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Design >> RF Design >> Low Pass Filter Problem . Expert advice needed.
https://designers-guide.org/forum/YaBB.pl?num=1274343076

Message started by Digital on May 20^th, 2010, 1:11am

Title: Low Pass Filter Problem . Expert advice needed.
Post by Digital on May 20^th, 2010, 1:11am

I am trying to remove the 1st and 2nd harmonic from a transmitter I have designed in the 433mhz band. It puts out 17dbm.. The transmitter is all surface mount based around the SX1230 chip.

Ive built two 5 element low pass filters with SMA in and out connectors and tested them with a spectrum analyzer with tracking gen. Both perform well with less than 1 db of insertion attenuation at 433mhz and around 25 db at 860mhz.

The problem is when I test them with my transmitter there seems to be no visible reduction in the harmonics when viewed with the spec ann.

This makes absolutely no sense to me infact it even sounds ridiculous.

Has anyone got any ideas of where I’m going wrong.???

Title: Re: Low Pass Filter Problem . Expert advice needed.
Post by wave on May 20^th, 2010, 1:20pm

Can you add some more details:
Is the 433M a carrier or the signal as well (ie baseband frequency?)

What is the BW of the 2 LPF by spec and confirmed by measurement on network analyzer?

Do all the impedances match when connected together in the system? ie 50 ohm terminations?

What do you expect to transmit at?
A 1st harmonic is the fundamental, so perhaps you actually mean the 2nd and 3rd harmnonics? That would transmit more of sinewave as opposed to a squarewave.

cheers!
Wave

Digital wrote on May 20^th, 2010, 1:11am:

Title: Re: Low Pass Filter Problem . Expert advice needed.
Post by Digital on May 20^th, 2010, 6:12pm

Wave..

1st harmonic is the fundamental.. :D

That would transmit more of sinewave as opposed to a squarewave. ;D

Sorry but i need expert advice

Title: Re: Low Pass Filter Problem . Expert advice needed.
Post by pancho_hideboo on May 21^st, 2010, 2:42am

To make LC filter work effectively, termination impedances for passband and stopband are very important.

Digital wrote on May 20^th, 2010, 1:11am:

The transmitter is all surface mount based around the SX1230 chip.

Show me output reflection coefficient of transmitter by network analyzer measurement.

Digital wrote on May 20^th, 2010, 1:11am:

Ive built two 5 element low pass filters with SMA in and out connectors and tested them with a spectrum analyzer with tracking gen.
Both perform well with less than 1 db of insertion attenuation at 433mhz and around 25 db at 860mhz.

Show me S-parameters(S11,S12,S21,S22) of one LPF and cascaded two LPF.

Digital wrote on May 20^th, 2010, 1:11am:

The problem is when I test them with my transmitter there seems to be no visible reduction in the harmonics when viewed with the spec ann.

What do you mean by "ann" ?
Don't use unfamiliar abbreviation.

Try to insert lossy matching pad between transmitter and LPF or between two LPFs.
http://www.rfcafe.com/references/electrical/attenuators.htm

Title: Re: Low Pass Filter Problem . Expert advice needed.
Post by aaron_do on May 21^st, 2010, 8:41am

Quote:

Wave..

1st harmonic is the fundamental.. :D

That would transmit more of sinewave as opposed to a squarewave. ;D

Sorry but i need expert advice

As far as I know the 1st harmonic is the fundamental. Regardless, you obviously need some humility to go along with your expert advice.

Title: Re: Low Pass Filter Problem . Expert advice needed.
Post by rfcooltools.com on May 21^st, 2010, 10:35am

Are you sure that your transmitter is getting distortion from the spec analyzer, try setting the attenuation in the spec analyzer 10dB more and if your 866MHz spur drops more than 10dB then that is a contributor. Also goto http://www.rfcooltools.com and experiment with the filter synthesis tool.