Before we may design a crystal filter it is necessary to know the crystal's data.
Unfortunately this information is not in the datasheet - or we have no datasheet
because the crystals were bought on a market some decades ago ...
Fig.1: A crystal, it's symbol and equivalent circuit
Fortunately these values may be measured using a small adapter - which may be built on the fly.
We measured our crystals using the circuit below :
Fig.2: Measurement Adapter
Fig.3: Measurement Adapter Schematics
The Capacitors used are Mica (Glimmer) obtained from www.reichelt.de
With a jumper either the short or one of the series capacitors ( 15 pF or 33 pF ) may be selected.
The tree positions of the jumper result in 3 different Transmission curves when using
a Network Analyser or Spectrum Analyser with built in generator.
( of course you may use a generator and a hf-voltmeter or anything equivalent ... )
The Amplitude response looks somehow like this :
Fig.4: Measured Amplitude Response S21
You will notice, that the maximum (fs) will move while the minimum (fp) remains constant.
With the short selected we measure the lowest frequency where Transmission
is maximum. The maximum ( with 33 pF selected ) is next and then the
maximum ( with 15 pF selected ).