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:

*Lm*=13,8mH

*Rm*=6,0 Ohm

*Q*=120000

**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:

8214kHz

LCFD – *Lm*=13,570mH

Dishal – *Lm*=14,116mH

8215kHz

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.

Tags: crystal filters, crystal ladder, crystal parameters, measurement

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