ECGR4124 Project 10, DSP Demodulator
Form the same groups of 3 as before. Members of any group must
be
all graduate or all undergrad.
Do NOT use "zoom" feature on
any
plots unless specifically requested.
Preliminary tasks
- Run NetBeans
5.5, or a later version
(Start>AllPrograms>MosaicXp>Programming>Netbeans>NetBeans55)
- Make a copy of your previous project from within NetBeans, and
rename
the new project.
- Make
sure
that your 20log10|FFT| is working correctly, with dc
(zero-frequency) in the center of the plot.
- The quadrature data on a QPSK signal can be represented as
f(t) = i(t) cos( w t ) + q(t) sin( w t )
where:
i(t) and q(t) are the data signals with levels +/- 1 volt
w is the carrier frequency in radians/second.
- Use the following block diagram to demodulate the signal
and
recover the data.
- In the above block diagram, the input signal iqmodint.au is
first multiplied by two local oscillators, LO1 and LO2. The
output of
the ideal multiplier is then lowpass filtered to obtain the I
and Q
data outputs corresponding to the transmitted data i(t) and
q(t).
Ideally, the outputs would be voltages of +/-1 volts, but
noise and
interference will be present. The output levels may also
differ from
values of +/- 1, depending on your particular design.
- Note: placing a filter at the input will cause phase delay
on
the carrier and induce errors in the outputs at I and Q. Do
not attempt
to filter the signal ahead of the multipliers.
- LO1 is a 1 KHz cosine wave, and LO2 is a 1 KHz sine wave.
- The basic theory for the above system can be seen by
evaluating the I channel. First, the signal is multiplied by
cos( w t
), giving the following output at point A:
cos( w t ) { i(t) cos( w t ) + q(t) sin( w t ) }
= 1/2 i(t) { cos( 2 w t ) + cos( 0 ) } + 1/2 q(t) { sin( 2 w
t ) + sin(
0 ) }
With a properly designed lowpass filter F1, the high
frequency components 1/2 i(t) cos( 2 w t ) and 1/2 q(t) sin(
2 w t )
are removed, leaving only 1/2 i(t) cos( 0 ) = i(t)/2.
- Use the above block digram to design a DSP algorithm to
recover the data from the two quadrature channels in the
signal
iqmodint.au.
DO NOT USE FFT or frequency domain methods.
Work in the time domain to implement the multipliers and the
filters.
Use a moving average filter as in Eq. 2.91 to implement F1 and
F2.
- Design F1 and F2 to pass the desired signal and to
attenuate
unwanted high frequencies. Design the second
null
of the filter
response to fall in the middle of the DSB-SC
interference. Be
careful that you also consider that the frequency is shifted
at point
A, so that the second null must must fall at the frequency of
interference after
this
frequency shift has occurred.
- Plot the spectrum of the input signal iqmodint.au. ( P1 )
- Plot the spectrum of the signal at point A above. ( P2 )
- Observe the spectrum at point A relative to the observed
spectrum at the input, considering the frequency shift
property of the
Fourier transform. State the observed frequency shift in
Hz. ( Q1 )
- Plot the recovered data from the I and Q channels. ( P3, P4 )
- Hint: the first few bits of I-channel output should look
like:
- Plot the spectrum of the recovered data from the I and Q
channels. ( P5, P6 )
- Estimate the data rate of the I channel in bits/second
based
on the recovered data, and the shortest duration pulses
observed. ( Q2 )
- Sketch a system block diagram of a hardware implementation
of
the above system. Use only digital adders, digital
multipliers, shift
registers (you may assume as many bits wide as needed), A/D
converters,
D/A converters. You may assume that digital signal sources are
available for LO1 and LO2. The input and outputs of the block
diagram
are to be analog. ( Q3 )
Graduate
students: see extra Project Presentation required in the last
project.
Report
Use the Project
Report Template
(also shown as pdf file)
Include the plot numbers P1, P2, etc
in
captions, along with a short description!
Write a cover sheet + 1 page executive
summary ONLY!!! Describe any unresolved issues.
Place answers to all questions on one or more consequitive sheets
of paper at the front of the report!
Do NOT scatter
the question answer sheets throughout the report, keep them
together
Do NOT use "zoom" feature on
any
plots unless specifically requested.
Make sure that you include question
numbers and plot numbers on all sheets!
I will only look on the question answer-sheets for any discussion.
All extraneous material WILL BE
IGNORED!
Do not write any discussion, except directly on the question
answer-sheets or 1 page summary.
Do not add extraneous pages.
Do not put explanations on plots unless
specifically directed to do so.
Include any problems or unresolved issues in the summary.
Copyright 2009 T.P. Weldon