Intermodulation

INTERMODULATION 

1. Introduction

The INTMOD program performs harmonic and intermodulation analysis of 2 and/or 3 signals of 3^rd, 5^th or 7^th order mixing.  The user enters the transmitter and receiver frequencies in MHz with their corresponding bandwidths in kHz.  The user may enter the lists interactively or use a database built from a previous run.

2. Harmonics and Intermodulation analysis

            Background

Mutual interference may occur between two circuits operating at different frequencies because of radiation at frequencies other than the operating frequencies.  A transmitter may radiate harmonics of its operating frequency, which may affect a receiver tuned to one of the harmonics.

Intermodulation products result in interference, when two or more signals combine in a nonlinear device and produce an undesired signal on or near the tuned frequency of the victim receiver.  The combination process can occur in the final stage of a transmitter or in the RF of first mixer circuitry of a receiver.

When the number of transmitters is increased, the number of possible intermodulation frequencies increases rapidly.  The most serious of these frequencies are the third order products of the form 2f₁ – f₂ or f₁ + f₂ – f₃, where f₁, f₂ and f₃ are operating frequencies of the transmitters.  The interference will be most serious when all or several of the frequencies, both transmitting and receiving, are in close proximity.  To minimize intermodulation, frequencies should be selected such that the frequency difference between any pair of frequencies is unlike the difference between any other pair.  In some cases, the specific operating frequencies can be chosen so that no third order product frequency coincides with a receiving channel frequency at the same or a nearby site.

Harmonic Analysis Equation

The mixing of frequencies whereby the largest frequency is greater than twice the smallest frequency is not analyzed.  Harmonic interference is defined below.

A f_t = f_r ± BW                                                             (2.1)

where, A is either 1 or 2, f_t is the transmitter frequency, f_r is the receiver frequency and BW is the receiver bandwidth.

Intermodulation Analysis Equations

The interference due to intermodulation is defined by the following equations.  The analysis only considers in-band modulation products.

Two Signal Case

3^rd order:                            2 f_t1 – f_t2 = f_r ± BW                                                     (2.2)

5^th order:                          3 f_t1 – 2 f_t2 = f_r ± BW                                                    (2.3)

7^th order:                          4 f_t1 – 3 f_t2 = f_r ± BW                                                    (2.4)

Three Signal Case

3^rd order:                        f_t1 – f_t2 + f_t3 = f_r ± BW                                                    (2.5)

5^th order:                    2 f_t1 - 2f_t2 + f_t3 = f_r ± BW                                                    (2.6)

                                   3 f_t1 – f_t2 – f_t3 = f_r ± BW                                                     (2.7)

7^th order:               2 f_t1 – 3 f_t2 + 2 f_t3 = f_r ± BW                                                    (2.8)

                                3 f_t1 – 3 f_t2 + f_t3 = f_r ± BW                                                    (2.9)

                                4 f_t1 – 2 f_t2 – f_t3 = f_r ± BW                                                     (2.10)