Lab 3: ACPR Simulations using CE

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9  Lab 3: ACPR Measurements using Circuit Envelope

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 Measure Channel Powers: 9-13

9.3.2.2 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

9.1  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.

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.
  1. Open up the design “a_xmtr_basic_amps_filters”, which is identical to the one last created in Lab. 1.
  1. “Save As…” the design with the new name “b_ACPR”.
  1. Delete the 50 ohm termination connected to the output of the BPF_Chebyshev, named BPF2.
  1. Edit the existing “VAR” equation as shown below. You will have to add more variables to reflect the digital modulation parameters:


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.

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.

9.2.3  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: value=”Plo” and units=“dBm” OR

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” OR

type in, directly in the schematic window: dbmtow(Pmod)

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 - units set to None

·  Delay to 0 nsec

·  Taps to 6538

·  Seed to 27.

This source will provide the psuedo random NRZ bit sequence into the modulator.

  1. 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.


9.3  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 to 30 dB

·  TOI to 30 _dBm

b.  Poweramp:

·  S21 to 12 dB

·  TOI to 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

·  Fcenter to RF_freq-(30 kHz) – units set to None – bottom filter

b.  Set the following parameter values for all three filters:

·  SymbolRate to Sym_rate - units set to None

·  DelaySymbols to Filt_delay_syms

·  SincE to no

·  DutyCycle to 0.

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.

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:

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:

9.3.3  ACPR Schematic and Simulation:

1.  “Save” the “b_ACPR” design, which should look as follows.


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:

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.

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.

9.4.2  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.


As a result, the ACPR can still be calculated with the same formulas:

9.4.4  The “acpr_vr” Function:


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:

The parameters are defined as follows:

·  voltage component mix(Vout,{1,1})

·  Load resistance 50 – unit defaults to Ohms

·  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:

9.4.5  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:

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.


Please note the ACPR measurements are identical, regardless of which method was used.

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.

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