RF Filters

Hi all,

I want to clear some of my queries on RF filters.

Query : 1
What is the difference between Group Delay Ripple and Group Delay Flatness?


Query: 2
How to measure Group Delay Ripple for a Low Pass Filter?

If anyone know the answer for these questions plz reply..

 

I'll take a shot at this, but I may be wrong. Ripple implies that there is at least one dip in the group delay within the passband. A flatness spec might define a filter with monotonic group delay response, i.e., it might not have ripple.

The best way is with a VNA (vector network analyzer). I think any other method would be tedious.

 

thankx for fast reply..

But regarding my first query I am still not clear...

If ripple indicates dip in the passband, then flatness is also used to measure the dips in the passband or how flat the passband is?

Regarding second query:
I want to know whether I have to measure group delay by:
1. Calculating max-min value of the group delay in the passband range will give the Group delay ripple?
or
2. For every peaks and dips (withing the pass band) calculate the difference and max value we got in the passband range will the group delay ripple. Is it correct?

 

I Googled "group delay flatness ripple" and got the following definitions from RF Filter Glossary at Trilithic RF and Microwave Components web site.

I think you have to specify whether measurements are peak-to-peak, RMS, or whatever is appropriate for your appplication.

Flatness
Absolute limits of variation in loss through the passband of the filter. Includes ripple, slope, and rolloff at band edges.

Passband Flatness
The difference between the minimum loss point and the maximum loss point (usually in dB) over a specified passband or bandwidth. This value includes the ripple and the overall slope of the passband.

Ripple
The deviation from a linear function or fundamental shape. This is usually applied to amplitude, phase, or group delay in filter terminology.

Passband Ripple
The variations in amplitude (transmission loss) in the passband of the filter, superimposed upon the fundamental shape of the passband.

 

Thank you very much....

Now I understand the differnce between the ripple and flatness..