This is three video series on single diode mixers.
Design and simulation using Microwave Office.
In today's video, we are going to talk about what is RF mixers and where do we use them in RF and microwave
engineering.
And in the next section of this video we will talk about what is single diode mixer, its application
and how to create a single diode mixer circuit using microwave office.
So let's get started.
Now let's start with what are RF mixers?
RF mixers are three ports active or passive devices.
They are designed to yield both a sum and a difference of frequencies at a single output ports.
When we insert two distinct frequencies on input ports.
Now as you can see in the block diagram, as we insert frequency one on input one port frequency two
on input two ports, then the output will be sum and difference of frequency one and frequency two.
Now let's understand about these ports and naming we use in RF mixers.
So if we see from the picture below this is the symbol of RF mixers.
So here is one declaration.
Before moving forward.
I want to make it clear that in this entire course of mixer discussion, I'll be talking with respect
to radio frequency receivers, not the transmitters.
Now let's go back to our port discussion.
So as you can see, the first port which is RF input signal is coming from antenna.
I just want to highlight one more point between mixers.
And antenna will have other circuit blocks as well.
So it's not just you can follow this block directly.
So there will be other circuit blocks.
For example there will be some amplifiers, filters, etc. for now we will focus on mixers only.
Now in the second port there is another input.
We denote it with local oscillator input.
All right.
So that local oscillator circuit we just introduce some frequency based on our output requirement.
So we design our local oscillator circuit based on the desired frequency of this mixer.
We'll talk about it in detail when we'll be working on or when will be designing the circuit of mixer.
Now the third port is an output port.
As you can see we denote it with intermediate frequency output.
Now if Now, if we'll talk about from the definitions or from the poor discussion of, uh, RF mixers,
the output of the mixer will be.
So we can just finalize the formula based on our previous discussion will be the sum and subtraction
of input frequencies.
So if I just write it down f I f which is the frequency of I of port will be is equal to flow, which
is the frequency of local oscillator port plus minus f rf.
Next we will understand the concept of up and down conversion.
So these are just the terms that we will use in this entire series of videos.
And yeah so in general application of mixer we don't keep both frequencies.
So as I've told you using mixers we can produce two frequencies.
The sum and subtraction of input port frequencies.
Right.
So in general we don't keep those both of those frequencies.
We reject one of those.
Either we reject flow plus plus FRF or we just reject flow minus FRF.
And that is again based on the requirement of our mixer design.
So if we want to produce an output frequency that is lower than the input RF frequencies, then it is
called the down conversion.
So if we have input frequencies coming from antenna is higher than the desired frequency at the output
of mixer, then it will be.
Then we have to down convert the frequency right.
As its name suggests.
And similarly, if we want to produce an output signal whose frequency is higher than the input frequency
at the RF port, then we call it up conversion of the signal.
Let's try to understand this concept better with the help of one example.
So let's say we want to up convert from 2GHz to 10GHz signal.
So we'll simply see if we'll add eight gigahertz into this input frequency we can achieve the target
frequency of ten gigahertz.
Right.
So let's apply that into the formula that we just discussed.
So here RF input will be is equal to two gigahertz.
Low input will be eight gigahertz.
And the formula is f I f is equal to f l plus minus FRF.
So then we'll produce two frequencies ten gigahertz and six gigahertz.
Regards.
All right.
We want to keep the ten gigahertz.
Right.
Then what we'll do, we'll just add a high pass filter and reject the six gigahertz of frequency.
So this is an example of upconversion because we have converted frequency from 2GHz to 10GHz.
Now there is another example.
Let's say we want to produce 0.5GHz from two gigahertz input.
So let me know in the comment section what will be the low frequency.
Now before I start discussing the single diode mixer, let's talk about what is the difference between
mixers and frequency multipliers divider or RF synthesizer.
Because we can also achieve the desired signal with the help of frequency multiplier, divider and synthesizers.
Right?
So in contrast to frequency multiplier and dividers which also change the signal frequencies.
So the advantage with mixers is mixers theoretically preserve the amplitude and phase without affecting
the modulation properties of signal and its port, which is not the case in case of multiplier and dividers.
So that's why whenever we are using multiplier and dividers, we need some sort of feedback or control
loop.
Let's say PLL, by which we can able to sync the phase of the signals.
And we also use amplifiers to maintain the amplitude of the signal.
Right.
So that makes this circuit a bit difficult to place when you are dealing with RF section where we need
really neat and clean layout and small circuits.
Right.
So that's why we always prefer mixers for RF whenever we need up and down conversion in the RF application.
Next we will understand what are single diode mixers or diode mixers in general.
And pros and cons of diode mixers over other types of mixers.
So diode mixers are the most modern mixer design.
Uses Schottky diodes.
The main reason for this is the Schottky diode has major carrier device.
That means it has higher switching speed than other p-n junction diodes, which is suitable for designing
mixers up to 13GHz or more.
The other advantage of using the diode mixer is by adjusting the barrier voltage, we can adjust the
yellow drive, which will be required to obtain low loss mixing, and that leads to have higher linearity
compared to other types of mixers.
Now let's understand that with the help of block diagram.
So there are four building blocks of a diode mixer, RF isolation circuit, RF matching network diode,
or the DC biased diode circuit.
And the last block is filter block.
Let's start with RF isolation circuit.
So as their name suggests, RF isolation circuit is responsible for isolating the input ports which
are RF input or RF frequency input and low frequency input signals.
Next building block is the matching network, which is responsible for matching the input impedance
of DC biased diode circuit and the output impedance of RF isolation circuit.
So whatever is coming out of RF isolation circuit, it should be impedance matched or both ports should
be impedance matched with the DC bias circuit.
And there we use a matching network.
It could be a different type of matching.
We'll understand that.
How to design that matching network in the next step of this video.
And at last we'll have filters.
So as we all know that we have some desired frequency of this mixer.
So to keep that desired frequency and reject all other types of frequency and spurs, we use these filters.
That will be the last stage of this mixer design.
Next we will understand these blocks in detail while designing and simulating the single diode mixer
and different blocks of it.
So step one is let's start with our simulations of diode, so that you can directly download using the
link given in the description, because I have already designed that in previous video.
And we are going to use the same diode model.
Once that project has been downloaded, you can go to our microwave office.
From your start menu, run the tool, go to File Open Project.
You can locate wherever you have saved it.
In my case it is on desktop and just select the single diode mixer amp file and click on open.
Here we go.
So this is the project which we have designed.
And this is nothing just the diode model and the IB curve simulation of the diode model.
So just to recall that this is the package parasitics of the diode which is here.
And we have downloaded the Spice model of this diode from the manufacturer's website.
And then we have created this test Testbench to plot the I-V curve.
And at last we have just matched the I-V curve or the output of this I-V curve with the datasheet to
verify that we have simulated it correctly.
So in the next step we are just going to create the diode DC biasing circuit.
Now before doing that I'm just going to add some of the requirement of our diode mixers.
So in case of this diode mixer the RF frequency.
So we'll just denote it with f RF will be 4.25GHz.
And the output frequency we need is 0.5GHz.
So F I f will be 0.5GHz.
All right.
So this is the requirement.
And to achieve that we need the flow of 3.75GHz.
Right.
So this is our sort of requirements.
You can add as much as information you want in this design notes, which is very helpful I find throughout
the designing and simulating different circuits because simulation can be as huge as possible.
So it's good to keep the design notes.
And yeah, just make make sure you're just noting down and mentioning all the information.
It could be impedance of the tracks.
Let's say you have designed a microstrip line.
I want to use it throughout the simulation.
That number or the thickness of or the width of that microstrip line.
So you can just note it down here.
And then it would be helpful to recall.
All right.
So these are our requirements as you can see on your screen.
Now in the next step we are just going to create the diode DC biasing circuit.
To do that we'll just simply go to our circuit schematic.
Right click Create New schematic I'm going to name it Diode Biasing.
And click over create button.
Now in the next step we need to create the Subcircuit.
As you already know from the previous tutorials of our design environment.
Now, instead of going through with the process again, as we already know that we have the subcircuit
of diode here.
So if I just select that and go to inner layer, it will lead me to this diode circuit.
Right.
Which are which is the package parasitics of the diode.
So instead of creating it again from scratch I'll just copy that from the diode I-V curve circuit.
So again to go or it's not open I guess.
So we'll just.
Here we go.
So we'll just copy it and go back to diode Biasing and paste it here.
So I want to rotate it.
So to rotate that we just press Ctrl R.
And I want to rotate it like that.
There we go.
And we'll add the ground port at the cathode of the diode.
Let's place it here.
Now we can't just directly connect the anode of the diode to DC biasing circuit, because we know that
the the input of the anode will be a RF signal.
And if we just connect that directly to a DC biasing circuit, then there will be a RF current flow
from this to the DC circuit, which is not good.
So to avoid that we have to add an inductor or we call it choke.
And that choke will act as a high impedance whenever a AC current will try to flow into the DC section.
To do that we'll just go to elements from elements.
We'll go to lumped model, go to inductor and select a closed form of inductor.
Now to rotate it we'll just right click and place it here.
Now to limit the current we need to add a resistor.
As we all know that resistor has the property to limit the current.
We just select a closed form of resistor right click and place it in series with this inductor.
Now in the next step we are just going to add a DC supply.
To add that we'll just go to source.
And from the sources we'll find the DC voltage source.
We'll just drag it, rotate and place it here.
All right, so as we all know that we need to add a couple of capacitors to this DC supply to avoid
all the transient noise due to this DC voltage.
All right.
Now to add that we'll again go back to our lumped model capacitor close form of capacitor.
And we'll add three parallel capacitors to it.
So for now I'm just making some space here.
Let's keep it here.
Control C.
Control v.
Here we go.
And next we are just going to connect all these.
So we'll just connect it.
This this and these all will be connected to ground.
So we'll just place one ground port here which is here.
Let's place it here and that's it.
So this is a sort of DC biasing of a diode.
Next we are just going to enter the value.
So as you remember from the I-V curve that yeah.
So we have looked for couple of voltages across the diode.
And what will the amount of current will be flowing through that.
So we'll target the ten milliamps of current.
So as you can see can see here.
So this is the point where we'll have approx ten milliamp of current.
And then the voltage across the diode will be 0.369V.
And so if I'll be precise then at ten milliamps the voltage across the diode will be 0.3685V.
And you can also tally that from the datasheet of the diode.
So let's go back to diode biasing.
Now if you simply apply the Ohm's law V is equal to IR.
Here V will be 3.3 minus the voltage across the diode or the target voltage across the diode will be
0.3685 is equal to I will be ten milliamps and r will be unknown.
So from that we'll get the value of resistance will be two 90 ohm.
And if we just look for that resistance let's say on different distributors website we'll find the nearest
resistance of two 70 ohm.
Right.
So we'll just make it 270 here.
And similarly if you want to calculate the value of inductance.
So the purpose of inductance as I told you it is going to act like a high impedance path at the input
frequency.
So our input frequency is to 4.25GHz.
If you recall that from the design notes then we can simply calculate the impedance is equal to two
pi f into L or omega L.
Right.
So it is coming across 1.8 kilo ohm or something around that.
If the inductance will be 68 nano Henry.
Right.
So I'm going to use 68 nano Henry here.
It could be any value which is sort of making it high impedance path for the input frequencies right.
So here we have all these values.
Next I'm just going to put the value of capacitors.
So let's make it.
It is in Picofarad.
So we are going to add let's say one microfarad capacitor.
So it would be one E6.
And then we'll have 1200 picofarad.
And let's add one more smaller capacitor.
Let's say 100 picofarad.
So these values are not going to impact anything.
These are just to remove the transient noise from the DC source.
So our circuit is pretty much ready.
And next we are going to just quickly run the simulation so, as you already know, to run the simulation
to get the voltage and current, we have to annotate the circuit, right.
To do that, we'll just go to project and go to DC biasing.
From here we'll click on Add Annotation.
So here as you can see we are going to annotate it for DC which is the DC input current for all the
elements.
All right.
Click on apply.
And then all the DC voltage for all the nodes apply.
And okay.
And at last we'll just run the analysis.
And if I zoom in here as you can see, the amount of current which will be flowing through this diode
will be 10.8 milliamps.
And the voltage across the diode will be 0.381V.
Make sure the supply voltage is 3.3V.
So yeah.
So from that our circuit is sort of verified.
And next we are going to add the DC block capacitors.
Because we also do not want any DC current will be flowing in the direction of RF path.
So we have avoided the current flow from this direction to this direction by using this inductor.
Now, to avoid the current flow from this DC path to RF path by adding the DC blocking capacitors so
we can simply place two closed form capacitors.
One will be here and just copy it.
Paste one will be here.
Let's make the circuit and the value of these capacitors will be 100 picofarad.
And that is again based on the frequency we are going to pass from these capacitors.
So it should be low impedance for that frequency.
And for DC it should be high impedance.
And next we are going to add ports.
So let's place one port here and we just copy it rotate.
Place another port here.
So this is our DC biasing circuit.
I'm just going to name these ports.
So this is port one.
Double click on this.
This window will open.
So this will be the input of the DC bias circuit.
And this port will be the output of the DC bias circuit.
So I'll just double click here and this will be out.
Next we are just going to save the project.
So this will be the biasing circuit of the diode which we are going to use it in another schematic as
in sub circuit block.
So that's it for this video.
So next we are going to design the RF isolation circuit and their matching network.