This is three video series on single diode mixer design and its optimization using Microwave Office.
In this video, we are going to create the matching network between bias diode circuit and RF isolator
circuit.
And in the second section of this video we are going to design the RF isolator circuit using Wilkinson
Power combiner.
So let's get started.
In the first step of this video we are going to download the demo files using the link given in the
description and open our design environment from your start menu.
Then we'll go to File Open Project.
In my case I have saved the file on desktop.
Select the single diode mixer amp file and click on open button.
So if I recall it from previous tutorial till here we have designed the DC bias circuit for a diode.
In the next step we are going to create the matching circuit for the diodes input port.
To do that first we are going to create a new schematic.
So go back to your project panel circuit schematic.
Right click click on new schematic I'm going to name it Matching circuit and click over create button.
Now here we are going to place the Subcircuit block from the diode bias circuit module.
To do that just click on this Subcircuit block.
Now from here make sure you have selected the Diode biasing and click on okay button.
And next we are going to assign the ports to it.
So first before designing the matching circuit I just want to run the simulation for return loss.
So from that we can able to visualize the ports are not matched to 50 ohm.
We'll place another port here.
Now to plot the S1 one parameter we need to create a new graph.
But before that I want to set the frequency for this project so that it will simulate for the input
frequency which is 4.25GHz in our case.
Right.
So instead of assigning the frequency or a single point frequency to the entire project, I'm going
to assign it for a for the schematic.
So right click go to options.
Now here we'll just uncheck this use project defaults.
Click a single point.
Make it 4.25GHz and click on okay button.
So now the frequency single point is set.
Next we are going to create the new graph.
To do that you'll simply click over this Create or Add New Graph button.
Now I'm going to name it let's say s11 RF.
Make sure it should be selected at Smith Chart and click over create button.
Now I'm going to add the measurement.
To do that we'll just simply right click here.
Go to add new measurement.
From here you have to go to linear then port parameters.
Then make sure you've selected S here and source will be matching circuit.
So let's select that.
We are going to plot for S1 one.
So it's also selected.
And we are going to plot for a complex number so that it will have some real and imaginary number.
But from that from Smith chart we can easily see if it will be not at the center line.
Then there will be some impedance mismatch and we are going to tune that using a matching network.
Click on okay and run analysis.
So as you can see at 4.25GHz we have this location on Smith chart.
If I'm going to check the real and imaginary value on that, we can simply add marker and we can clearly
see the real part is 0.5338 and imaginary part is 0.418.
Ideally, we want this point at the center line.
Then our input impedance will be matched to port impedance which is 50 ohm.
Right.
And input impedance I'm talking about the input impedance of a diode bias circuit.
So here we are going to use single step matching technique to match the input port to diode circuit
at 4.25GHz.
Create a single stub matching network.
First we are going to add the microstrip line in series to the input port of the subcircuit.
Let's go back to our matching circuit schematic.
Go to elements from elements.
You have to search for microchip, and from here we'll just select our line and search for Emlin.
So let's select this Emlin drag and drop here we'll make some space, delete this connection and place
it in series with the port.
Now the microstrip line should have some sort of substrate definition, which should be exactly similar
as your PCB.
All right.
To do that I'm just going to define a global definition of the substrate.
So we'll just go to global definition double click here.
And here we are going to add the substrate element.
Again go back to your element panel.
Here you have to search for substrate I'm just going to close it.
Search for substrate.
Select that and place Msub.
All right I'm going to zoom out a bit.
Here we go.
Now we are going to add the parameters of Msub.
So I'll just double click here.
Now er in our case will be 2.2.
Height of the substrate will be 0.787.
Thickness of copper will be 35 micron.
So it will be 0.035.
And then tangent loss will be 0.0007.
This will be 2.2 as well.
And the name should be sub one.
We'll click on okay.
And here we go.
Now we'll just save the project, go back to project and go back to our schematic which is matching
circuit.
Now here we have to assign the substrate to this microstrip transmission line.
To do that we'll just double click on that click on MSB.
And from here make sure selected sub one.
So we have automatically assigned the properties from the substrate to this microstrip line.
Next we are going to define some variables.
So let's say the width of the substrate will be the w 50 ohm.
So that will calculate using the transmission line tool.
I'll show you later.
And let's make the length another variable which will be L line.
And this is what we are going to tune.
So we'll just set some initial value.
And later on we'll tune that.
And similarly we'll just place these variables here as an equation.
Now before doing that I'm just going to calculate the value of the width using the substrate parameters.
So we'll go to tool transmission line.
And here we are going to change the properties.
So make sure the dielectric constant should be 2.2.
It's the same value which you have set for the substrate conductor should be copper frequency 4.25GHz.
Uh, yeah.
Impedance.
We are looking for 50 ohm.
This doesn't matter.
Height should be this.
And the thickness we want is 0.035.
Make sure it should be selected as.
Mhm.
So then this will be 0.035.
All right.
And then we'll calculate it.
So the calculated width is 2.38mm.
Right.
And if we'll do that for the substrate we'll get the impedance of 50 ohm.
So let's remember that now we are going to create the equations.
So we'll just click on this equation button.
Let's place it here.
So w 50 will be 2.38.
Copy and paste.
Next equation will be for the length of the line which is L line.
For now I'm just making it 16.85mm.
Why so specific that we'll see later, but this is a tunable parameter which will tune to make sure
that the impedance or the input impedance of the diode bias circuit should be 50 ohm.
All right.
So for now I'll just leave it 16.85.
Next we are going to add the stub next to this microstrip transmission line.
To do that we'll go to elements.
And instead of placing the sub we'll just go to again microstrip line.
Select line from here.
And if you scroll down you'll find MLF.
So if you check the description of that this is an open microstrip line with an end effect.
So this is a stub which we are going to place it here.
I'm just going to connect it.
And as you can see the substrate for this stub transmission line is a sub one.
This is the same substrate which we have created a couple of minutes ago.
And now the width of the transmission line will be same as the 50 ohm width which we have calculated
using the transmission line tool.
Line of the of the substrate will just make it another variable.
Let's make it elstub.
And we'll copy paste here.
So this is another tunable parameter which we are going to tune.
For now I'm just placing the value here which is the exact value.
We want to make it time efficient.
Let's make it 19.5.
Now we are going to run the simulation again and see if it is match to 50Ω or not.
So we'll run the analysis.
And as you can see the imaginary part is almost zero.
And it is on the line right.
So the impedance is 50 ohm.
Now how this you have to do from scratch because I got these values L and L line from the tuning.
So you can just create the tunable parameter for yourself.
And just click on this tune tool.
Select these two variables.
Go back to your plot and click on this tuner.
Here you can see the lowest value is set to 9.75.
Maximum value is set to let's say 39 right.
So you So you can just.
Yeah check it.
And as you can see it's moving right.
Similarly you can move it in this direction.
All right.
So as you can see on this particular value only you can able to place it correctly at the center line.
All right.
So you can just go through with this and play along with these tunable parameters and see if you can
able to match it to 50 ohm or not.
All right.
So this is what I did.
And from that I got these values 19.5 and 16.85.
So if I just enter here it is correctly matched right.
So our matching circuit is ready.
Next we are just going to create an independent matching circuit which we can use it as a sub circuit
when we'll be creating the mixer connection.
All right.
At later stage we will be creating the mixer circuit.
We'll be using all these schematic which we have created right.
So to do that I'm just going to create a new schematic and let's name it RF Match Network and click
on create button.
We'll just copy our matching circuit here.
So I'll just copy it.
Ctrl c, go back to RF match network Ctrl V and we'll just delete this diode circuit block.
There we go.
So this is our independent matching circuit which we can use as a sub circuit.
Right.
Because it's a two port network.
In the next step we are going to design a RF isolation circuit.
So there are different type of isolator circuit or network or structure are possible.
So we can use to isolate two input ports.
But in this case we are simply going to use power combiner structure as an RF isolator.
So first we will just create a new schematic.
Go to your project panel.
Right click on schematic.
Click on new schematic.
We are going to name it RF isolator and click on create button.
Then we'll go back to elements from elements.
We have to go to microstrip line here.
Instead of selecting the line we'll select Power Divider.
And here we have this Wilkinson Power splitter and divider.
We'll just select this will key one.
We'll just drag and drop.
Rotate it like this.
Now to edit the parameters.
The width of this Wilkinson divider will be 50 ohm.
So we'll make it w 50.
This is the branch length so we'll just make it WB later assign the parameters to it.
Length will be we call it quad length.
So we'll just name another parameter here L quad R will be 100 and substrate will be sub one.
Next we are going to define all the equations or the values to these variables.
So w 50 as we have calculated it was 2.38.
Copy and paste.
WB will be 1.34.
You can simply calculate that based on the impedance of 15 root two.
All right.
So you can just go back to your tool.
Enter the the value of impedance 50 root two.
And based on that the width will come out 1.34.
And L quad will be lambda by four.
So we'll just copy paste here.
just name it El Quad and its value will be 14.8.
Right.
So this is the parameter which we need to tune.
So I'm just putting a rough value here.
And this will be WB.
So we have entered all these parameters.
Next we are going to add ports.
So port will be.
So this will be first port.
And I will just copy paste this one.
This is second port.
Okay.
Don't go with these these uh numbers here.
All right.
I'm assigning just ports based on.
The RF flow and RF flow parameters.
So this will be our RF input, the port one.
So I'll double click here and make this ID to RF.
Right.
Double click here.
This will be our low input and the output will be RF plus a low.
So let's make it RF plus low and click on okay button.
Uh before going further I just want to arrange these ports Sports parameters a bit, so there will be
no overlap and clearly see things.
All right.
Next we are going to plot the response and tune the alcohol.
So to do that we are going to create the new plot.
But before that I'm just going to add the frequency sweep on that.
So we'll go to project right click here.
Go back to options here.
Make sure you have uncheck this use Project defaults.
This will be our sweep and start frequency will be three gigahertz.
Stop will be let's say 5.5GHz.
And step size will be 0.05GHz will apply.
And we can see there are 51 points.
We are going to simulate and click on okay button.
Next we are going to add the graph.
Just click on this Add new graph button.
Let's name it as parameter of RF flow.
And this time this will be our rectangular plot.
Click on create button.
Right click add new measurement.
Our measurement will be same port as parameters.
From here we have to select the RF isolator this time.
And so we are going to simulate for S21.
So two port.
And this will be port two.
Make sure the amplitude will be in db and click on add if you want.
You can also simulate for S11 but that's not required and we'll just close it.
Next we are going to run the analysis.
And as you can see there is this isolation of more than 50dB between the port one and two.
But it is coming at certain frequency.
So I'm just going to add a marker.
Here it is at 3.552MHz.
But in our case we want this at 3.75GHz.
If we go back to our design notes.
So the low frequency is 3.75GHz.
And we want the isolation between these two ports at that frequency.
So then that means we have to tune the parameter.
So we'll go to our RF isolator circuit.
And we have to tune the quad.
So we'll just select this go back to plot.
Open the tuner.
and I'm just going to remove other tuners.
So we'll just uncheck from here and keep the code.
So from the tuning parameters the chord length which is coming close to 13.8.
So we'll just we'll just go below.
You can see it's moving in that direction.
That is what we want.
And at 13.8 the frequency will be pretty much if I just add another marker to this point.
So as you can see this is approx 3.8GHz right.
You can just tune it further.
But again that's a time taking.
And you can just increase the value up to some certain decimal and it will be fully matched.
Right.
So we want this notch ideally at 3.75GHz which is here right.
And the isolation between port one and port two at this frequency is approx 50dB.
Right.
So we'll just close it.
So that's it for the isolation circuit.
Next we are going to add everything together to create the mixer circuit for simulation.
Next we are going to create the mixer circuit.
To do that, we'll again go back to our project panel.
Right click and create new Schematic.
I'm just going to name it mixer and click on create button.
Now here, if you recall that our mixer circuit consists of three blocks flow isolator matching network
and diode biasing circuit.
And all the three blocks we have created here.
Right.
So we'll just add our first sub circuit which will be our flow isolator.
So we'll just place it here I'm just going to rotate it.
So here we have this RF flow and RF flow output.
This will go to our matching network.
So we'll place another sub circuit that will be our matching circuit.
Alright.
So make sure you are using the independent matching circuit that we have created without diode.
Right.
We just place it here.
And next we are going to add the diode bias sub circuit.
So we'll click on Diode biasing and click on okay.
Here we go.
So this is our mixer circuit I'm just going to connect all these ports.
In the next step we are going to add input and output ports to this mixer circuit.
To do that, we will again go back to elements.
From elements, we'll go to ports and select harmonic balance ports from the harmonic balance port.
I'm going to add the one tone HB source right.
So we'll just place it on.
So first port I'm going to assign to RF.
So just rotate it right.
Then I'm going to add the output port which will be port number two.
And then I'm going to add just copy this place another harmonic balance port at the L0.
That will be our port three.
Next we are going to configure the parameters of these ports.
But before that I'm just going to set the frequency of the project.
So let's go back to the schematic right click option.
We are going to run the simulation for a single point frequency which is an input frequency of RF signal
which is 4.25GHz.
And click on okay.
So that will be the frequency of this RF port.
Now to set the frequency of the yellow port, we have to double click on this and go to port parameter.
From here let's say second tone and click on okay.
Now as you can see we are getting the frequency option.
Here it is in megahertz.
And the low frequency is 3.75GHz.
So it will be 3.750MHz.
And the low power I'm just going to make it a variable.
So let's make it yellow which we are going to tune when we'll see the spectrum or when we plot for spectrum,
we'll just tune the power yellow and see what is happening in the amplitude of the signal.
And similarly we are going to define this variable here.
For now I'm just going to make it zero DBM.
So p underscore low will be zero.
All right.
So that's pretty much for this video.
In the next video we are going to simulate this and see the spectrum of this mixer output.
And later we'll optimize that spectrum by changing the PLO power and adding some bandpass filter to
remove all the extra frequencies, which is not required.
Right.
So that we'll discuss in next video.