LowPass Filter - Ham Radio Projects

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LowPass Filter

IM MEASURING KIT

For measuring own IMD (not only) of the kit it is very important to suppress the second harmonic of amplified signal at the combiner input as much as possible. At the output of the amplifier Chebyshev 7 elements Lowpass filter in every section was added


When designing the PCB, I tried to follow all the principles of design such circuits.


Next picture shows harmonics view of pure VNA output signal. When I decided to use VNA as a signal source for IMD measuring, I measured the suppression of the second harmonic directly at the VNA output first. Second harmonic is supressed abt. 55 dBc (+3.3 dBm of output level). OIP3 of the VNA varies from +41 to +45 dBm depending on the combiner type used.


I used next simplified consideration: If I get at the combiner input comparable suppression of the second harmonic with the input signal level abt. +20dBm, own IP3 will increase and can meet the requirements for the measurement of passive circuits, probably. Next pictures show the harmonics of the input signal level +20 dBm without LPF and with LPF.



Suppression of the second harmonic is comparable (54 dBc), but the signal level is substantially higher than at the output of the VNA, so we expect significantly higher OIP3 at the combiner output.....


 
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