BP filter design

[kaszpore]

Hi Everybody,

I would really appreciate if somebody could help me with the my problem. I have to design a BP filter between 40 and 80 MHz with the following special features:

1. The noise level is around -90 dBm.
2. The signal is known to have a strong (60 dBm above noise) peak at exactly 80 MHz. This peak must be eliminated. Apart from that it has some standalone peaks in the 0-40 MHz region (~ 30 dBm above noise), and a peak in 40-80 MHz which I would like to isolate (~ 25 dBm above noise).

3. The filtered signal will be used in a feedback loop, so the phase shift should be accordingly small.

Unfortunately, I do not have much experience in filter designing, any help would be highly appreciated. First I have designed a high-order Chebyshev filter, but I have realized that it screws up the phase. Shall I dwell into active filters (any good literature?), can they help? I am absolutely clueless...

 

[Harald Kapp]

The signal is known to have a strong (60 dBm above noise) peak at exactly 80 MHz. This peak must be eliminated.

To eliminate a peak you need a notch filter (also named band stop filter), not a bandpass.

Shall I dwell into active filters

You're going to have a hard time building an active filter for 40 MHz..80 MHz.

Here's a website that deals with filter design for frequencies up to 500 MHz (also ist says 500mHz, but thats for sure 500 MHz).

 

[kaszpore]

Thank you, I'll look into it. I have never worked with active filters, what is the difficulty, what shall I pay attention to?

I have my reasons why I want a BP, I just did not want to overcomplicate the specs. Actually I have every integer multiples of 80 MHz present, and they are "real" peaks, not just higher harmonics, so eliminating the 80 peak alone will not erase them.

 

[Harald Kapp]

what is the difficulty, what shall I pay attention to?

Active filtes require active compöonents (amplifiers) that work in the frequency range of interest.
Typical active filter circuits use operational amplifiers as the active element. Operational amplifiers are rarely useful at 40 Mhz...80 MHz

 

[Harald Kapp]

I have my reasons why I want a BP, I just did not want to overcomplicate the specs. Actually I have every integer multiples of 80 MHz present, and they are "real" peaks, not just higher harmonics, so eliminating the 80 peak alone will not erase them.

1. Sorry, imho you failed in your intent. By not stating the full requirements you actually make the matter more complicated, not simpler.
2. To suppress 80 MHz plus multiples you can either use a comb-filter
   or a lowpass filter to remove the frequencies > 80 MHz followed by a notch filter to remove the 80 MHz component.

 

[LvW]

I have to design a BP filter between 40 and 80 MHz with the following special features:
.........
2. The signal is known to have a strong (60 dBm above noise) peak at exactly 80 MHz. This peak must be eliminated..

...."between 40 and 80 MHz " normally means: Passband between 40 and 80 MHz.
In this case, however, you cannot eliminate any 80 MHz peak.
Please, solve this contradictory specification.
In general, you can design a bandpass with a real zero at 80 MHz (Chebyshev invers or elliptical response).
However, in this case the upper passband edge is, of course, below 80 MHz.