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2016 Extra Class study guide: E7F - DSP and SDR

Posted: 09 Feb 2016 12:03 PM PST
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This section is almost entirely new. Im far from being an expert on digital
signal processing (DSP) and software-defined radio (SDR), so if you have
some expertise in these areas, please feel free to comment on this
section.Dan
E7F DSP filtering and other operations; Software Defined Radio
fundamentals; DSP modulation and demodulation

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. As applied to software defined
radios, direct digital conversion means incoming RF is digitized by an
analog-to-digital converter without being mixed with a local oscillator
signal. (E7F01)

Analog-to-digital converter specifications are crucial for a
software-defined radio. For example, sample rate is the aspect of receiver
analog-to-digital conversion that determines the maximum receive bandwidth
of a Direct Digital Conversion SDR. (E7F10) 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. (E7F05)

Voltage resolution is also important. The reference voltage level and
sample width in bits sets the minimum detectable signal level for an SDR in
the absence of atmospheric or thermal noise. (E7F11) 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 is 10 bits. (E7F06)

Modern software defined radios convert an incoming signal into two data
streams: I and Q. The letters I and Q in I/Q modulation (and demodulation)
represent In-phase and Quadrature. (E7F17). 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.

The digital process that is applied to I and Q signals in order to recover
the baseband modulation information is the Fast Fourier Transform. (E7F12)
Converting digital signals from the time domain to the frequency domain is
the function that a Fast Fourier Transform performs. (E7F07)

Once a signal has been digitized, or converted into a series of numbers, it
can be digitally filtered. The kind of digital signal processing audio
filter used to remove unwanted noise from a received SSB signal is an
adaptive filter. (E7F02) Another type of digital filter, one that is often
used in a direct digital conversion receiver, 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. (E7F15)

The FIR filter in a software defined radio might also be a decimating
filter. The decimation function reduces the effective sample rate by
removing samples when using a digital filter. (E7F08) SDRs perform
decimation because the signal of interest will usually have a significantly
lower bandwidth than the digitized signal, and reducing the sample rate
allows SDRs to use less-powerful processors. One way the sampling rate of
an existing digital signal might be adjusted by a factor of 3/4 is to
interpolate by a factor of three, then decimate by a factor of four. (E7F16)

An anti-aliasing digital filter is required in a digital decimator because
it removes high-frequency signal components which would otherwise be
reproduced as lower frequency components. (E7F09)

Taps in a digital signal processing filter provide incremental signal
delays for filter algorithms. (E7F13) More taps would allow a digital
signal processing filter to create a sharper filter response. (E7F14)

Signals can also be generated using SDR techniques. A common method of
generating an SSB signal using digital signal processing is to combine
signals with a quadrature phase relationship. (E7F04) The type of digital
signal processing filter used to generate an SSB signal is a
Hilbert-transform filter. (E7F03)

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