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Homebrew Crystal Filter Design

❶ MEASURE CRYSTAL DATA

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 ).


f_S3 (short) < f_S2 ( C = 33 pF ) < f_S1 ( C = 15 pF )  <  f_p ( parallel )

Enter the measured values below to calculate the crystal data :

parallel resonance freq. fp		[MHz]
series resonance freq. fs3		[MHz] with "short"
series resonance freq. fs2		[MHz] with 33 pF
series resonance freq. fs1		[MHz] with 15 pF
Series Capacitor C1		[pF] with different Cap
Series Capacitor C2		[pF] with different Cap
Cs		[fF]
Ls		[mH]
Cp		[pF]

As we don't dare to stuff your valuable disk-space with a cookie, please note the values
(C_s, L_s, C_p) and proceed to Step 2.

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