Design and Characterization of a Transistor Mixer -RF and Microwave Electronic

nsnkoduru
Sep 8, 2024
2 min read

Transistor Mixer

Objectives: The objective of this lab is to Simulating with the Cadence toolset by designing a simple Mixer. We use Mixer which is drawn with a multiplier symbol. A real mixer can not be driven by arbitrary inputs. Instead, one port, the LO port is driven by a local oscillator with a fixed amplitude sinusoid. In a down conversion mixer, the other input port is driven by the RF signal and the output is at a lower IF signal. In a up conversion mixer the other input is the IF signal, and the output is the RF signal.

Design Process:

We need a total of 9 NMOS transistors, 2 for biasing, 1 for the tail current, 2 for the differential LO input, and 4 for the differential RF input. Also, 4 resistors will be needed, 2 for the load and 2 for isolate the AC input nodes from the DC (AC ground).

To draw the schematic, I have followed some rules which are given bellow

• In the schematic working space, I pressed “I” on the keyboard to add any electronic components to my schematic

• To edit the properties of a component, pressed “Q” on the keyboard.

• To create wires, I pressed “W”.

• Pressed “L” to add a wire name.

• Pressed “P” to add a pin

Simulation results:

After inserting all the design components and make their connection we got the following diagram in the schematic window.

After designing the schematic diagram, we get our symbol by going to “Create” -> “Cellview” -> “From cellview”. Click “OK” if the pin allocation is correct. So, I can see a generated symbol of my mixer which is shown bellow.

In this design process we need to create the testbench schematic for the Mixer, which is used for the simulation. We create this new schematic in the same way I did before, but this time name it “Mixer_TB”. Apart from the mixer I just created, I would also need 3 “vdc” as bias voltage, 1 “idc” as bias current, 3 “port”, 2 resistors, 2 capacitors,1 “vcvs”, 1 “balun” and 1 “ideal Balun. We set the parameters in the input and output port and connected the components together using wires and properly label the wires, after finishing wiring, my testbench will be like bellow.

Now I Selected “Launch” -> “ADE L” then to “Analyses” -> “Choose” any desired parameters such as SP(S-parameters) or NF (Noise figure) or 1-dB compression points or IIP3 point in the window that pops up. We can then find the values we want. Voltage conversion gain is the difference in amplitude of the available RF signal to the IF signal output (downconverter), or from the IF signal to the RF signal

(upconverter). So initially we have found voltage conversion gain in IF port shown below

Similarly, we can plot the voltage conversion gain in RF port like below

Here after choosing SP (s-parameters) we got S11 value which has shown below.

We also plotted noise figure (NF) which has been shown below

When we click on the Positive net of LO port RF-to-LO feedthrough vs. frequency we found the following graph

To plot the RF-to-IF feedthrough curve, I clicked on the positive net of the IF port and found the following plot

To plot the LO-to-IFfeedthrough curve, I clicked on the LO port and found

To plot the LO-to-RF feedthrough, keeping configuration unchanged, I have selected the RF port and got the plot shown bellow.

After clicking on the IF port to we found the 1-dB compression point curve

Based on the same simulation, by clicking on the IF port we found the IIP3 plot which has been shown bellow. Because the IIP3 increase with the cube of the input change as compared to the linear change for the fundamentals, the higher the signal level input, then higher the ratio of intermodulation products to fundamental. There is a theoretical point where the output level of intermodulation products equals the output level of the fundamental. This point is called 3rd order intercept point, and this is often specified to define the 3rd order intermodulation performance of mixer.

Discussion:

Our mixing device operates in a non-linear mode to carry out its function as a mixer, it can also generate intermodulation products (IM) from unwanted signals at its input. The products might result from mixing our signal ω1 with some other signal ω2 or mixing two entirely different signal ω1 and ω2. The most troublesome of these are what are called the third order products (2ω1- ω2) or (2ω2- ω1). These are troublesome because they are normally the closest intermodulation products to our desired signal ω1. To overcome this problem we need to keep IM products low as it is necessary to operate the input signal ω1 at low level.