Measuring IMD

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Described is a method to measure the IMD mixers, amplifiers, etc. without any sophisticated equipment. A PC and a simple circuit is all you need. Two stable free-running VFOs are set to generate frequencies in the 14 Mhz bad separated by around 20 KHz. The two signals are combined in the 6db hybrid coupler and filtered through a low-pass filter. Either or both the RF sources can be switched on, one at a time to set their frequencies and measure their individual outputs. In our case, the output of either of the oscillators was -15dbm. (70mV peak on a 50 ohms resistor connected across the output).

The circuit is as follows: Two-tone-lores.png

Test Setup

The individual VFOs are set at 14.000 KHz and 14.020KHz (don't worry about the exact frequeny, as long as it is approximatley 20 KHz away). The output of this two-tone generator is fed to the receiver through a home-made RF attenuator that has a range of 80db in 1db steps. The receiver output is viewed through a PC based audio spectrum analyzer/oscillpscope.

Method

The corresponding to the 14.000 and 14.020 input frequencies, the receiver front-end will generate responses at 14.040 and 13.980 (20 Khz on either sides of the input signals). We have to measure the ratio between the input signal and these responses. The higher the ratio indicates better receiver performance. Ideally, we should not hear anything at all except at 14.000 and 14.020. We first tune to the the unwanted response, let's say at 14.040 KHz, note the amplitude of this on the PC scope, then we tune to the input signal at 14.020 KHz and flip on the RF attenuator until the amplitude at 14.020 also reads exactly the same levels. This half of this ratio added to the input signal strength gives you the IIP3. For instance, each RF signal was -10dbm and it needed an RF attenuation of 20 db of the original at 14.020 to match the 14.040 response, then, the IIP3 = -10 + (20/2) = 0dbm.

Some meausurements