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Experiments with crystal filters

I have made several types of crystal filters. All crystals type LSB Japanese production. To assess and image frequency characteristics filter I introduced the concept of "factor sharpness" and "bandwidth". This is my personal definitions that differ from those of professional terminology. See Fig.1
All filters are made on the crystals frequency 10.645 MHz.

I have tested the following 6 types of filters:

  • Fig2 : 1 crystal : a simple filter (proposed W1FB)
  • Fig3 : 2 crystals : basic scheme
  • Fig4 : 4 crystals : basic scheme
  • Fig5 : 4 crystal : with large capacitors
  • Fig6 : 4 crystal : with more capacitors
  • Fig7 : 4 crystal : small capacitors Fig1. shows the definitions on this page.
    f1 = bandwidth.
    Single-chip filter (fig.2) Point Frequency Voltage Attenuation No. MHz V dB 1 10.62337 0.06 -20 2 10.64392 0.54 -0.9 3 10.64510 0.6 0 4 10.64641 0.54 -0.9 5 10.65474 0.06 -20 charactor
    * impedance 500 Ohm (at least in the center of the passband)
    * bandwidth f2=2.49 kHz
    * factor sharpness 8%
    * flatness (ripple) - no
    * this filter is good for AM receivers Two-crystal filter (fig.3) Point Frequency Voltage Attenuation No. MHz V dB 1 10.63578 0.055 -20 2 10.64400 0.5 -0.9 3 10.64488 0.55 0 4 10.64592 0.53 -0.3 5 10.64628 0.54 -0.1 6 10.64696 0.5 -0.9 7 10.65083 0.055 -20 charactor
    * the impedance of 82 Ohms (min in the center of the passband)
    * bandwidth f2=2.96 kHz
    * factor sharpness 20%
    * uneven bandwidth 0.3 dB
    * this filter is good for simple SSB receiver. Triple filter (fig.4) Point Frequency Voltage Attenuation No. MHz V dB 1 10.64309 0.05 -20 2 10.64441 0.45 -0.9 3 10.64484 0.5 0 4 10.64564 0.43 -0.13 5 10.64650 0.5 0 6 10.64693 0.45 -0.9 7 10.64786 0.05 -20 charactor
    * impedance 27 Ohms (min in the center of the passband)
    * bandwidth f2=2.52 kHz
    * factor sharpness 52%
    * this filter is good for SSB receivers and transmitters. Chetyrehkolenny filter (fig.5)
    with large capacitors Point Frequency Voltage Attenuation No. MHz V dB 1 10.64319 0.04 -20 2 10.64408 0.36 -0.9 3 10.64509 0.4 0 4 10.64555 0.36 -0.9 5 10.64618 0.058 -20 charactor
    * impedance 21 Ohms (min in the center of the passband)
    * bandwidth f2=1.47 kHz
    * a factor in the severity of 49%
    * this filter is good for narrow-band filters in SSB receivers. Chetyrehkolenny filter (fig.6)
    with more capacitors Point Frequency Voltage Attenuation No. MHz V dB 1 10.64350 0.02 -20 2 10.64373 0.18 -0.9 3 10.64395 0.2 0 4 10.64405 0.18 -0.9 5 10.64473 0.02 -20 charactor
    * impedance 40 Ohm (min in the center of the passband)
    * bandwidth f2=0.32 kHz=320Hz
    * factor sharpness 26%
    * this filter is good for narrowband filters CW receivers.

    Chetyrehkolenny filter (fig.7)
    small capacitors Point Frequency Voltage Attenuation No. MHz V dB 1 10.64219 0.05 -20 2 10.64493 0.40 -1.9 3 10.64668 0.35 -3.1 4 10.64848 0.55 0 5 10.64906 0.50 -0.9 6 10.65014 0.05 -20 charactor
    * impedance 82ohm (min in the center of the passband)
    * bandwidth f2=4.13 kHz
    * factor sharpness - 52%
    * this filter is good for AM and narrowband FM receivers.

    My findings:

  • A large number of crystals improves the shape of the frequency characteristic of the filter (increases Squareness).
  • Large capacitors make a narrower bandwidth.
  • Input impedance of this type of crystal filter of about 10 Ohms.
  • Impedance changes in the bandwidth limits.
  • Minimum impedance is in the center of the bandwidth.

    Author: Kazuhiro Sunamura, JF1OZL, Japan, translation of Nikolai Bolshakov, RA3TOX; Publication: N. Bolshakov, rf.atnn.ru

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