Crystal ladder filters – measuring crystal parameters

Values of crystal parameters needs to be determined to correctly  design crystal ladder filter.

Measurement examples are for custom ordered resonators 8214 i 8215kHz (RS-3017X) made by polish company Omig.


1) Selecting crystals based on series resonance frequency (fs)

– crystals for CW filter should be selected to have fs +/- 10Hz
– for SSB filter +/- 50Hz
In general allowed difference between crystals fs is +/-2% of designed filter bandwidth.

To measure fs we can use wobbluscope (fs is a maximum signal frequency) or simple oscillator and frequency meter with resolution at least 10Hz.
When using wobbluscope, we can also measure all crystal parameters using 3dB method.

It is important to not mix the crystals after measurements. Small plastic box, in which crystals were delivered, was very helpful.

2) Crystal parallel capacitance (Cp)

It is not necessary to measure Cp, but it allows to compute filter elements more accurately. Not all filter design programs allows to insert Cp value.

To measure Cp, C meter with 10pF range is needed. I used  LC meter by VK3BHR.

We can assume Cp=4pF if we don’t have C meter.

You should measure couple of resonators from entire set and calculate average value.

The measured RS-3017X resonators had Cp=5.8pF.

3) Measuring crystal parameters using 3dB method.

Wobbluscope is needed for this method. I used NWT7 v2.1, you can use any amateur or professional wobbluscope. It can also be substituted with precise hf generator, accurate frequency meter and hf signal meter.

It is very easy to determine series (maximum signal) and parallel (minimum signal) resonance.

To measure crystal parameters, typical 50 Ohm input/output impedance can be used. More precise results can be achieved using lower impedance. Most of crystals at IF frequencies has too low series resistance to achieve exact results at 50 Ohm. 50 Ohm.

To measure at 12,5 Ohm you can use two bifilar transformers (10 bifilar windings on FT37-43 core).

Matching to lower impedance can be also achieved using attenuators. Values of Pi attenuator (50 to 10 Ohm) were computed using online calculator. When using resistor values from E12 series input impedance is 52 Ohm and output is 10,7 Ohm. R1 i R6 have such big values that the can be omitted.  The disadvantage of this solution is high attenuation – about 25dB.

Using both types of matching you cannot forget about wobbluscope calibration (replace crystal with wire jumper).

I used attenuator match (Z=10,7 Ohm) for measuring my crystals.

Attenuation, 3dB bandwidth, and series resonant frequency fs (fm) measured by wobbluscope software.

Be careful: when amplitude peak is flat, then software is assuming lowest frequency with maximum signal as a fmax. It is better to take fm (middle frequency of 3dB bandwidth) as a fs or use cursor and select series resonance frequency ,manually.

Crystal parameters computed using 3dB module of “Dishal” software:

It is good to measure all crystals in a set. Some of crystals can have lower Q (more attenuation at fs). For crystal ladder we need to select crystals with highest Q and fs in range of +/-2% filter bandwidth.

The results for three selected crystals from a set. You can see differences in crystal Q:

X1:  Lm=13,31mH  Rm=4,7  Q=146279
X2:  Lm=13,96mH  Rm=7,1  Q=100798
X3:  Lm=14,0mH  Rm=6,0  Q=119682

Average values:

Rm=6,0 Ohm

4) Measuring crystal parameters using G3UUR method

This simple method is based on measuring frequency of oscillator with crystal connected directly and through capacitor. This allows to compute motional inductance  Lm and capacitance Cm of crystal series resonance.
Knowing Lm (or Cm) and fs is enough to design filter using software “LCFD” i “Dishal”.
Disadvantage of this method is no possibility to determine crystal series resistance Rs and Q.

Schematics of oscillator circuit can be found in LCFD software.

Circuit can be mounted on universal board.

Capacitance values are not critical, you should insert into blue fields actual values used. Cs should be also specified (default value can be left). LCFD Software will compute crystal; parameters after entering measured values of F1 and F2.

Dishal software also contains module for G3UUR method.

The results:

LCFD – Lm=13,570mH
Dishal – Lm=14,116mH

LCFD – Lm=13,560mH
Dishal – Lm=14,114mH

Average value Lm= 13,8mH

To provide accurate results it is important to know exact value of capacitor in series with crystal.


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