Electronics > Beginners

Does the "shape" of an FM bandpass filter matter?

**vk6zgo**:

One problem with using 50 Ohm Z for low frequency filters is that the series resistance of the inductors becomes significant,so that a real filter won't look like your modelled version.

You might have to wind up a sample inductor,measure R,then insert that into your model.

Another thing you can do is put impedance conversion sections at the input & output of the filter,& design it for a somewhat higher impedance.

This of course,makes the whole thing more complex,& larger.

I had occasion to design & test a 50 Ohm filter for a bit lower in frequency (around 530kHz),& it was a nightmare.

The idea was to allow a Broadcast station to use another station "off air" as a standby program source,without interference from the station's local transmitter.

A filter designed from the standard tables (No modelling software back then),looked good,but the series R bugbear raised its ugly head,& I had to rethink the whole thing.

Empirical design lead to a filter which did the job,but reduced one sideband of the required signal more than I would have liked.

In retrospect,I would have designed it from the tables,but for a higher Z,& used impedance conversion sections.

VK6ZGO

**SoftwareSamurai**:

--- Quote from: vk6zgo on October 27, 2011, 12:22:33 am ---One problem with using 50 Ohm Z for low frequency filters is that the series resistance of the inductors becomes significant,so that a real filter won't look like your modelled version.

--- End quote ---

Good point. That's why I'm sticking to known, manufactured inductors, so I can add their spec'ed resistance into the LTSpice model. (I considered adding their spec'ed capacitance too, but that's so small it doesn't seem to make too much difference.)

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