Spectrum Software
Industrial Strength Simulation




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Analyzing Mixers Using Intermodulation Distortion Analysis


Mixers are widely used in RF applications. Their main function is to simply multiply two signals together, typically the LO (local oscillator) signal and the RF (radio frequency) signal. Multiplication is accomplished by passing the signals through a nonlinear stage. The resulting output waveform contains signal content at the sum and the difference of the LO and RF frequencies.

One of the many mixer topologies is based on two diodes. This type of circuit is known as a single balanced diode mixer. Because the inputs are balanced it provides good rejection of the input signals at the output.

This type of mixer is singly balanced because it does not isolate the two input ports. Signal from the LO node may leak onto the RF node (or vice versa), producing intermodulation distortion. However, for many applications this circuit operates quite well. A double balanced mixer can be used instead if input leakage is a problem.

In order to assess the degree of IM distortion we can run Intermodulation Distortion analysis from the Analysis menu.

Intermodulation Distortion analysis is a type of transient analysis. It applies two sinusoidal signals to the named input source with user-specified non-harmonic frequencies and amplitudes, and then measures the resulting second and third order distortion in the specified load resistor using the HARM function.

The figure below shows an example of such a diode mixer.

Single-balanced diode mixer

To assess its intermodulation distortion, select the Intermodulation Distortion analysis option from the Analysis menu. Its analysis limits menu looks like this:

Diode mixer analysis limits

The system will apply the specified two frequencies, 400MHz and 450MHz, to the source named VRF. Its voltage will be log stepped from 50mV to 1.0V. The VLO source will use the frequency and voltage level specified in the schematic (1 Volt and 500MHz)

Press F2 to start the run and in a few seconds the following plot will appear.

Distortion plot

This shows a plot of the H1 harmonic in blue and IM3 (3írd order intermodulation) in green. The program will find the region on the H1 plot where the slope is closest to 1.0 and also on the IM3 plot where the slope is closest to 3.0. It will then draw lines of slope 1.0 and 3.0 respectively, and compute where they intersect. That point of intersection is the IP3 point. It also computes and marks the point where the H1 curve drops by 1dB from the ideal slope=1 line. Both of these are important figures of merit for a mixer.

In this case the values are P1dBm=2.6 and IP3=18.4.

The second plot window holds plots of the waveforms which are used to compute the distortion and harmonic variables. The program applies the FFT to these basic waveforms and computes the necessary distortion plot values. This "working plot window" looks like this.

Diode mixer working plots

Here we've plotted the following waveforms.

HARM(V(RL)) The frequency spectrum of the output voltage waveform V(RL)
V(RL) The output waveform
HARM(I(VRF)) The frequency spectrum of the input current waveform V(RL)
HARM(V(VRF)) The frequency spectrum of the RF input voltage waveform
HARM(V(VLO)) The frequency spectrum of the LO voltage waveform

Not all of these are necessary. The later two are for information only to show what spectra are actually applied during the run.

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