This is three videos series on single diode mixer design and optimization using Microwave Office.
In this video, we are going to simulate the mixer circuit and optimize the output spectrum by changing
parameters, and at the end we are going to design the RF filter to remove all the extra frequencies
or spurs.
So let's get started.
In the first step we are going to download the demo project using the link given in the description.
Then run or design environment from start menu.
Let's open the project first.
So I have saved that on desktop.
Go to Diode mixer folder and from there you will find this single diode mixer amp file.
Let's click on open button.
So as you can see till here we have designed this mixer circuit.
And next we are going to run the harmonic balance simulation.
To run the simulation we are going to create a new graph.
And that we can easily do by clicking over this Add New Graph button.
I'm going to name it Eve out because this is the intermediate frequency output from the mixer.
Make sure you have selected the selected the rectangular plot and click over create button.
Next we are going to add the measurements.
So right click on the plot and click on Add New Measurements.
This time the measurements will be nonlinear.
So we'll just go to nonlinear tab.
And from here we have to select the power.
And then we have to select the power harmonics which is here.
Data source should be mixer and the harmonics.
We are going to calculate on port two which is the output port of the mixer.
Here we go.
And make sure you have selected the DBM here and click on okay.
Now just I want to verify the frequency.
So just go to the option of your mixer schematic.
And here as you can see the single point frequency should be 4.25 because this is the project frequency.
So this is fine.
Just cancel that and run the analysis.
Right.
So as you can see this is our if output spectrum.
And here we have lot of extra frequencies as you can see.
Because as of now we are not using any attenuators or filters in the design.
So I just want to show you what will happen if we change the power of the local oscillator.
So as of now the PLO is zero DBM will go back to your mixer circuit.
Here you can clearly see PLO is equal to zero which is assigned to the local oscillator power.
Right.
So if I go back to our IAF output plot first I'm just going to adjust the parameters.
So go to options go to access.
So I want to optimize the left axis.
And it is set to -100.
So instead of going for auto limit I'm just going to uncheck that.
And let's put the parameter let's say 100 and make this plus 20.
Click on apply.
And okay now I'm going to add the marker at the output frequency.
So if you remember from the previous discussion the target or the desired output frequency of mixer
should be 0.5GHz right.
So I'll just right click add marker.
And this is our Point five gigahertz.
Output frequency.
Right.
So apart from that, we are also getting a lot of strong signals, which we can easily remove by adding
bandpass filter so that we'll see later.
Next I'm just going to add a tuner on the low power.
So we can clearly see the effect of low power on the mixer output.
So let's click over this tune button add tuner.
Go back to I.F. out.
Click over this tuner.
Now I'm going to remove this L code.
It's not required.
Let's make it zero to let's say 5 or 10 DBM.
Right.
So as you can see, as I'm changing the low power, you can just look at this marker.
The output power of the mixer is increasing.
All right.
Which totally makes sense.
So as we are increasing the low power the output power is increasing.
Next I'm going to design a bandpass filter which will use on our mixer circuit and see the effect of
that on the spectrum.
Next we are going to design the filter for the if output to remove all the extra frequencies or spurs,
and keep the signal which is at 0.5GHz.
Now to design that IO filter, we are going to use an open source tool, which is RF SIM 99 that you
can easily find inside the same folder I have shared with you.
So if we go to the folder and there you will find this RF sim 99 standalone.
Here you will find this RF sim 99 x file.
Just double click on that.
Now to design a filter we will just simply go to tools.
Click on Design Filters.
And here make sure you have selected the Chebyshev filter which will be a bandpass.
So let's select the bandpass topology.
First branch will be in series.
Everything else will be same.
The center frequency we want should be 500MHz or 0.5GHz.
The bandwidth of the filter should be ten megahertz.
And we'll just leave everything else as it is and click on Calculate.
So from this tool we got all these inductor and capacitance values.
So I'm going to design the exact same filter on our design environment.
So go back to or just close this tuner.
Go to your project panel and I'm going to create another schematic.
Let's name it I of filter and click on create button.
Now to place all the components we'll simply go back to elements.
And these are all the lumped elements.
And so first we will start with placing inductor.
So let's go back to inductor.
We have to place the closed form of inductor.
Select it drag and drop here.
Then we'll need capacitor.
So I'll go to close form of capacitor drag and drop on schematic canvas.
And then we'll just copy paste.
So one capacitor here there will be another inductor.
So you can clearly see the schematic which we are designing here.
And then we'll have another inductor like that and a capacitor at the output.
And then I'm going to quickly assign all the values.
So this inductor will be 82.09 nano Henry.
This capacitor will be 1.234 picofarad.
Similarly this will be 36.523 Picofarad.
Then we have this parallel inductor 2.774 nano.
Henry will be precise here and this will be 82.091, which is same as here Nano Henry.
And then we'll have the capacitor 1.234 Picofarad.
Next I'm just going to make the connection here.
So let's connect all these components together.
I'm just going to shift it.
There we go.
And then this will be ground.
So I'm just going to add the ground port here.
And then I'm going to add two ports at the input and output of the filter.
Just rotate it by right click.
Here we go.
So here we have added the filter.
Now I'm just going to verify the filter by adding a graph and simulate it for S21 and S21.
And we'll see what is happening exactly at 0.5GHz.
All right.
To do that we will just simply add new graph.
Let's name it filter as parameters should be rectangular plot.
Click on create.
And next we are going to add the measurement which will be our linear measurement.
So we just go to linear port parameters as parameters.
Source will be I of filter.
So first we will plot for S1 one.
We will just add that and then we'll plot for S2 one and add this one as well and close.
And next I'm just going to run the simulation.
Now before running the simulation.
As you can see I'm getting just some garbage value.
I just want to add the frequencies.
So let's go back to the schematic which is our air filter.
Right click go to options.
Here I'm just going to add the sweep frequency which will be start from let's say 0 to 1GHz and step
size.
Let's keep it 0.01 apply.
So we'll have 101 points.
Click on okay and rerun the analysis.
Here we go.
Now here we have to adjust the.
Since you can see the amplitude is going up to -800 which we don't want here.
So I'll just go to options.
Go to access left.
Remove the auto limit.
Let's make it -100.
And the maximum value can go up to let's say 20.
Apply.
And okay.
So as you can clearly see the return loss at 0.5GHz is minimum.
All right.
So as you can see and all other frequencies you can see this band, all the other frequencies will be
rejected except this band of between.
I'm just going to add marker here so we can clearly see.
So this band will be approx 450MHz to just move it here.
553.8MHz.
Right.
And the center frequency will be 500MHz.
So if I add another marker on this return loss parameter, you can clearly see it's at 500MHz.
It is minimum which is approx 60dB.
So there will be approximately negligible return loss at this frequency.
That means all the signal will be passed from port one to port two.
So we are going to implement this filter to our mixer circuit.
So let's go back to our mixer circuit.
Let's make some space after diode biasing.
Just going to shift it a bit toward left side.
Make some space here.
Delete this.
And I'm going to add another sub circuit which will be our air filter.
Okay.
Connect it.
Like that.
Now go back to our RF output spectrum.
Here we go.
And now I'm going to run the analysis again and close it.
Here we go.
So as you can clearly see we have removed all the extra frequencies right.
So you can simply compare the plot with and without filter.
And you can clearly see the bandpass filter we have added is able to pass 500MHz frequency and rejecting
all other frequencies.
So this is the desired output we want amplitude.
We can simply increase by adding amplifiers.
Or you can increase the Alo and RF power of the mixer.
So you can simply play with these values and you can plot for different spectrums.
And you can also try different filters at the output of the mixer.
Similarly, you can plot for different Alo powers and RF power and run the different sweeps to plot
for SFR phase shift and other measurements.
So hope you can design and customize diode Mixer for your mixer design requirements.
Using our microwave.