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)