oscillation.
QUESTION: Which of the following are basic components of a sine wave
oscillator? (G7B07)

ANSWER: A filter and an amplifier operating in a feedback loop

An “LC� oscillator uses an inductor and a capacitor connected so that they
form what’s called a tank circuit to provide feedback. The tank circuit is
the filter that determines the frequency of oscillation.
QUESTION: What determines the frequency of an LC oscillator? (G7B09)

ANSWER: The inductance and capacitance in the tank circuit

There are many different types of amplifiers. These types are often
categorized by class: Class A, Class B, Class C, and Class D amplifiers.
Each class of amplifiers has certain characteristics. Linear amplifiers,
for example, are usually Class A amplifiers. Class A amplifiers add very
little distortion and the output waveform is very close to the input
waveform, just larger. This makes them the best choice for amplifying AM
and SSB phone signals.
QUESTION: Which of the following describes a linear amplifier? (G7B10)

ANSWER: An amplifier in which the output preserves the input waveform

The Class C amplifier, on the other hand, is not linear. Fortunately, an
amplifier need not be linear to amplify FM and CW signals. They are a good
choice for FM and CW because they are more efficient than Class A, Class B,
and Class AB amplifiers.
QUESTION: Which of these classes of amplifiers has the highest efficiency?
(G7B02)

ANSWER: Class C
QUESTION: For which of the following modes is a Class C power stage
appropriate for amplifying a modulated signal? (G7B11)

ANSWER: FM
QUESTION: How is the efficiency of an RF power amplifier determined? (G7B08)

ANSWER: Divide the RF output power by the DC input power

High-power amplifiers are often prone to self-oscillation due to stray
capacitive feedback. To prevent this from happening, you induce some
feedback that is out of phase with the stray capacitive feedback to
neutralize it.
QUESTION: What is the reason for neutralizing the final amplifier stage of
a transmitter? (G7B01)

ANSWER: To eliminate self-oscillations

Receivers and transmitters, filters, oscillators

Filters are very important circuits in amateur radio equipment. As the name
implies, these circuits are used to clarify or process radio signals. For
example, one type of filter—a low-pass filter—passes all signals whose
frequencies are below a certain frequency, called the “cutoff frequency.�
QUESTION: What is the frequency above which a low-pass filters output power
is less than half the input power? (G7C12)

ANSWER: Cutoff frequency

One application of a low-pass filter is to block the VHF and UHF harmonics
produced by an amateur transceiver from reaching the antenna. To do this,
you would connect the input of the filter to the output of your transceiver
and the output of the filter to your antenna system.
QUESTION: What should be the impedance of a low-pass filter as compared to
the impedance of the transmission line into which it is inserted? (G7C06)

ANSWER: About the same

Band-pass filters are designed to pass signals between two
frequencies—called the passband—while rejecting signals outside this range.
The width of this frequency range is the difference between the two
frequencies at which the signal power is one-half the maximum power. The
difference between the upper and lower half-power frequencies is called the
bandwidth of the filter.
QUESTION: The bandwidth of a band-pass filter is measured between what two
frequencies? (G7C14)

ANSWER: Upper and lower half-power

How well a filter rejects signals outside the passband is called ultimate
rejection.
QUESTION: What term specifies a filters maximum ability to reject signals
outside its passband? (G7C13)

ANSWER: Ultimate rejection

Of course, filters arent perfect. They will reduce, or attenuate, the input
signal level of the input signal, by some amount, even if the signal is
within the passband?
QUESTION: What term specifies a filters attenuation inside its passband?
(G7C15)

ANSWER: Insertion loss

Filters are also used in amateur radio transmitters. For example, to
produce a single sideband signal, you combine the audio signal and the
signal from the carrier oscillator in a balanced modulator. The output of
the balanced modulator output includes both the upper and lower sidebands.
To obtain a single sideband, you filter out the sideband you dont want.
QUESTION: 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? (G7C02)

ANSWER: Balanced modulator
QUESTION: 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? (G7C01)

ANSWER: Filter

For many years, the superheterodyne receiver has been the most popular type
of amateur radio receiver. Superheterodyne receivers convert the received
signal to an intermediate frequency (IF), using a circuit called a mixer.
The mixer combines the incoming signal with the signal from an HF
oscillator, called the local oscillator, to produce the IF signal. In some
SSB receivers, a product detector then demodulates that IF signal.
QUESTION: 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? (G7C03)

ANSWER: Mixer
QUESTION: 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? (G7C04)

ANSWER: Product detector
QUESTION: What is the simplest combination of stages that implement a
superheterodyne receiver? (G7C07)

ANSWER: HF oscillator, mixer, detector

FM receivers have different types of circuits than the superheterodyne
receivers designed for AM, CW, and SSB reception. In FM receivers, the
detector is called a discriminator.
QUESTION: What circuit is used in analog FM receivers to convert IF output
signals to audio? (G7C08)

ANSWER: Discriminator

Most modern transceivers use circuits called a direct digital synthesizer
to control the receive and transmit frequencies. This gives them much more
stability than receivers that use analog local oscillators.
QUESTION: Which of the following is a typical application for a Direct
Digital Synthesizer? (G7C16)

ANSWER: A high-stability variable frequency oscillator in a transceiver
QUESTION: Which of the following is an advantage of a direct digital
synthesizer (DDS)? (G7C05)

ANSWER: Variable frequency with the stability of a crystal oscillator

Digital techniques have proven to be so effective at generating and
receiving radio signals, that some transceivers now implement most
functions using digital signal processors. We even have a special term for
this type of radio: software defined radio, or SDR.
QUESTION: What is meant by the term “software-defined radio� (SDR)? (G7C11)

ANSWER: A radio in which most major signal processing functions are
performed by software

Software-defined radios work with two signals, called I (in phase) and Q
(quadrature), that are 90 degrees out of phase. When an SDR is receiving,
the receivers analog-to-digital converters convert the incoming signal into
a series of values, representing the I and Q signals. When an SDR is
transmitting, it does the reverse. It uses the digital representations of
the I and Q signals to produce an RF output.
QUESTION: What is the phase difference between the I and Q signals that
software-defined radio (SDR) equipment uses for modulation and
demodulation? (G7C09)

ANSWER: 90 degrees
QUESTION: What is an advantage of using I and Q signals in software-defined
radios (SDRs)? (G7C10)

ANSWER: All types of modulation can be created with appropriate processing

The post 2019 No Nonsense General Class Study Guide: Practical Circuits
appeared first on KB6NUs Ham Radio Blog.