Microwave Office Training
Linear Simulation – Low Noise Amplifier
Summary
• Linear Simulation
– S parameters at Ports - and derived parameters ... Gain, NF
• Low Noise Amplifier as an Example to show:
–
–
–
–
–
–
Creating schematics
Data libraries
Editing schematic symbols (adding explicit ground nodes)
Creating graphs and adding measurements
Measurements: S parameters, Noise, Gain
Tuning Parameters
• Step Response (Optional)
2
Linear Simulation
1
Linear Simulation
• Linear Simulation solves for S parameters in the
frequency domain at ports.
– Internally, it solves the Y matrix.
• It is an ideal choice for:
– Linear Amplifier Analysis
– Linear Noise Analysis
– Interconnect / Passive elements
• It Cannot:
– Show currents and voltages at nodes. (Use linear HB)
– Show time domain transients and start up conditions.
• It Can:
– Show step/pulse response for passive circuits.
– Work with biased devices - if they have a model, not just S
parameter data. (For example - Gummel Poon)
3
Linear Simulation
Example - A Low Noise Amplifier
• Design Goals of a 5 GHz amplifier are:
– Gain > 10 dB.
– Noise Figure < 1.15 (1.2 in dB)
• We will learn:
– Insert S parameter file for a transistor.
– Add graphs and measurements to look at:
• S parameters
• Noise figures - NF and NFmin
• Stability figures: K, B1
– Smith Chart Graphs and Circle Measurements
• Stability Circles, Gain Circles, Noise Figure Circles
4
Linear Simulation
2
Example - A Low Noise Amplifier 2
Step 1: Create a Schematic “ Device” in a new project
Step 2: Import the S parameter data set for
the HEMT transistor - FHX35LG.s2p
Tip: Make sure you’re looking for Touchstone files or you might not see the file in the directory.
5
Linear Simulation
Example - A Low Noise Amplifier 3
Step 3: Look at the S parameter for the device.
double click on the data file - to see the S parameters and the the
Noise data.
The S parameters are
from 100 MHz to 20
GHz
The noise data are
from 2 to 18 GHz.
6
Linear Simulation
3
Example - A Low Noise Amplifier 4
Step 4: Insert the data set as a sub-circuit into the schematic “Device”.
Note: You can insert a sub-circuit
by:
•Draw - Insert Sub-circuit
•Hotkey is: Ctrl K
•Subcircuits in Elements Library
Tip: Make sure you select Explicit Ground
Node. This is used for transistor S
parameter data - so you expose the 3rd port
(Usu. the source in common source.) For
interconnect S parameter data - use
Normal.
7
Linear Simulation
Example - A Low Noise Amplifier 5
Step 5: Change the Symbol to a FET.
It looks more like a transistor and is less confusing which port is which.
Right Click (RC)
Select the FET symbol
Select the Sub-Circuit’s
Properties
Tip: Make sure the port numbers of the symbol - match what you expect.
8
Linear Simulation
4
Example - A Low Noise Amplifier 6
Step 6: Complete the amplifier by attaching ports.
Tip: Useful Hot Keys:
•Add a port: Ctrl P
•Add a ground: Ctrl G
You can rotate, flip, and mirror the port before placing it.
•To rotate - right click (RC)
•To Flip About the Horizontal Axis - Shift RC.
•To Flip About the Vertical Axis - Alt Shift RC.
9
Linear Simulation
Example - A Low Noise Amplifier 7
Step 7: Set the simulation frequencies from 0.1 to 20 GHz - in steps of 0.1
GHz.
Select Project Options
Note: This sets the
frequencies for all project
simulations - unless you
override them. You can do
so at the individual circuit
level, or EM simulation
level.
10
A very common error is to forget the Apply Button.
Linear Simulation
5
Example - A Low Noise Amplifier 8
Step 8: Create the Graphs and Measurements
Create a Graph called Input Port
Create a Measurement - S11
Measurement
Added to the Graph
11
Linear Simulation
Example - A Low Noise Amplifier 9
Add a measurement S22 to the same Graph.
Data source name
Various Control
Fields
This is a linear measurement.
Measurements in MWO are carried
out in the Graphs.
12
We are sweeping the x - axis
with Frequency
Linear Simulation
6
Example - A Low Noise Amplifier 10
Step 9: Run the simulator.
Three ways to do this:
•Lightning Bolt in the Toolbar
•Simulate > Analyze
•F8
Standard Toolbar
You will get a warning
message. It’s saying you
don’t have any noise data
at 20 GHz - so it’s
extrapolating.
13
Linear Simulation
Example - A Low Noise Amplifier 11
Tip: You can RC on the graph - and bring up properties.
There you can change the Graph’s appearance: traces, axes, fonts, etc.
14
Linear Simulation
7
Example - A Low Noise Amplifier 12
Step 10: Change the graph to a Smith Chart.
RC on the Graph in the Browser
15
Linear Simulation
Example - A Low Noise Amplifier 13
Step 11: Add a graph - “Two Port Gain”.
-Add measurement: S21 (Measurement type Linear > Port Parameters)
-Add measurement: Maximum Stable Gain (MSG) (Measurement Type Linear > Gain)
Maximum Stable Gain is in dB.
16
Linear Simulation
8
Example - A Low Noise Amplifier 14
Step 12: Make the Graph Look Nicer.
RC on the Graph > Properties
Change the x axis limits to 0 to 20
GHz.
We will use the left axis for S21 and the right axis for MSG.
17
Linear Simulation
Example - A Low Noise Amplifier 15
Setting the MSG to the right axis.
18
Linear Simulation
9
Example - A Low Noise Amplifier 16
Tip - Interpolation between data
points
The wiggle in the S21 curve
is because we are using
linear interpolation.
This can be changed in Project Options
> Interpolation.
Change it to Spline or Rational
It’s Smoother!
19
Linear Simulation
Example - A Low Noise Amplifier 17
Step 13: Make a Graph “ Two Port Noise Parameters”
-Add measurements for Noise Figure (NF) and Noise Figure
Minimum (NFmin)
(The measurement type is Noise.)
- Make the graph nice by resetting the lower frequency to 0.
20
Linear Simulation
10
Example - A Low Noise Amplifier 18
Step 14: Create a graph “Stability Data”
- Add measurements for K and B1. (Measurement type is linear.)
Note: Don’t use dB.
21
Linear Simulation
Example - A Low Noise Amplifier 19
Step 15: Add a marker to the trace for K.
RC on the graph and select
Add Marker ( Ctrl M).
Select the K trace and add the marker.
RC on the marker and select search - for
1.
Remember K < 1 and/or B1 < 0 is unstable.
22
Linear Simulation
11
Example - A Low Noise Amplifier 20
Step 16: Stabilize the amplifier by adding resistance.
Copy the schematic and rename the copy “Stable Device”.
Tip: The easiest way to do this is drag Device into Circuit Schematics
- and Copy of Device will be created.
This line takes out the resistor R3 at 5
GHz.
23
Stable Device
Linear Simulation
Stable Device
24
Linear Simulation
12
Example - A Low Noise Amplifier 21
Step 17: Update the graphs with the new Stable Device S
parameters, Gain, Nose, and Stability measurements.
Tip: A quick way to do this - is any measurement can be dragged on top of the
graph in the browser - creating a copy. Then open it, and change the Data
Source Name to Stable Device.
K and B1
K > 1 and B1 > 0
Note: Autoscaling is
turned off for the left
axis, as K gets very
large near DC.
25
Linear Simulation
Example - A Low Noise Amplifier 22
S22 has gotten smaller
S11 and S22
26
Linear Simulation
13
Example - A Low Noise Amplifier 23
The Gain is lower - as expected.
27
Linear Simulation
Example - A Low Noise Amplifier 24
Step 18: Stability Circles
Create a graph - Smith Chart type, named “Stability Circles”
-Add the measurement: SCIR1 (Measurement Type - Circle).
-Use the Source Name “ Device”. (It’s more interesting - not stable.)
Unstable regions.
A circle has been added for
each frequency.
28
Linear Simulation
14
Example - A Low Noise Amplifier 25
Optional
What if I want only a few circles?
Method 1: Have fewer frequencies.
- You can do this in Options at the Project level or the circuit level.
Method 2: In the “Device” Schematic - insert a SWPFREQ block.
- For values: stepped(1.0e9,10.0e9,1e9) - 1 to 10 GHz in 1 GHz increments
Now we can use these frequencies - instead of all the project frequencies.
In the measurement dialog box
- Change the Sweep Freq to
SWPFRQ.FWP1
Located under Simulation Control.
Click the arrow to select FSWP1.
29
Linear Simulation
Example - A Low Noise Amplifier 26
Optional
Fewer Circles - Much Easier to Read.
Tip: The dotted side of the circles is the unstable region.
30
Linear Simulation
15
Example - A Low Noise Amplifier 27
Optional
Method 3: Create a dummy sweep variable.
Circle measurements require a swept variable - which by default is frequency.
Idea: Create a dummy sweep variable - and sweep on that - in effect disabling
the sweep.
Insert the SWPVAR into the
schematic “device” and create a
variable “foo”.
Create an equation for “foo”.
To insert an equation - Ctrl E or
Draw > Insert Equation
31
Located under Simulation Control.
Linear Simulation
Example - A Low Noise Amplifier 28
Optional
Change the measurement settings
The frequency is
selected by the
tuner.
The swept variable
foo is set to the x
axis. (So - we are
using a dummy for
the x axis sweep.)
32
Linear Simulation
16
Example - A Low Noise Amplifier 29
Optional
Equations Toolbar
You can now use the tuner to shift through
frequencies and stability circles.
The Tuner
33
Linear Simulation
Example - A Low Noise Amplifier 30
Step 19: Add a graph called “Noise and Gain
Circles”. We will work with the Stable Device data.
Set the Project Frequency to
1 point - 5 GHz. This is where we want to
tune.
Add NFCIR (Linear > Circle) with
2 circles and 0.5 dB steps.
34
Linear Simulation
17
Example - A Low Noise Amplifier 31
Add GAC_MAX - Available Gain (Linear > Circle)
with a Gain step 1 and 2 circles.
35
Linear Simulation
Example - A Low Noise Amplifier 32
Let’s make a matching input circuit.
Step 20: Create a schematic “Input Matching Circuit”
36
Linear Simulation
18
Example - A Low Noise Amplifier 33
Step 21: Create a schematic - “Amp”
- Include Stable Device and Input Matching Circuit as
subcircuits.
Amp Schematic
37
Linear Simulation
Example - A Low Noise Amplifier 34
Optional
Window in Window Technology
Allows you to “see” other circuits, and graphs in your schematic.
Open up “Amp”.
LC on Stable Device in the browser and drag it into the schematic.
Drag - to create the area. You see the window Stable
Device.
Looks Pretty!
38
Linear Simulation
19
Example - A Low Noise Amplifier 35
Step 22: Tune the L’s to get a good compromise between Gain and NF.
In “Input Matching” select L1 and L2 for tuning.
-Method 1: Use the tuning tool. Select each of them.
-Method 2: Select Properties for each of them.
Equations Toolbar
Set to tune 0 to 10 nH.
When selected for tuning - the parameters
turn blue.
39
Linear Simulation
Example - A Low Noise Amplifier 36
Step 23: Tune the L’s to get a reasonable compromise between NF and Gain.
- Add S22 of Input matching to the graph Noise and gain Circles.
Equations Toolbar
Tuner
40
S22 of your input
matching will equal S11
of your stable device.
Tune until there is a
reasonable compromise
between Gain and Noise
Figure.
Linear Simulation
20
Example - A Low Noise Amplifier 37
Optional
Step 24: Add an “Output Matching Circuit”.
Tune the values to get maximum gain.
You can add - S21 of the Amp to
the Two Port Gain for example,
and tune over the C and L’s.
Note: We have not used simultaneous, conjugate match
designs - so the output circuit will influence the design
of the input circuit.
41
Linear Simulation
Example - A Low Noise Amplifier 38
Optional
Step 25: Set the simulation frequencies back to 1 to 20 GHz
and look at the Gain and NF.
42
Linear Simulation
21
Example - Step Response
Optional
Step Response to the input of the device
Step 1: Make a new schematic - “Test Loads”.
These are some test loads for us to try to make sure we understand step
response.
43
Linear Simulation
Example - Step Response - 2
Step 2: Make a graph - “Step Response”
Add step response measurements: (Linear > TDR) TDR_LPS.
- Try S11, S22, and S33 of Test Loads.
Select Real
44
Linear Simulation
22
Example - Step Response - 3
A 100 Ohm Load
A short circuit
An RC circuit.
The results are the same as a Time Domain Reflectometer.
Tip: Linear analysis can do this because it’s taking the
FFT of the S parameters.
45
Linear Simulation
Example - Step Response - 4
Step 3: Add the Step Response of the “Device”. Convince
yourself it looks like an RC curve.
46
Linear Simulation
23