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Theory of Intermodulation Distortion Measurement (Imd)

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Theory of Intermodulation Distortion Measurement (Imd) Model MAURY MICROWAVE MT993A CORPORATION 27 Jul 1999 THEORY OF INTERMODULATION DISTORTION MEASUREMENT (IMD) Introduction The ten measurement parameters of interest here are as follows: Intermodulation distortion occurs when the non-lin- earity of a device or system with multiple input Carrier (C): This is the power of the carrier signal frequencies causes undesired outputs at other frequen- in dBm units. It is similar to the Pout parameter, cies. In a communications system, this means that except that C is measured with a spectrum ana- signals in one channel can cause interference with lyzer, and Pout is measured with the power adjacent channels. As the spectrum becomes busier and meter with only one RF input signal. the channels become more tightly spaced, minimizing intermodulation distortion becomes more important. 3rd Order Intermodulation Product (I3): This is Two-Tone Intermodulation Testing the power of the spurious 3rd order intermodulation signal in dBm units, measured A convenient way to measure IMD is to combine two with the spectrum analyzer. equal power signals with a set frequency spacing at the input of the device-under-test (DUT). The output spec- Carrier to 3rd Order Intermodulation Ratio trum will look like Figure 1. The two largest signals are (C/I3): This is the ratio of the carrier power to the the amplified carrier signals, and the smaller signals, spurious 3rd order intermodulation power in moving away from the carriers in both directions, are dB. the 3rd order, 5th order, and 7th order intermodulation products, respectively. The frequency spacing between 3rd Order Intercept Point (IP3): This is a figure of all of the signals is equal. Figure 3 shows a typical IMD merit in dBm for the device and normally varies measurement setup. with tuning. Figure 1: Output Spectrum of Two-Tone Intermodulation Test 2900 Inland Empire Blvd. • Ontario, California 91764-4804 application note 5C-043 Tel: 909-987-4715 • Fax: 909-987-1112 • http://www.maurymw.com Copyright 1999 Maury Microwave Inc., all rights reserved. SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 1 of 3 27 Jul 1999 5th Order Intermodulation Product (I5): This is 3rd Order Intercept Point the power of the spurious 5th order The non-linear transfer function of a device or sys- intermodulation signal in dBm units, measured tem can be expressed as a series expansion: with the spectrum analyzer. 2 3 4 Carrier to 5th Order Intermodulation Ratio (C/ f(x) = a0 + a1(x) + a2(x) + a3(x) + a4(x) + ... I5): This is the ratio of the carrier power to the spurious 5th order intermodulation power in dB. The intermodulation signals shown in Figure 1 come from the 3rd order term in the series expansion, 5th Order Intercept Point (IP5): This is a figure of hence the name 3rd order intermodulation product. merit in dBm for the device and normally varies Because of the 3rd order, it will increase with input with tuning. power much faster than the carrier as shown in Figure 2. In dBm units, which is a logarithmic func- 7th Order Intermodulation Product (I7): This is tion, the intermodulation signal will increase with the power of the spurious 7th order power three times as fast as the carrier signal. intermodulation signal in dBm units, measured with the spectrum analyzer. The measurement of the 3rd order intercept point is illustrated in Figure 2. If the linear portion of the C v. Carrier to 7th Order Intermodulation Ratio Pin and the I v. Pin curves are extrapolated out, the (C/I7): This is the ratio of the carrier power to the interception point is called the 3rd order intercept spurious 7th order intermodulation power in dB. point (IP3). In Figure 2, this is the intersection of the dotted lines. This theoretical point is never reached 7th Order Intercept Point (IP7): This is a figure of in practice, because both curves will saturate before merit in dBm for the device and normally varies reaching it. However, IP3 is often used as a merit with tuning. function for the device. Figure 2: Measurement of 3rd Order Intercept Point 5C-043 application note 2900 Inland Empire Blvd. • Ontario, California 91764-4804 Tel: 909-987-4715 • Fax: 909-987-1112 • http://www.maurymw.com SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 2 of 3 MAURY MICROWAVE 27 Jul 1999 CORPORATION If the 3:1 theoretical slope difference is assumed, 5th and 7th Order Intercept Points then the IP3 can be calculated from only one power level. If a power sweep is done, IP3 calculated this The 5th and 7th order intercept points (IP5 and IP7) way will be constant in the linear region if the 3:1 are determined in a manner to IP3. The main differ- slope assumption was correct. As the carrier and ence is that the 5th order intermodulation increases intermodulation signals saturate, the IP3 value will with input power five times as fast as the carrier (in usually drop off, indicating an invalid measurement. dBm units). The 7th order intermodulation increases At the lower power levels, IP3 will start to change as with input power seven times a fast as the carrier (in the spectrum analyzer noise floor is reached, also dBm units). indicating an invalid measurement. The correct mea- surement is therefore in the power range where IP3 stays constant. NOTE: Theoretically, IP3 is not a function of power level. However, the dynamic range is limited by the spectrum analyzer noise floor on the low end, and DUT saturation or spectrum analyzer intermodula- tion on the high end. Looking at IP3 as a function of power provides a good way of checking the valid measurement range. GPIB ➣ ➣ ➣ ➣ ➣ ➣ ➣ ➣ ➣ PC RF SOURCE RF SOURCE INPUT REFLECTION BIAS SUPPLY TUNER SPECTRUM POWER COMPUTER #1 #2 POWER POWER MT960A CONTROLLER ANALYZER METER METER METER (OPTIONAL) (OPTIONAL) MT993A/D SOFTWARE AMP (OPTIONAL) BIAS FILTER POWER COUPLER COUPLER BIAS TTF TUNER TUNER TEE COUPLER (OPTIONAL) AMP COMBINER (OPTIONAL) (OPTIONAL) TEE (OPTIONAL) MAURY ATS Figure 3: Typical Setup for Performing Power/Intermodulation Measurements 2900 Inland Empire Blvd. • Ontario, California 91764-4804 application note 5C-043 Tel: 909-987-4715 • Fax: 909-987-1112 • http://www.maurymw.com Copyright 1999 Maury Microwave Inc., all rights reserved. SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE Page 3 of 3.
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