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2020 Extra Class study guide: E7F - DSP filtering and other operations;
software defined radio fundamentals; DSP modulation and demodulation

Posted: 25 Feb 2020 01:53 PM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


Some modern radios modulate and demodulate signals entirely in software.
This type of radio is called a software-defined radio, or SDR. One type of
SDR uses a process called direct digital conversion to convert the analog
radio signal into a series of numbers. What this type of SDR does is
digitize an incoming RF signal with an analog-to-digital converter without
being mixed with a local oscillator signal.
QUESTION: What is meant by direct digital conversion as applied to software
defined radios? (E7F01)

ANSWER: Incoming RF is digitized by an analog-to-digital converter without
being mixed with a local oscillator signal

Analog-to-digital converter specifications are crucial for a
software-defined radio. Sample rate is one such specification. The sample
rate determines the maximum receive bandwidth of a direct digital
conversion SDR. An analog signal must be sampled at twice the rate of the
highest frequency component of the signal by an analog-to-digital converter
so that the signal can be accurately reproduced.
QUESTION: What aspect of receiver analog-to-digital conversion determines
the maximum receive bandwidth of a Direct Digital Conversion SDR? (E7F10)

ANSWER: Sample rate
QUESTION: How frequently must an analog signal be sampled by an
analog-to-digital converter so that the signal can be accurately
reproduced? (E7F05)

ANSWER: At least twice the rate of the highest frequency component of the
signal

Voltage resolution is also important. In the absence of atmospheric or
thermal noise, the reference voltage level and sample width in bits sets
the minimum detectable signal level for an SDR. The reference voltage is
basically the maximum detectable voltage, while the number of bits
determines the resolution of the analog-to-digital converter. For example,
if an analog-to-digital converter had a reference voltage of 1 V, the
minimum number of bits required to sample a signal at a resolution of 1
millivolt is 10 bits. The reason for this is that 210 = 1,024, meaning that
each bit represents approximately 1 mV.
QUESTION: What sets the minimum detectable signal level for a
direct-sampling SDR receiver in the absence of atmospheric or thermal
noise? (E7F11)

ANSWER: Reference voltage level and sample width in bits
QUESTION: What is the minimum number of bits required for an
analog-to-digital converter to sample a signal with a range of 1 volt at a
resolution of 1 millivolt? (E7F06)

ANSWER: 10 bits

Modern software defined radios convert an incoming signal into two data
streams: I and Q. The I and Q data streams are 90 degrees out of phase with
one another, and as a result, the two data streams not only show how the
amplitude of a signal is changing, but how the phase of a signal is
changing. Perhaps the most common operation that is performed on the I and
Q signals is a Fast Fourier Transform (FFT). The Fast Fourier Transform
converts digital signals from the time domain to the frequency domain.
QUESTION: What function is performed by a Fast Fourier Transform? (E7F07)

ANSWER: Converting digital signals from the time domain to the frequency
domain

Once a signal has been digitized, or converted into a series of numbers, it
can be digitally filtered. We call this digital signal processing, or DSP
For example, to remove unwanted noise from a received SSB signal you would
use a DSP audio filter called an adaptive filter. Another type of digital
filter used in SDRS is the finite impulse, or FIR, filter. An advantage of
a Finite Impulse Response (FIR) filter vs an Infinite Impulse Response
(IIR) digital filter is that FIR filters delay all frequency components of
the signal by the same amount.
QUESTION: What kind of digital signal processing audio filter is used to
remove unwanted noise from a received SSB signal? (E7F02)

ANSWER: An adaptive filter
QUESTION: Which of the following is an advantage of a Finite Impulse
Response (FIR) filter vs an Infinite Impulse Response (IIR) digital filter?
(E7F15)

ANSWER: FIR filters can delay all frequency components of the signal by the
same amount

Taps in a digital signal processing filter provide incremental signal
delays for filter algorithms. More taps would allow a digital signal
processing filter to create a sharper filter response.
QUESTION: What is the function of taps in a digital signal processing
filter? (E7F13)

ANSWER: Provide incremental signal delays for filter algorithms
QUESTION: Which of the following would allow a digital signal processing
filter to create a sharper filter response? (E7F14)

ANSWER: More taps

Because so many calculations are required to do digital signal processing,
SDRs sometimes use a technique called decimation, which allows them to use
less-powerful processors. Decimation reduces the effective sample rate by
removing samples when using a digital filter. They can get away with this
because the signal of interest will usually have a significantly lower
bandwidth than the digitized signal. An anti-aliasing digital filter must
be used with a digital decimator because it peaks the response of the
decimator, improving bandwidth.
QUESTION: What is the function of decimation? (E7F08)

ANSWER: Reducing the effective sample rate by removing samples
QUESTION: Why is an anti-aliasing digital filter required in a digital
decimator? (E7F09)

ANSWER: It removes high-frequency signal components that would otherwise be
reproduced as lower frequency components

Signals can also be generated using SDR techniques. For example, a common
method of generating an SSB signal using digital signal processing is to
combine signals with a quadrature phase relationship. The type of digital
signal processing filter used to generate an SSB signal is a
Hilbert-transform filter.
QUESTION: What is a common method of generating an SSB signal using digital
signal processing? (E7F04)

ANSWER: Signals are combined in quadrature phase relationship
QUESTION: What type of digital signal processing filter is used to generate
an SSB signal? (E7F03)

ANSWER: A Hilbert-transform filter

The post 2020 Extra Class study guide: E7F – DSP filtering and other
operations; software defined radio fundamentals; DSP modulation and
demodulation appeared first on KB6NUs Ham Radio Blog.