15
15 Lab 6: Agilent Ptolemy - QPSK Simulation
Lab6: Agilent Ptolemy- QPSK Simulation
15-2
Lab 6: Agilent Ptolemy- QPSK
Simulation
15
LAB 6: AGILENT PTOLEMY - QPSK SIMULATION................................................................... 15-1
15.1
OBJECTIVES: .................................................................................................................................. 15-5
15.2
THE DATA SOURCE: ....................................................................................................................... 15-5
15.2.1 Schematic Capture: .................................................................................................................. 15-5
15.2.2 Dynamic Signal Monitoring (TkPlot and TkXYPlot): ............................................................... 15-7
15.2.3 Simulation Setup (Data Flow Controller): ............................................................................... 15-8
15.2.4 Simulate and Observe the Results: ........................................................................................... 15-9
15.3
ADD BASEBAND FILTERS AND QAM MODULATOR: ..................................................................... 15-10
15.3.1 Add Baseband Filters: ............................................................................................................ 15-10
15.3.2 Add QAM Modulator: ............................................................................................................. 15-11
15.3.3 Add FFT Analyzer And TkPlots: ............................................................................................. 15-12
15.3.4 View the Constellation Diagram: ........................................................................................... 15-13
15.4
SAMPLED CONSTELLATION:.......................................................................................................... 15-14
15.4.1 Sampling Clock: ..................................................................................................................... 15-14
15.4.2 Sample and Hold Circuits: ..................................................................................................... 15-15
15.4.3 View the Sampled Constellation: ............................................................................................ 15-16
15.4.4 View the Modulated Spectrum : .............................................................................................. 15-17
15.5
QPSK DEMODULATOR: ................................................................................................................ 15-18
15.5.1 Add the QPSK demodulator: .................................................................................................. 15-18
15.5.2 Connect Timed Sinks: ............................................................................................................. 15-21
15.5.3 View Demodulated I and Q: ................................................................................................... 15-22
15.6
OUTPUT CONSTELLATION: ............................................................................................................ 15-22
15.6.1 Sample the Output I and Q: .................................................................................................... 15-22
15.6.2 View the Sampled Output Constellation: ................................................................................ 15-23
15.6.3 View the Output Eye Diagram:............................................................................................... 15-24
15.7
PHASE NOISE EFFECTS: ................................................................................................................ 15-25
15.8
LAB REVIEW: ............................................................................................................................... 15-26
15-3
Lab6: Agilent Ptolemy- QPSK Simulation
15-4
Lab 6: Agilent Ptolemy- QPSK
Simulation
15.1 Objectives:

Perform an Agilent Ptolemy simulation of a QPSK system using the DSP
schematic page

Set up TkXYPlots and TkPlots to view constellation and eye diagrams

Use SampleAndHold circuits to sample Nyquist bandlimited symbols
NOTE about this lab: This lab and the remaining labs will use QPSK and
PI4DQPSK modulation to demonstrate the use of Agilent Ptolemy for digital
communication systems analysis. The techniques used in the labs will be directly
applicable to applications such as CDMA, GSM, PCS, etc.
15.2 The Data Source:
15.2.1
Schematic Capture:
1. Open the project named “d:\users\ads\CommSys_Lab6_prj”.
Refer to the pictures and follow the steps to create the QPSK modulator using timed
elements. Throughout this lab, you can use the F5 key (Edit > Text > Move
Component Text) to move any component text if it makes connecting the circuits
easier.
2. Open a schematic window and open a the design a_QPSK_Sys.
15-5
Lab6: Agilent Ptolemy- QPSK Simulation
3. The only thing on the schematic should be a “VAR” component (from the
“Controllers” library/palette). The “VAR” block should look like the figure
shown below:
NOTE about variable units:
Units such as kHz and MHz can be assigned either in the VAR item or when used
as a parameter. They act as a multiplier (e.g. MHz= Hz * 1E6), so they should only
be assigned in either the VAR item or when used as a parameter, but not both.
Using variables and equations to establish the key relationships such as bit rate,
symbol rate, the minimum Nyquist frequency, etc., is a good technique to ensure
that element parameters are set up correctly. It is also flexible when parameters
such as the bit rate or the number of samples per symbols need to be changed.
4. Insert a “Data” source element from the “Timed Sources” library. Set the
folowing parameters:

TStep
to
Tstep
– units set to None

BitTime
to
Bit_time
– units set to None
Attention: The time units for the parameters should be set to None as they are
already set to the correct units in the “VAR” item.
15-6
Lab 6: Agilent Ptolemy- QPSK
Simulation
a. Insert a “Symbol Splitter” from the “Timed Data Processing” library.
Set the following parameters:

SymbolTime
to
Bit_time
- units set to None

Delay
to
-1 sec
- for automatic synchronization.
15.2.2
Dynamic Signal Monitoring (TkPlot and TkXYPlot):
1. Insert a “TkPlot” from the “Interactive Controls and Displays” palette. Set
the following parameters:

Label
to
Input Bits

XTitle
to
Time

YTitle
to
Amplitude.
Wire the TkPlot to the output of the Data source.
2. Insert a “TkXYPlot” from the “Interactive Controls and Displays” library.
Set the following parameters:

Label
to
Ideal Constellation

XTitle
to
Re

YTitle
to
Im

Style
to
connect
Wire the TkXYPlot to the Data splitter output.
15-7
Lab6: Agilent Ptolemy- QPSK Simulation
About the Symbol Splitter:
The SymbolSplitter will input two bits at a time and output the first bit to the Q
symbol output and the second bit to the I symbol output. The resulting I & Q output
symbols will have twice the duration (e.g. the output symbol duration will be 2uS for
a 1uS bit time). The SymbolTime parameter references the input, or the bit time.
This element essentially functions as a serial-to-parallel converter to take in two bits
to define four possible symbol states (2^2= 4 possible combinations) for QPSK.
15.2.3
Simulation Setup (Data Flow Controller):
Insert a “DF” (Data Flow) controller element from the “Controllers” library. Set the
parameter:

15-8
DefaultTimeStop
to
Tstop.
Lab 6: Agilent Ptolemy- QPSK
Simulation
15.2.4
Simulate and Observe the Results:
Look at the NRZ data stream from the data source and the ideal constellation from
the data splitter.
Perform a “View All” in the TkXYPlot to see the constellation.
The TkPlots have a high input impedance and the data splitter output has a 50 ohm
output impedance, so the symbol amplitudes are +/- 2 volts instead of +/- 1 volts.
Notice that the NRZ symbols are rectangular and that the constellation is ideal since
there is not yet any baseband filtering.
About Tk Plots:
TkPlots provide a quick and easy way to interactively display data. They are
particularly useful for validating the basic configuration of a system before using the
Data Display window to perform quantitative measurements.
Try stopping the simulation and changing the Style for the constellation to dot and
rerunning the analysis.
End the simulation (click “Quit”).
15-9
Lab6: Agilent Ptolemy- QPSK Simulation
15.3 Add Baseband Filters and QAM Modulator:
15.3.1
Add Baseband Filters:
Select a “LPF_RaisedCosineTimed” (Raised Cosine Lowpass filter) from the “Timed
Filters” library. Set the following parameters:

CornerFreq
to
Filt_Nyquist_freq

ExcessBw
to
0.35

Type
to
Model with pulse equalization

SquareRoot
to
Yes

Delay
to
Filt_delay_time
- units set to None
- units set to None.
Connect a filter to both the I and Q outputs of the SymbolSplitter as shown.
15-10
Lab 6: Agilent Ptolemy- QPSK
Simulation
About the filter:
The ideal rectangular symbols seen with the TkPlots will result in a wide spectrum
when the I & Q symbols are modulated onto a carrier. Nyquist root raised cosine
filters are typically used to filter the rectangular symbols and significantly reduce the
spectral bandwidth required for transmission, with minimal degradation to the
system performance.
The optimal Nyquist cutoff frequency is defined to be 1/(2*symbol time) for minimal
intersymbol interference. Since a brick-wall filter response is not realizable, an
excess bandwidth factor is specified such as 0.35 for this example (equivalent to
having 35% more bandwidth than a filter with a brickwall response). The pulse
equalization setting on the filter applies a x/ (sinx) equalization to provide the correct
filter response for rectangular input symbols instead of impulses. The square root
setting applies an exponent of 0.5 to the filter response, allowing it to be used in
both the modulator and demodulator (the product of the two filters results in one
raised cosine filter response).
It is typical to have one root raised cosine filter in the demodulator to provide some
noise filtering in the receiver. This filter is non-causal and a delay must be added to
make it causal. The filter model accuracy is proportional to the specified delay,
where modeling accuracy increases with delay.
15.3.2
Add QAM Modulator:
Select a “QAM_Mod” from the “Timed Modem” library and set the following
parameters:

Fcarrier
to
IF_freq
- units set to None

Power
to
+10 dBm
- or type in: dbmtow(10)
Connect the I & Q inputs to the raised cosine filter outputs as shown.
15-11
Lab6: Agilent Ptolemy- QPSK Simulation
15.3.3
Add Spectrum Analyzer And TkPlots:
1. Select a “SpectrumAnalyzer” from the “Sinks” library. Give it the label:
Mod_Spec. Leave all the parameters at their default values. Connect it to
the output of the QAM_Mod element:
2. Place a termination resistor from the “Timed Linear” library. Connect it to
the Spectrum Analyzer's input and ground the other end. Set R to 50
ohms.
3. From the Interactive Controls and Displays add a TkPower meter.
4. Connect a TkXYPlot to the output of the raised cosine filters to look at the
Nyquist bandlimited constellation. You can copy the TkXYplot that is
connected to the output of the data splitter. Make sure you set the
following parameters:

Label
to
Filtered Constellation

XTitle
to
Re

YTitle
to
Im

Style
to
connect
Perform the Simulation.
15-12
Lab 6: Agilent Ptolemy- QPSK
Simulation
15.3.4
View the Constellation Diagram:
You should see the Filtered Constellation. After analyzing it, answer the questions
about this Nyquist bandlimited constellation.
QUESTIONS:

Why does the constellation look so distorted?
__________________________________________________________
________
15-13
Lab6: Agilent Ptolemy- QPSK Simulation

Why wasn't this seen for the ideal constellation with rectangular
symbols?
__________________________________________________________
________

Using a TkPlot to view the Nyquist filtered symbol stream may provide
some clues. TkPlots display every sample they receive...is this
desirable for Nyquist bandlimited symbols?
_____________________________________________________________
_____

Why is the Modulator output power +13 dBm when it was set for +10
dBm?
End the simulation and save your work (the name should already be
“a_QPSK_Sys”).
15.4 Sampled Constellation:
15.4.1
Sampling Clock:
1. Select a “Clock” element from the “Timed Sources” library and set the
following parameters:

TStep
to
Tstep
- units set to None

Period
to
Sym_time
- units set to None

Delay
to
Filt_delay_time+(0.5*Sym_time) - units set to None.
15-14
Lab 6: Agilent Ptolemy- QPSK
Simulation
2. Select a “SplitterRF” element from the “Timed Linear” library and connect
it to the output of the Clock element.
15.4.2
Sample and Hold Circuits:
1. Place two “SampleAndHold” elements from the “Timed Linear” library and
connect the clock inputs to the outputs of the “SplitterRF”. NOTE: Use the
command Edit > Advanced Rotate/Mirror > Mirror About X to properly
orient the upper element.
Set the following parameters:

Rin
to
1 GOhm

DroopRate
to
0
15-15
Lab6: Agilent Ptolemy- QPSK Simulation
Setting Rin to 1GOhm will allow the SampleAndHold circuits to be connected to the
system without loading it down. Setting the DroopRate to 0 will hold the sampled
voltage at a constant level until the next sample.
2. Connect the “Filtered Constellation” TkXYPlot to the output of the
SampleAndHold circuits.
The input of the TkXYPlot is high impedance so insert 50  resistors to ground at
each input to match the output impedance of the SampleAndHold circuits.
Connect the input of the SampleAndHold circuits to the outputs of the raised cosine
filters. The final circuit should look like shown here:
15.4.3
View the Sampled Constellation:
Check Simulation/Simulation Setup… and make sure Open Data Display when
simulation completes is checked. Run the simulation and look at the “Filtered
Constellation” plot.
Why are the SampleAndHold circuits required here to look at the constellation of a
Nyquist bandlimited system? Later on we’ll learn a simpler way to do this.
15-16
Lab 6: Agilent Ptolemy- QPSK
Simulation
About Sampled Constellations:
SampleAndHold circuits are required to view a Nyquist bandlimited constellation
because it is not valid to display every sample for filtered symbols. The filtered
symbols have overshoot and ringing because the high frequency content has been
removed from the rectangular symbols. This distortion causes minimal degradation
to the system performance if the symbols are sampled in the middle of a symbol
time, where the filtered symbols have a value close to the ideal rectangular symbols
(+/- 1 V). This requires the symbol stream be selectively sampled starting in the
middle of a symbol with a sampling period of a symbol time. The delay parameter
for the Clock elements establishes the start time for the sampling using the
equation:
Delay=Filt_delay_time+(0.5*Sym_time)
This equation accounts for the delay through the raised cosine filter and begins
sampling in the middle of the first valid symbol out of the filter. The period
parameter for the Clock element establishes the sampling period and is set to a
symbol time so that the middle of each subsequent symbol is sampled.
Downsamplers could also be used to accomplish the same functionality as the
SampleAnd Hold circuits.
15.4.4
View the Modulated Spectrum :
It is important to let the simulation run long enough as to allow the Spectrum
Analyzer to collect sufficient data and send it to the Data Display window. A warning
message will appear if the TkPlots are dismissed prematurely and a dialog box will
prompt the user for a choice of continuing or quitting. Only partial data will be sent to
sinks if the simulation is ended prematurely. Click on Quit to end the simulation.
15-17
Lab6: Agilent Ptolemy- QPSK Simulation
A Data Display window should automatically open and place a plot of
Mod_Spectrum. It may be necessary to add a dBm modifier to get the plot as
shown below:
dBm(Mod_Spec)
0
-10
-20
-30
-40
-50
-60
69.85 69.90 69.95 70.00 70.05 70.10 70.15
freq, MHz
NOTE on timestep:
The time step for the timed elements is set to the variable equation
Tstep=1/(Sym_rate*Sam_per_sym), which yields a timestep of 4.115uS. The
simulation bandwidth is a function of time step, where Simulation BW= Fcarrier +/1/(2*timestep). Thus, the 4.115uS time step yields a simulation bandwidth of 121.5
kHz around the carrier frequency of 70 MHz.
15.5 QPSK Demodulator:
15.5.1
Add the QPSK demodulator:
1. Disconnect the “Mod_Spectrum” Spectrum Analyzer sink and termination
resistor from the output of the “QAM_Mod”. Place a “SplitterRF” from the
“Timed Linear” library, and connect it between the QAM_Mod and the
Spectrum Analyzer sink as shown:
15-18
Lab 6: Agilent Ptolemy- QPSK
Simulation
2. Place a “QPSK_Demod” element from the “Timed Modem” library and set
the following parameters:

SymbolTime
to
Sym_time
- units set to None

ExcessBW
to
0.35
- for the raised cosine filters
Connect the RF input of the “QPSK_Demod” element to the lower output of the
“SplitterRF”.
3. Place an “N_Tones” source from the “Timed Sources” library and set the
following parameters:

TStep
to
Tstep
- units set to None

Frequency1
to
IF_freq
- units set to None

Power1
to
+12 dBm
- or type in: dbmtow(12)
NOTE:
The N_Tones source allows phase noise at different frequency offsets to be
specified using the parameter PhaseNoiseData.
15-19
Lab6: Agilent Ptolemy- QPSK Simulation
4. Connect the output of the “N_Tones” source to the “Osc” input of the
“QPSK_Demod” as shown below.
15-20
Lab 6: Agilent Ptolemy- QPSK
Simulation
15.5.2
Connect Timed Sinks:
1. Select a “TimedSink” element from the “Sinks” library and change its
instance name to “Iout”. Connect it to the I output of the demodulator.
2. Copy the TimedSink and connect it to the Q output of the demodulator,
changing its name to “Qout”.
3. Set the input impedances of the sinks to 50 ohms by shunting 50 resistors
to ground at the inputs of the sinks as shown in the diagram.
4.
Perform the simulation. Start the simulation and wait until it is complete.
15-21
Lab6: Agilent Ptolemy- QPSK Simulation
15.5.3
View Demodulated I and Q:
Open a DDS window and place a Stacked Rectangular Plot to display the
demodulated I & Q symbol streams, Iout and Qout.
QUESTIONS:

What is causing the .493 mS delay? Hint: Push into the demodulator.

How should this delay be accounted for when setting up Error Vector Magnitude
and Bit Error Rate Measurements?
15.6 Output Constellation:
15.6.1
Sample the Output I and Q:
1. Insert a TkConstellation element at the output of the QPSK Demod
element. Set its parameters to:
15-22

Label=“Demodulated Constellation”

NumSamplesPerSymbol=10

Amplitude=2

SampleDelay=5

Sytle=dot
Lab 6: Agilent Ptolemy- QPSK
Simulation
a. Place a TkEye and Set the following parameters:

Label=”Tk Eye”

NumSamplesPerSymbol=10

NumSymbols=2

Amplitude=2
Connect it to the Q output of the demodulator.
5. Simulate and view the results. Examine the demodulated constellation
and eye diagram.
15.6.2
View the Sampled Output Constellation:
15-23
Lab6: Agilent Ptolemy- QPSK Simulation
QUESTION:

15.6.3
Why does the demodulated constellation look better than the
"Filtered Constellation" in the modulator? Hint: Is the raised cosine
filter response distributed between the modulator and demodulator?
View the Output Eye Diagram:
Select “View>all” to see the eye diagram.
About the plot: The x-axis represents samples, not time. There are 10 samples per
symbol in this exercise, so the xRange is set from 0 to 10 to overlay 10 samples (or
one symbol duration) on top of each other. The x-axis can be converted to time by
multiplying the timestep by the sample number (e.g. 0 to 10 samples = 0 to
10samples*(4.115uS/sample) = 0 to 41.15uS.
15-24
Lab 6: Agilent Ptolemy- QPSK
Simulation
15.7 Phase Noise Effects:
Add phase noise to the “N_Tones” source and observe its effect on the constellation
and eye diagrams. Edit the parameter:

PhaseNoiseData
to
100 -50 1000 -60 10000 -70 100000 –80
where the frequency offset is specified first (in Hz), with the phase noise following (in
dBc/Hz).
N_Tones
N1
TStep=Tstep
Frequency1=IF_freq
Power1=dbmtow(12)
Phase1=0.0
AdditionalTones=""
RandomPhase=No
PhaseNoiseData="100 -50 1000 -60 10000 -70 100000 -80"
ROut=50.0 Ohm
RTemp=-273.15
PN_Type=Random PN
The results (constellation and eye diagrams) for the demodulated output are shown
here.
15-25
Lab6: Agilent Ptolemy- QPSK Simulation
The points in the constellation are spread out more, so there is more “uncertainty” in
deciding the actual state.
Also, the eye diagram does not have a definite “point” at the sampling moment
(reason for which the constellation is spread out more).
All these will finally translate into a degradation of the bit error rate (BER) for the
same level of signal to noise ratio (SNR).
15.8 Lab Review:
In this lab, a basic QPSK system was examined using the HP Ptolemy simulator.
Fundamental relationships were established using variables and equations for
accuracy and flexibility.
TkPlots were used to examine constellation diagrams and eye diagrams.
SampleAndHold circuits were added to selectively sample in the center of the
symbol time to obtain a valid constellation for the Nyquist bandlimited system.
A QPSK demodulator was used to demodulate the signal and recover the I and Q
data. The demodulated I and Q symbol streams were examined and the delay
resulting from the root raised cosine filters in the modulator and demodulator were
observed. This delay will be important when considering qualitative measurements
such as Error Vector Magnitude and Bit Error Rate measurements.
The effects of local oscillator phase noise on the constellation and eye diagrams
were also analyzed.
15-26