Filtering Harmonics

Thread Starter

tenacity2986

Joined Oct 31, 2012
9
When working with a 900MHz signal (902MHz to 928MHz passband) with allowances by the FCC for 1 W Output Power, I'm having trouble filtering the harmonics. What is the best way to filter harmonics to get minimum roll-off and maintaining bandwidth? Currently have a 5 element band pass filter before amplification and after. The first and second harmonic seemingly are very strong.
 

Papabravo

Joined Feb 24, 2006
21,159
When you say 'element', what are we talking about exactly? At those frequencies you can't be talking about actual inductors in your filters.

Why are you using a bandpass filter? Harmonics are integer multiples of the fundamental so your second harmonic will be at 1.8 GHz. It doesn't take much to kill signals in that frequency range. Usually you get more bang for the buck with a low pass filter.
 

Danm1

Joined Jul 19, 2010
69
I think designing filters is a black art so I would use a design tool that does all the work. I've recently used the Filter Design Wizard on the Analog Devices web site, it's very nice.
 

Papabravo

Joined Feb 24, 2006
21,159
I think designing filters is a black art so I would use a design tool that does all the work. I've recently used the Filter Design Wizard on the Analog Devices web site, it's very nice.
It is not a black art although it requires more mathematics background than the average design task.

And you would never know if the tool was feeding you garbage. Stugeon's law "90% of every thing is crap" applies to the internet of things as well.
 

LvW

Joined Jun 13, 2013
1,752
I think designing filters is a black art so I would use a design tool that does all the work. I've recently used the Filter Design Wizard on the Analog Devices web site, it's very nice.
Yes - it is very nice. And there are other filter design packages which also are very nice.
However, if I remember well you intend to design a filter around 1GHz, don`t you? Don`t forget, that the AD program is for ACTIVE filters and you need an opamp that is able to cope with these frequencies.
Perhaps you should consider also a PASSIVE filter structure.
 

KL7AJ

Joined Nov 4, 2008
2,229
When working with a 900MHz signal (902MHz to 928MHz passband) with allowances by the FCC for 1 W Output Power, I'm having trouble filtering the harmonics. What is the best way to filter harmonics to get minimum roll-off and maintaining bandwidth? Currently have a 5 element band pass filter before amplification and after. The first and second harmonic seemingly are very strong.
At that frequency, you'll have to use cavity resonators to get the kind of filter response you need.

Eric
 
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