TFS321_0106 Datasheet, PDF (3/3 Pages) Vectron International, Inc – Application Note

Ltp: tft025
version 1.0
19.06.2001
VI TELEFILTER
Application Note
TFS 321
3/3
- 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
TELEFILTER GmbH
Potsdamer Straße 18
D 14 513 TELTOW / Germany
Tel: [phone] / Fax: [phone]
E-Mail: [e-mail]
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