KB6NU's Ham Radio Blog

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2016 Extra Class study guide: E7H - oscillators and signal sources

Posted: 12 Feb 2016 05:00 PM PST
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E7H Oscillators and signal sources: types of oscillators; synthesizers and
phase-locked loops; direct digital synthesizers; stabilizing thermal drift;
microphonics; high accuracy oscillators

Oscillator circuits are one of the basic building blocks of amateur radio
equipment. Oscillator circuits are not only used to generate the signals we
transmit. They are also an integral part of receivers, such as the
superheterodyne receiver.

You can think of an oscillator as an amplifier with a tuned circuit at the
input. This tuned circuit might be an LC circuit or a crystal. The values
of the components in the tuned circuit determine the output frequency of
the oscillator. There are three types of oscillator circuits commonly used
in Amateur Radio equipment Colpitts, Hartley and Pierce. (E7H01) Colpitts
and Hartley oscillator circuits are commonly used in VFOs. (E7H06)

For a circuit to oscillate, it must have positive feedback. In a Hartley
oscillator (shown in the figure below), positive feedback is supplied
through a tapped coil. (E7H03)



In a Colpitts oscillator, positive feedback is supplied through a
capacitive divider. (E7H04)



In a Pierce oscillator, positive feedback is supplied through a quartz
crystal. (E7H05) To ensure that a crystal oscillator provides the frequency
specified by the crystal manufacturer, you must provide the crystal with a
specified parallel capacitance. (E7H12) NPO capacitors are components that
can be used to reduce thermal drift in crystal oscillators. (E7H08)



One problem that oscillators sometimes have is called microphonics. Changes
in oscillator frequency due to mechanical vibration describes a
microphonic. (E7H02) An oscillators microphonic responses can be reduced by
mechanically isolating the oscillator from its enclosure. (EH707)

Digital frequency synthesizers

Most modern amateur radio transceivers use digital frequency synthesizers
instead of analog oscillators to generate RF signals. One reason for this
is that they are much more stable than analog oscillators. The two main
types of digital frequency synthesizers are the direct digital synthesizer
and the phase-locked loop synthesizer.

A direct digital synthesizer is the type of frequency synthesizer circuit
that uses a phase accumulator, lookup table, digital to analog converter
and a low-pass anti-alias filter. (E7H09) The information contained in the
lookup table of a direct digital frequency synthesizer is the amplitude
values that represent a sine-wave output. (E7H10)

Another type of frequency synthesizer that’s popular are those that use a
phase-locked loop. A phase-locked loop circuit is an electronic servo loop
consisting of a phase detector, a low-pass filter, a voltage-controlled
oscillator, and a stable reference oscillator. (E7H14) Frequency synthesis,
FM demodulation are two functions that can be performed by a phase-locked
loop. (E7H15)

Both direct digital synthesizers and phase-locked loop synthesizers have
issues with spectral purity. The major spectral impurity components of
direct digital synthesizers are spurious signals at discrete frequencies.
(E7H11)

Because frequency multipliers are often used for generating RF signals at
microwave frequencies, it is very important that the oscillators used in
microwave transmitters are accurate and stable. Any inaccuracy or
instability will be multiplied along with the desired frequency. All of
these choices are correct when talking about techniques for providing
highly accurate and stable oscillators needed for microwave transmission
and reception: (E7H13)

Use a GPS signal reference
Use a rubidium stabilized reference oscillator
Use a temperature-controlled high Q dielectric resonator


The post 2016 Extra Class study guide: E7H oscillators and signal sources
appeared first on KB6NUs Ham Radio Blog.


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Learn the math of DSP

Posted: 12 Feb 2016 11:49 AM PST
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Digital signal processing is, obviously, all about the math. This series is
from EE Times, and consists of excerpts from the bookÂ*Digital Signal
Processing: Instant Access.
The math of DSP, part 1: Series, integration, and frequency.Â*Part 1
introduces the basic math needed for DSP. Topics covered include
polynomials, transcendentals, series, limits, integration, polar notation,
and frequency.
The math of DSP, part 2: Complex numbers.Â*Part 2 explains complex numbers.
Topics covered include real and imaginary numbers, periodic signals,
digital frequencies, and discrete arithmetic.
The math of DSP, part 3: Filters.Â*Part 3 explains the basics of low-pass
and high-pass filters. It also explains the concept of causality.
The math of DSP, part 4: Convolution, Fourier, and Nyquist.Â*Part 4 looks at
convolution, the Fourier series, and the Nyquist sampling theorem.
The math of DSP, part 5: Orthogonality.Â*Part 5 explains the concept of
orthogonality and introduces quadrature signals.

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