Sunday, 29 January 2012

Low Pass Filter (Chebyshev) for 88-108 FM Broadcast Band

Here is a little project i have been working on today, it is a low pass filter for the FM broadcast band (88-108 in the UK, although the filter as shown would be suitable for use in the extended (e.g. Japanese) band too.

A low pass filter is a very important piece of kit when using broadcast transmitters of anything over a few hundred milliwatts. NO PIECE OF TRANSMISSION EQUIPMENT WILL PASS TECHNICAL REQUIREMENTS WITHOUT PROPER FILTERING, AND A TRANSMITTER WITHOUT THIS WILL CAUSE INTERFERENCE TO OUT OF BAND SERVICES.

Filters do exactly that, they filter. In this case they filter frequencies above and beyond 108MHz (actually about 115MHZ to allow plenty of headroom). There are several different characteristics/design parameters for filters, the one shown here is a LOW PASS FILTER, meaning it passes frequencies lower that a given level, and attenuates frequencies above. Three important design considerations are the frequency which the filter begins to act, how hard it filters and pass band ripple (the amount of effect the filter has on frequencies within the pass band).

The design shown here is Chebyshev architecture, known for it's good cutoff and pass band characteristics. It is a 7 pole filter (meaning there are 7 different sections to the circuit).

I have included schematic as well as some of the software analyses of the filter itself. The inductors can be wound of wither enamelled copper, or tinned copper wire, if you have an LCR meter you can adjust the spacing to get the value exactly right, if not just wind them carefully and measure them as accurately as possible. Capacitors can be standard ceramic up to about 70 watts, anything over that i would recommend mica capacitors, or metal clad for really high powers (unlikely! If you need this much power you will need to recalculate the coils using thicker wire). I have included a Monte Carlo analysis which randomly adjust the component values within a given tolerance and shows the changes in filter characteristics (shown as red lines with the original design shown as a black line in the middle).

Input/Output characteristic impedance is 50 ohm, this is closest at the top and bottom of the band and worst in the middle (about 58 ohm), this is perfectly normal for a filter of this type and will not affect the output power too much.

M.A.S.T. Chebyshev Low Pass Filter for the 88-108MHz FM Broadcast Band, 0.1dB pass band ripple, 3rd Harmonic -60dB or better:


  1. Why you say with -60dB or better if at ~170MHz it is only about -30dB?

  2. Hello. This is interesting I'll do it soon.

  3. I've build this filter but instead of using capacitors I used trimmers. This works awesomely!! You can tune the filter in such a way that your desired frequency lies at the top of a passband ripple and thus there is practically no loss of power. Plus you can get the SWR down to real 1 to 1 I mean even if I turn the forward power knob all the way open the needle for reverse power doesn't move. So it also acts as an antenna matcher although your antenna does need to have a good SWR without the filter. This filter can't work miracles. I encourage everyone to build this and use trimmers instead of capacitors and you'll see what I mean. I tested this in simulation software and the filter keeps doing its job perfectly.