Ltp: tft025

version 1.0

19.06.2001

Application Note

TFS 321

3/3

TELEFILTER GmbH

Potsdamer Straße 18

D 14 513 TELTOW / Germany

Tel: (+49) 3328 4784-0 / Fax: (+49) 3328 4784-30

E-Mail:

tft@telefilter.com

VI TELEFILTER reserves the right to make changes to the product(s) and/or information contained herein without notice. No liability is

assumed as a result of their use or application. No rights under any patent accompany the sale of any such product(s) or information.

- The shunt capacitors mainly influences the pass band tilt an increase of the capacity value

will change the tilt to lower attenuation on the high frequency side of the pass band

The coupling coil mainly influences the bandwidth in pass band of the filter

The strategy to match the filter on the customers board should be as follows:

-

match the filter according to theoretical values to 50 Ohm.

Use your final PCB for this matching to be sure to have the stray elements of the PCB

then before.

-

Adjust external coil according to stray elements of the PCB

The external coil influences the bandwidth of the filter by influencing the upper transition

band of the filter only. The position of the lower transition band is not influenced. That

means a wrong value for the external coil will lead to wrong bandwidth and wrong centre

frequency. The group delay of the filter is influenced in the same way. There are two

peaks in group delay behaviour for the matched filter. The peaks in group delay are

located at the pass band edges of the filter. Finally this means that only the higher

frequency peak in group delay is influenced by the external coil. This makes the

adjustment of this coil very easy. Make sure that the distance in frequency of the two

group delay peaks is identical with the distance shown in the delivered plots. (around 340

kHz) The matching of the filter will influence the position of the filter only slightly. The

height of the peaks is influenced mainly.

-

Adjust matching at filter input and filter output

Check VSWR for filter input and output.

-

Carry out the matching for the actual source and load impedance in three steps.

Steps one and two are matching of the filter for input or output to source or load

impedance respectively. The result of the matching of the filter input to the actual source

can be checked via the VSWR at the filter output which is still matched to 50 Ohms and

vice versa.

The third step is the matching of the filter input to the actual source and the filter output to

the actual load.

If you match the filter immediately to your source and load impedance without following the

steps before you will probably find a quite good matching for filter amplitude and group delay

performance. But there is no guarantee that VSWR is as specified. This may increase

insertion loss and the stability of filter parameters in production.

In case of questions please contact us to