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Design Notes

Gilbert Cell Mixer using IAM-81

This example shows mixer noise analysis for a Gilbert Cell mixer.  This is an example taken from the HP Application Note AB-0013: SPICE Models for the IAM-81 and IAM-82 Active Mixers.


The “Gilbert_Cell Testbench” schematic shows the simulation setup for nonlinear noise analysis. After the simulation is complete, the Tuner (Simulate > Tune) can be used to sort through some of the swept data - in particular, the LO Power level used on the “Noise Spectral Density Graph”

This project is showing the Aplac HB simulator since this simulator supports the nonlinear noise contributor measurements.  

Nonliner noise analysis requires that a NLNOISE component on the schematic.  The NLNOISE component has the following parameters:

PortTo: Output port index, usually the IF port in the case of mixers.

PortFrom: The port where the noise source is applied. This is usually the RF port in the case of mixers. By convention, the source is a resistor at 290K.

NFstart: Start of the noise frequency sweep.

NFend: End of the noise frequency sweep.

NFsteps: Number of sweep points in noise analysis.

SwpType: Linear or log sweep.

LSTone: Tone index of large signal excitation.

SSTone: Tone index of noise signal excitation.

NoiseContributors: Type of individual noise contributors to collect.

The noise is analyzed at all frequencies of the form n x f_LO ± f_noise, where f_LO is the LO frequency and f_noise is the smallest difference between any of the LO products taken into account in the simulation and the specified noise frequency sweep.  Therefore it is easiest to specify noise frequencies as IF frequencies so that the noise will be analyzed at all frequencies of the form n x f_LO ± f_noise_frequency_sweep.  In the same case, assuming that the mixer is an upper-sideband downconverter, the noise input frequency of interest in the NF calculation is f_LO + f_noise_frequency_sweep and the noise output frequency of interest is f_noise_frequency_sweep.  The harmonic index pairs of these two noise components are (1,1) and (0,1).

In the NLNOISE block, the user must specify the PortFrom (source of the noise signal) and the PortTo (measurement of the noise signal). The PortFrom is usually the RF port and the PortTo is the IF port. NFstart, NFend, and NFsteps specify the noise frequencies in the NLNOISE block. In this example, NFstart and NFend are set to “250 MHz” (IF frequency). The noise frequencies are relative to one of the HB tones, which is usually the LO tone. This HB tone is specified in the LSTone. In this example, the LO port has been assigned Tone 1. To verify which tone is assigned to the LO port, right-click on the LO Port (PORT_PS1) and select Properties. Click on the Port tab and check that the Tone type is set to Tone 1. The SSTone is assigned a tone type number higher than the one used in the LSTone list. In this case, since there is only one LSTone and the tone type is Tone 1, then the SSTone is assigned tone “2”.

Nonlinear Noise Contributors

Nonlinear noise contributors allow plots of how much noise each individual model contributes to the total noise at the output.   The requirements are:

1.   Aplac HB simulator.

2.   Must have a V_NSMTR  at the output of the circuit.

3.   Must use the NLNoiseCon measurement on a tabular graph.  

The graph All Noise Sources Contributors has all the noise contributors sorted by highest value with all the individual noise types (Shot, Thermal, etc).  The graph Total Noise Contributors has the noise contributor sorted by the highest value but only for the total noise for each model.  

Nonlinear Noise Analysis notes

The measurements for conversion gain and NF are setup using the Output large-signal harmonic and Output sideband and the Input large-signal harmonic and Input sideband. The Output large-signal harmonic is at DC (HB index 0) and the Output sideband corresponds to frequency offset for the noise measurements. In this case, the offset is for the upper sideband which was setup to be equal to the IF frequency. The Input large-signal harmonic is set to the LO frequency (1.75 GHz, HB index 1) and Input sideband is also the upper sideband.

Some notes on Nonlinear Noise analysis

• The same results are obtained if a linear port or a HB signal port is used for the RF port.

• One may sweep the LO power, the LO frequency, and the noise frequency simultaneously.

• Currently available measurements of interest are noise figure, noise temperature, noise voltage, and the noise spectral density at the output.  Note that the noise of the output termination is excluded from the analysis, as is conventional for the purpose of noise figure simulation.  Approximate conversion gain is included as well.  It is approximate in the sense that it is applicable in cases where the LO signal is much larger than the RF signal.  This is the usual case.  If the RF signal is less than 15dB below the LO signal, it is a good idea to verify the accuracy of the conversion gain measurement against large signal S parameter predictions.

Conversion Gain vs  LO Power Graphv

This graph shows the mixer conversion gain versus LO drive level.

Noise Spectral Density Graph

This graph shows noise spectral density versus frequency at a single LO drive level.  The drive level can be selected by using the tuner (Simulate > Tune) after the simulation has run.

Noise vs LO Power Graph

This graph shows noise figure versus LO drive level.

Schematic - Gilbert_Cell

Graph - Conversion Gain vs LO Power

Graph - Noise Figure vs LO Power

Graph - Noise Spectral Density