Receiver Metrics: Theory and Practice. Intermodulation and the 3 rd Order Intercept Point. Carl Ferguson, W4UOA. John Drum, W4BXI John Krupsky, WA5MLF. Recent Advertisement. Example Receiver Types & Specs.
the 3rd Order Intercept Point
Carl Ferguson, W4UOA
John Drum, W4BXI John Krupsky, WA5MLF
Linear gain in a circuit is normally represented by a straight line.
The scale on the Input and Output axis reflect the gain through the circuit. In this example, a gain of 2:1.
However, all RF & IF circuits are inherently nonlinear.
At low input levels, receiver RF and IF stage gain will be generally linear—approaching a level called the small-signal asymptotic value.
But as the input level increases, gain through the stage becomes increasingly nonlinear. When the gain falls n dB below the small-signal asymptotic value, it has said to have reached its compression point (CP). The compression point, stated in dB, is frequently given as either 1 dBor 3 dB below the small-signal asymptotic value.
Token math slides
The performance of an ideal amplifier can be represented by the transfer function:
An amplifier with some distortion due to nonlinearities can be expressed by a transfer function in the form of a power series expansion:
An input signal with two frequencies 1 and 2may be shown as:
The first order term
gives the fundamental products
The second order term
determines the second order products:
2nd harmonic terms
2nd order IMD terms
The third order term
determines the third order products:
Fundamental frequency terms
3rd harmonic terms
3rd order IMD terms – The troublemakers
Our graph illustrates that the 3rd order intercept point is defined by the intersection of two hypothetical lines. Each line is an extension of a linear gain figure: first of the signal of interest; and second, of the 3rd order intermodulation distortion product—from which IP3 gets its name.
While there are several ways that IP3 can be located, you will note that of the several illustrated here, they all fall in the same general area. The larger the value of IP3, the less likely the receiver will be adversely affected by 3rd order intermodulation products. More on this later.
Source: QEX, July/August, 2002.
The upper bound is
set by the 3rd order
IMD equal to theMDS.Spurious-Free Dynamic Range
Why is it important to have a wide dynamic range?
Notice below that an input signal of -110 dBm will produce 20 dB output in our signal of interest. To achieve 20 dB output in the third order product, the off channel test signals f1 and f2 must be 80 dB (110 dBm – 30 dBm) greater than our signal of interest. An unlikely occurrence except in unique circumstance.
The lower bound is
Defined as a signal
of interest 3 dB greater than the noise floor.
FT-1000 IPO Button
Normally, the front-end FET RF amplifiers provide maximum sensitivity for weak signals. During
typical conditions on lower frequencies (such as strong overloading from signals on adjacent frequencies), the RF amplifiers can be bypassed by pressing the [IPO] button so the green LED is on.
This improves the dynamic range and IMD (intermodulation distortion) characteristics of the receiver, at a slight reduction of sensitivity. On frequencies below about 10 MHz, you generally will want to keep the [IPO] button engaged, as the preamplifiers are usually not needed at these frequencies.
From ARRL test reports in QST magazine(at 14 MHz w preamp off)
Intercept point and undesired responses
Sagers, R.C. Motorola, Inc., Fort Worth, Texas;
This paper appears in: Vehicular Technology Conference, 1982. 32nd IEEEPublication Date: 23-26 May 1982Volume: 32, On page(s): 219- 230Posted online: 2006-06-19 10:22:27.0
AbstractThis paper presents a method for using the concept of intercept point to calculate the undesired-response rejection ratio of a single stage. Single stages may be cascaded together to form a system and the undesired response rejection ratio of the system may be found using a procedure similar to cascaded noise figure. When applied to receiver system design, this method allows easy calculation of such undesired receiver responses as intermodulation distortion and spurious responses.
Two-Tone Nonlinearity Testing - The Intercept Point
Fulton, F.F.This paper appears in: Microwave Symposium Digest, G-MTT InternationalPublication Date: Jun 1973Volume: 73 , On page(s): 112 - 112 Posted online: 2003-01-06 17:17:01.0
AbstractWhen a nonlinearity is modeled as memoryless with a three-term power series, a convenient way of expressing the characteristics is by the use of intercept points. An intercept point is the output power level at which the fundamental tone and the distortion tone have equal amplitudes. For many practical system problems, specification of an intercept point permits very quick calculation of distortion tone levels; in particular, given two equal amplitude fundamental tones at similar frequencies, the adjacent third order distortion product is down from a fundamental by twice the number of decibels that the fundamental is down from the third order intercept point. Even more simply, the second order distortion is down from a fundamental by an amount equal to the number of decibels that the fundamental is down from the appropriate intercept point.