urgent, LPF to suppress the hamonics

Dear all,

I want to filter the all harmonics above 125MHz (m8, attached file) using Low pass filter (BW : 300MHz, specification) but I could not suppress them even I used a good response LPF (attached filter). Note that, this harmonics are from mixing an RF signal and an LO signal.

Is an idea about it ?

Thnak you in advance

N

Good choice is to place at the output of the mixer a Diplexer.
The Diplexer will absorb undesired mixer output signals so that they are not reflected back into the mixer (and producing more spurious), and let the desired IF signal to go through.

If you can stand the in-band ripple you can use an elliptical filter. They have the
best cutoff performance. But you will have to make sure the filter impedance
does not generate spurs in the mixer.


One other thing I just noticed. You say you want to suppress everything above
125MHz, BUT you want a LPF Band width of 300MHz ?

If you start at DC (0Hz) and go to 300 MHz you will have passed the 2nd Harm
(2*125)MHz. It might be better to place your Cutoff frequency lower, ~175MHz.

Cheers

I guess it is nothing to do with filter as the signal is at 24GHz and the application is at 125MHz only.

Hello clleagues,

Thank you so much for clarifing me, say, how can i use a Diplexer ? i have reduced the cuttoff frequency to 175 MHz but no thing change...... the filter that i have used is stepped impedance LPF may be i have to use another LPF filter!
what you suggest to me ?
thnak you in advance for helping me
M

Added after 4 hours 40 minutes:

i have just found a paper (attached file) which talk about a diplexer in the output of the mixer... i will try to design a deplixer but is ther any tutorial about it ? is there based on microstrip line.......

thank you in advance

nassim

If you use a blanced mixer the near in armonics should be very low, or if it is possible use a cheby filter. You can find some exemple in pozar MICROWAVE ENGINEERING

can you share the schematic of your entire design? instead just filter

the whole shematic is in the attached file.

i hope to design a balanced mixer in microstrip but i do not have an adea..

1. i remind you that i'm student and i'm dealing to design diffrent mixers in microstrip
technology so this design is my first, is any one has a tutorail or otherkind of example ?

2. in my design i did not bias the diodes with DC, i'm not sure if i have to bias them or no so how can i know that ?

i appreciate your help!

thank you in advance..

nas