9
9 Lab 3: ACPR Measurements using Circuit Envelope
Lab 3: ACPR Simulations using CE
9-2
Lab 3: ACPR Simulations using CE
9
LAB 3: ACPR MEASUREMENTS USING CIRCUIT ENVELOPE ................................................. 9-1
9.1 OBJECTIVES: .......................................................................................................................................... 9-5
9.2 SCHEMATIC CAPTURE AND SIMULATION SETUP: .................................................................................... 9-6
9.2.1 Setting Up Variables - Digital Modulation parameters: .............................................................. 9-6
9.2.2 Simulation Control – Envelope Simulation: ................................................................................. 9-7
9.2.3 Build the Modulator Front End for the ACPR Simulation: .......................................................... 9-8
9.3 ACPR MEASUREMENTS WITH RECEIVER CHANNEL FILTERING:........................................................... 9-11
9.3.1 Separate the Channels at the Output: ......................................................................................... 9-11
9.3.2 Measurement Setup in Schematic Window: ................................................................................ 9-13
9.3.2.1
9.3.2.2
Measure Channel Powers: .................................................................................................................. 9-13
Measure ACPR: ................................................................................................................................. 9-13
9.3.3 ACPR Schematic and Simulation: .............................................................................................. 9-14
9.3.4 Display the Results – Spectrum, Power Levels and ACPR: ........................................................ 9-14
9.4 ACPR MEASUREMENTS WITHOUT RECEIVER CHANNEL FILTERING: .................................................... 9-16
9.4.1 Schematic Capture: .................................................................................................................... 9-16
9.4.2 Measurement Setup in Schematic Window: ................................................................................ 9-17
9.4.3 The “channel_power_vr” Function: .......................................................................................... 9-17
9.4.4 The “acpr_vr” Function: ........................................................................................................... 9-17
9.4.5 Display results: ........................................................................................................................... 9-19
9.5 REVIEW OF LAB 3: ................................................................................................................................ 9-21
9-3
Lab 3: ACPR Simulations using CE
9-4
Lab 3: ACPR Simulations using CE
9.1
Objectives:

Use a PI4DQPSK modulator in Circuit Envelope to generate a modulated
spectrum.

Measure the integrated signal power in the main channel, as well as the upper
and lower adjacent channels.

Calculate the upper and lower Adjacent Channel Power Rejection, or ACPR.
NOTE about this lab:
Please refer to the Circuit Simulation manual for details on Circuit Envelope simulation.
This lab will exploit the availability of modulators used with Circuit Envelope in the A/RF
window. These allow the circuit and system designers a convenience in evaluating digital
communication system performance parameters such as ACPR in the A/RF window before
being integrating the circuit or subsystem into the top level design in the DSP window.
In this lab we will modify the “a_xmtr_basic_amps_filters” schematic from Lab1 as shown
below and save it as “b_ACPR” schematic.
“a_xmtr_basic_amps_filters”
schematic
“b_ACPR” schematic
9-5
Lab 3: ACPR Simulations using CE
9.2
Schematic Capture and Simulation Setup:
9.2.1
Setting Up Variables - Digital Modulation parameters:
1. Open the project d:\users\ads\CommSys_Lab3_prj.
2. Open up the design “a_xmtr_basic_amps_filters”, which is identical to the one
last created in Lab. 1.
3. “Save As…” the design with the new name “b_ACPR”.
4. Delete the 50 ohm termination connected to the output of the BPF_Chebyshev,
named BPF2.
5. Edit the existing “VAR” equation as shown below. You will have to add more
variables to reflect the digital modulation parameters:
VAR
VAR1
Bit_rate=48.6 kHz
Sym_rate=Bit_rate/2
Numpts=512*2
Sam_per_sym=10
Tstep=1/(Sym_rate*Sam_per_sym)
Tstop=Numpts*Tstep
Pmod=-10 _dBm
Plo=7 _dBm
Filt_delay_syms=8
IF_freq=70 MHz
LO_freq=766.5 MHz
RF_freq=LO_freq+IF_freq
9-6
Lab 3: ACPR Simulations using CE
9.2.2
Simulation Control – Envelope Simulation:
1. Delete the “Harmonic Balance” controller and place an “Envelope” controller from
the “Simulation-Envelope” library.
2. Edit the “Envelope” controller and set the following parameters:

Freq[1]
to
LO_freq
– units set to None

Freq[2]
to
IF_freq
– units set to None

Order[1]
to
3

Order[2]
to
1

Stop
to
Tstop
– units set to None

Step
to
Tstep
– units set to None.
Envelope
Env1
Freq[1]=LO_freq
Freq[2]=IF_freq
Order[1]=3
Order[2]=1
Stop=Tstop
Step=Tstep
NOTE on Order and MaxOrder:
These are the same as the Harmonic Balance simulation controller. Order[1] is the number
of harmonics considered for Freq[1] and Order[2] is the number of harmonics considered
for Freq[2]. MaxOrder is the number of mixing products from Freq[1] and Freq[2]. MaxOrder
is set to 4 in this example, so the frequency 3*LO_freq-1*IF_freq (a fourth order product)
would be considered. Changing MaxOrder to 3 would preclude this product from being
considered.
9-7
Lab 3: ACPR Simulations using CE
9.2.3
Build the Modulator Front End for the ACPR Simulation:
Follow the steps below to build the modulator front end. Begin by inserting the components.
1. Insert a “BPF_RaisedCos” filter from the “Filters-Bandpass” library. Insert it in
front of the mixer, MIX1. Edit the filter and set the following parameters:

Alpha
to
0.35

Fcenter
to
IF_freq
– units set to None

SymbolRate
to
Sym_rate
– units set to None

DelaySymbols
to
Filt_delay_syms

Exponent
to
0.5

DutyCycle
to
100

SincE
to
yes
The raised cosine filter provides Nyquist filtering to bandlimit the spectrum, while resulting in
minimum intersymbol interference (ISI). The raised cosine filter and each of its parameters
will be explained in greater detail in another lab exercise.
NOTE:
Please ensure that the Raised Cosine filter is a bandpass “BPF_RaisedCos” filter and not a
lowpass LPF_RaisedCos filter, since the signal is a modulated carrier instead of a
baseband signal.
2. Insert a “PI4DQPSK_ModTuned” element from the “System-Mod/Demod”
library. Insert it in front of the BPF_RaisedCos filter introduced previously. Set
the following parameters:

Fnom
to
IF_freq
– units set to None

SymbolRate
to
Sym_rate
– units set to None
Connect the output of the modulator to the input of the “BPF_RaisedCos” filter.
3. Edit the LO source (P_1Tone named PORT3) and set the following parameters:

Instance Name
to
PORT2

Num
to
2

P
to
Plo dBm
In “Edit Component Parameters” window:
9-8
value=”Plo” and units=“dBm”
OR
Lab 3: ACPR Simulations using CE
type in, directly in the schematic window:
dbmtow(Plo)
Previously, port 2 was defined by the Term2, which was deleted. Port numbers must be
sequential (1, 2, 3…) so it is not valid to have a Port 1 and Port 3.
4. Edit the IF source (P_1Tone named PORT1) and set the following parameters:

P
to
Pmod dBm
In “Edit Component Parameters” window:
value=”Pmod” and units=“dBm”
type in, directly in the schematic window:
dbmtow(Pmod)
OR
5. Place a “VtLFSR_DT” element from the “Sources-Time Domain” library and
connect it to the modulating input (labeled with a B – for input bits) of the
PI4DQPSK_ModTuned, named MOD1. Set the following parameters:

Vlow
to
-1V

Vhigh
to
+1V

Rate
to
Bit _rate

Delay
to
0 nsec

Taps
to
6538

Seed
to
27.
- units set to None
This source will provide the psuedo random NRZ bit sequence into the modulator.
6. Wire the components together as shown. Connect a ground to the “VtLFSR_DT”
source. After this step, the input of the system should look like the figure shown
below.
9-9
Lab 3: ACPR Simulations using CE
9-10
Lab 3: ACPR Simulations using CE
9.3
ACPR Measurements with Receiver Channel Filtering:
9.3.1
Separate the Channels at the Output:
1. Inspect the amplifiers to ensure they have the correct parameter values. Check
the following parameter settings:
a. Preamp:

S21

TOI
to
to
30 dB
30 _dBm
b. Poweramp:

S21

TOI
to
to
12 dB
30 _dBm.
The other S-parameters are ideal.
2. Place a “PwrSplit3” from the “System-Passive” library and connect it to the
output of the BPF_Chebyshev BPF2. Set the following parameters:

S21
to
1

S31
to
1

S41
to
1.
The default value of 0.577 defines an ideal lossless power splitter (that divides the input
power into equal output powers, without any losses in the device). This would affect the
absolute output power measurement. To ensure correct absolute power measurements,
these parameters need to be modified to the value of 1. This would imply some gain in the
splitter, but will ensure the output power will have the correct value.
3. Insert three “BPF_RaisedCos” filters from the “Filters-Bandpass” library and
connect them to the splitter outputs. Also, terminate each filter with a 50 resistor
(from the “Lumped Components” library) to ground as shown below.
4. Set the parameters for the three raised-cosine band-pass filters:
a. To space the filters 30 kHz apart, set center frequencies:

Fcenter
to
RF_freq+(30 kHz)
– units set to None – top filter

Fcenter
to
RF_freq
– units set to None – middle filter
9-11
Lab 3: ACPR Simulations using CE

Fcenter
filter
to
RF_freq-(30 kHz)
– units set to None – bottom
b. Set the following parameter values for all three filters:

SymbolRate
to
Sym_rate

DelaySymbols
to
Filt_delay_syms

SincE
to
no

DutyCycle
to
0.
- units set to None
c. The other parameters should remain with their default values. Please check
these values to correspond with the schematic shown below.
5. Name each BPF node. Select “Component> Name Node” (or click the icon) and
name the output of the upper BPF_RaisedCos “Vupper”. Name the center
output “Vmain” and the lower output “Vlower”.
The output of the schematic should look like the following one.
PwrSplit3
PWR1
S21=1
S31=1
S41=1
BPF_RaisedCos
BPF5
Alpha=0.35
Fcenter=RF_freq+(30 kHz)
SymbolRate=Sym_rate
DelaySymbols=Filt_delay_syms
Exponent=0.5
DutyCycle=0
SincE=no
R
R1
R=50 Ohm
BPF_RaisedCos
BPF4
Alpha=0.35
Fcenter=RF_freq
SymbolRate=Sym_rate
DelaySymbols=Filt_delay_syms
Exponent=0.5
DutyCycle=0
SincE=no
R
R2
R=50 Ohm
BPF_RaisedCos
BPF6
Alpha=0.35
Fcenter=RF_freq-(30 kHz)
SymbolRate=Sym_rate
DelaySymbols=Filt_delay_syms
Exponent=0.5
DutyCycle=0
SincE=no
9-12
R
R3
R=50 Ohm
Lab 3: ACPR Simulations using CE
9.3.2
Measurement Setup in Schematic Window:
9.3.2.1 Measure Channel Powers:
Place a “MeasEqn” from the Envelope Simulation library and change the instance name to
“Channel_Powers”. Define the equations to calculate the integrated power in the upper,
main and lower channel, respectively, as shown below:
MeasEqn
Channel_Powers
Ch_power_Upper_dBm=10*log(channel_power_vr(mix(Vupper,{1,1}),50,{13.6 kHz,46.4 kHz},"Kaiser"))+30
Ch_power_Main_dBm=10*log(channel_power_vr(mix(Vmain,{1,1}),50,{-16.4 kHz,16.4 kHz},"Kaiser"))+30
Ch_power_Lower_dBm=10*log(channel_power_vr(mix(Vlower,{1,1}),50,{-46.4 kHz,-13.6 kHz},"Kaiser"))+30
NOTE on the equations:
These equations use the function channel_power_vr to integrate the signal power over the
defined bandwidth. The first parameter is the named node voltage at the RF frequency
(1*LO_freq + 1*IF_freq, as LO_freq and IF_freq are the simulation frequencies specified in
the Envelope simulation control item). This corresponds to the main channel center
frequency 836.5 MHz. The frequencies between the curly brackets {} define the frequency
window relative to 836.5 MHz over which the power will be integrated. Thus the
Main_ch_power is centered over +/-16.4 kHz, while the upper and lower channels are offset
from 836.5 MHz by one channel spacing (that is 30 kHz). The channel bandwidths are
defined by +/-(1+Alpha)/(2*Symbol Time), which yields a bandwidth of +/-16.4 kHz. The
time domain data is windowed by a Kaiser window and the power in Watts is converted to
dBm (by the 10*log(…) and adding 30).
9.3.2.2 Measure ACPR:
Place another “MeasEqn” from the “Simulation-Envelope” library and change it's instance
name to “ACPR_Measurements”. Define equations to calculate the upper/lower adjacent
channel power rejection. They should look like the ones shown below:
MeasEqn
ACPR_Measurements
ACPR_upper_dB=Ch_power_Upper_dBm-Ch_power_Main_dBm
ACPR_lower_dB=Ch_power_Lower_dBm-Ch_power_Main_dBm
9-13
Lab 3: ACPR Simulations using CE
9.3.3
ACPR Schematic and Simulation:
1. “Save” the “b_ACPR” design, which should look as follows.
P_1Tone
PORT1
PI4DQPSK_ModTunedBPF_RaisedCos MixerIMT
MOD1
BPF3
MIX1
VtLFSR_DT
SRC1
BPF_Cheby shev
BPF1
Amplif ier
Preamp
Amplif ier
Poweramp
BPF_Cheby shev
BPF2
BPF_RaisedCos
BPF5
R
R1
BPF_RaisedCos
BPF4
R
R2
PwrSplit3
PWR1
P_1Tone
PORT2
BPF_RaisedCos
BPF6
2. Simulate:
Simulate the design. The dataset name should default to b_ACPR.
9.3.4
Display the Results – Spectrum, Power Levels and ACPR:
1. Open a new DDS window and set the default dataset name to “b_ACPR”.
2. Insert three separate equations to display the main, upper, and lower spectrums:
Eqn UpperChSpectrum=dBm(fs(mix(Vupper,{1,1})))
Eqn MainChSpectrum=dBm(fs(mix(Vmain,{1,1})))
Eqn LowerChSpectrum=dBm(fs(mix(Vlower,{1,1})))
9-14
R
R3
Lab 3: ACPR Simulations using CE
3. Insert a grid and display the three spectrums.
4. Insert a list and display Ch_power_Upper_dBm, Ch_power_Main_dBm,
Ch_power_Lower_dBm, ACPR_upper_dB, and ACPR_lower_dB.
5. Save the DDS window as “b_ACPR” and save the schematic design.
Eqn UpperChSpectrum=dBm(fs(mix(Vupper,{1,1})))
Eqn MainChSpectrum=dBm(fs(mix(Vmain,{1,1})))
Eqn LowerChSpectrum=dBm(fs(mix(Vlower,{1,1})))
10
0
LowerChSpectrum
MainChSpectrum
UpperChSpectrum
-10
-20
-30
-40
-50
-60
-70
-80
-90
140
120
100
80
60
40
20
0
-20
-40
-60
-80
-100
-120
-140
freq, KHz
Ch_power_Lower_dBm
-5.526
Ch_power_Main_dBm
19.499
ACPR_lower_dB
-25.026
Ch_power_Upper_dBm
-5.565
ACPR_upper_dB
-25.065
9-15
Lab 3: ACPR Simulations using CE
NOTE about the spectrum:
The “fs” function used displays the spectrum centered about the frequency passed. The
frequency of mix(V,{1,1}) corresponds to a the center frequency 836.5 MHz which is
represented as 0 kHz on the x-axis. The main channel power meets the specification of
minimum +18 dBm, but the ACPR does not meet the specification of -26 dBc. Optimization
will be used in the next lab to meet the -26dBc specification.
9.4
ACPR Measurements without Receiver Channel Filtering:
The measurement method presented before is applied for standards that require the use of
the receiver channel filter during the ACPR measurement. But there are other standards
that do not have this requirement. In such a case, the simulation setup is a little different
and there are some additional functions that can be used to make this measurement.
9.4.1
Schematic Capture:
1. “Save As…” the schematic “b_ACPR” with the new name
“c_ACPR_no_Rx_filter”.
2. Delete the power splitter, the three output raised cosine channel filters and their
terminations.
3. Add a 50 Ohm resistor termination at the output of the band-pass filter BPF2.
4. Name the output node “Vout”.
The schematic should look like the following one.
9-16
Lab 3: ACPR Simulations using CE
9.4.2
Measurement Setup in Schematic Window:
9.4.3
The “channel_power_vr” Function:
The equations used to measure the channel power are based on the “channel_power_vr”
function. The band definition capability of this function is used to “filter” the appropriate
channel. In the previous example, the bandwidth defined in these equations had to be at
least as large as the channel filter, if not larger. In this example, the measurement relies on
the “filtering” performed by this function to make a correct measurement.
MeasEqn
Channel_Powers
Ch_power_Upper_dBm=10*log(channel_power_vr(mix(Vout,{1,1}),50,{13.6 kHz,46.4 kHz},"Kaiser"))+30
Ch_power_Main_dBm=10*log(channel_power_vr(mix(Vout,{1,1}),50,{-16.4 kHz,16.4 kHz},"Kaiser"))+30
Ch_power_Lower_dBm=10*log(channel_power_vr(mix(Vout,{1,1}),50,{-46.4 kHz,-13.6 kHz},"Kaiser"))+30
As a result, the ACPR can still be calculated with the same formulas:
MeasEqn
ACPR_Measurements
ACPR_upper_dB=Ch_power_Upper_dBm-Ch_power_Main_dBm
ACPR_lower_dB=Ch_power_Lower_dBm-Ch_power_Main_dBm
The “acpr_vr” Function:
9.4.4
There is a built-in function that can directly calculate the ACPR in the lower/upper channel.
This function is called “acpr_vr” and has similar parameter definitions like the
“channel_power_vr”. Add a MeasEqn element in the schematic window, name it
ACPR_Direct_Measurements and add the following equation:
MeasEqn
ACPR_Direct_Measurements
ACPR_direct_dB=acpr_vr(mix(Vout,{1,1}),50,{-16.4 kHz,16.4 kHz},{-46.4 kHz,-13.6 kHz},{13.6 kHz,46.4 kHz},"Kaiser")
The parameters are defined as follows:

voltage component
mix(Vout,{1,1})

Load resistance
50
– unit defaults to Ohms
9-17
Lab 3: ACPR Simulations using CE

Main channel definition
{-16.4 kHz, 16.4 kHz} – defined as freq. offset

Lower adjacent ch. definition
{-46.4 kHz, -13.6 kHz} – defined as freq. offset

Upper adjacent ch. definition
{13.6 kHz, 46.4 kHz} – defined as freq. offset

Window used
Kaiser
The schematic window should look like the following:
9-18
Lab 3: ACPR Simulations using CE
9.4.5
Display results:
1. Run the simulation.
2. “Save As…” the “b_ACPR” data display window with the new name
“c_ACPR_no_Rx_filter”.
3. Change the default data set to “c_ACPR_no_Rx_filter”.
4. Delete the three equations calculating the upper, main and lower channel
spectrum.
5. Add the following equation to calculate the output spectrum:
Eqn Spectrum=dBm(fs(mix(Vout,{1,1})))
6. Edit the grid plot. Remove all previous equations and add the “Spectrum”
equation to be displayed.
7. Add a new table display and show the ACPR_direct measurements in it. Place
under the previous ACPR measurements for easy comparison.
The data display window should look as shown on the next page.
9-19
Lab 3: ACPR Simulations using CE
Eqn Spectrum=dBm(fs(mix(Vout,{1,1})))
10
0
Spectrum
-10
-20
-30
-40
-50
140
120
100
80
60
40
20
0
-20
-40
-60
-80
-100
-120
-140
freq, KHz
Ch_power_Lower_dBm
4.835
Ch_power_Main_dBm
19.958
ACPR_lower_dB
-15.123
Ch_power_Upper_dBm
-1.931
ACPR_upper_dB
-21.889
ACPR_direct_dB
ACPR_direct_dB(1)
ACPR_direct_dB(2)
-15.123
-21.889
Please note the ACPR measurements are identical, regardless of which method was used.
9-20
Lab 3: ACPR Simulations using CE
9.5
Review of Lab 3:
In this lab, the transmitter used in Lab 2 was modified to input a PI4DQPSK modulated
signal.
Equations were used to measure the integrated signal power in the main, upper, and lower
channels and to calculate the Adjacent Channel Power Rejection, or ACPR.
An alternate method to measure the adjacent channel power directly was presented.
The design met the minimum output power specification of +18dBm, but did not meet the
minimum ACPR specification of -26 dBc. The equations used to calculate ACPR will be
used in the next lab to optimize the performance of the system to meet the ACPR
specification.
9-21