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# Low pass filter design

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#### manushanker20

##### Junior Member level 1
Hi

I want to design a low pass filter with a load impedance that changes with frequency.

The cut off frequency is 80 MHz. The circuit´works fine if you have a constant load impedance.

please give some ideas to over come this problem.

Regards

Manu

what is the range of change of the impedence

the load impedance corresponds to the impedance of a FM antenna. the antenna impedance is 35.3+j86.4 at 250 MHz and 0.8-j147.7 at 30 MHz.

At 80 MHz it is 55.8-j148.25. when i fixed the load impedance to be 55.8 then low pass network works fine but when i connect a S1p port (the s parameter file defined in ADS) then the results are not satisfactory.

I think you could try broadband matching network for
antenna to a around-fix load impedance in your band(30-250MHz), says 55Ohm. Then the LPF can be fixed.

I am have designed a matching network for the frequency band of interest. For the matching network I used a tunable capacitor (Varactor diode). the basing circuit of the varactor diode must be protected from the RF patch. To do this I am desingning a low pass filter to avoid the coupling of Rf signal into the dc path.

It is true that the input match will be disrupted by the changing impedance. Question is if you actually care ...

What really matters is the power that is transmitted into the air. To know this you can model the dipole as a series RLC circuit. The R value is the radiation resistance, the L and C are defined by the Q factor of the dipole. These can be approximated by the measurements you made.

Now if you drive this parallel circuit with a port with source impedance R, the maximum power is dissipated into the antenna and transmitted in air at the resonance freq. Outside the resonance frequency, the antenna is less efficient.

Now with the lowpass filter in the middle, the antenna will still radiate less efficient. You should now evaluate the S21 with the RLC circuit for the antenna instead of the simple R to find out the suppression due to filtering and antenna efficiency. Then you should evaluate the spurs that your transmitter generates and whether these don't violate any EMC rules.

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