Experiments with crystal filters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Quartz filters

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I have made several types of crystal filters. All LSB type crystals are made in Japan. To evaluate and display the frequency response of a filter, I introduced the concepts of "sharpness factor" and "bandwidth". These are my personal definitions, which differ from professional terminology. See Fig.1
All filters are made on crystals with a frequency of 10.645MHz.

I have experienced the following 6 types of filters:

1. Fig2 : 1 crystal : simple filter (suggested by W1FB)
2. Fig3 : 2 crystals : basic circuit
3. Fig4 : 4 crystals : basic circuit
4. Fig5 : 4 crystals : with large capacitors
5. Fig6 : 4 crystals : with even larger capacitors
6. Fig7 : 4 crystals : with small capacitors

	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	character * 500 ohm impedance (minimum at the center of the passband) * bandwidth f2=2.49kHz * spicy factor 8% * unevenness (ripple) - no * this filter is good for AM receivers
	Double 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	character * 82 ohm impedance (min at the center of the passband) * bandwidth f2=2.96kHz * spicy factor 20% * passband ripple 0.3dB * this filter is good for simple SSB receivers.
	Three-crystal 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	character * 27 ohm impedance (min at the center of the passband) * bandwidth f2=2.52kHz * spicy factor 52% * this filter is good for SSB receivers and transmitters.
	Quadruple 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	character * 21 ohm impedance (min at the center of the passband) * bandwidth f2=1.47kHz * spicy factor 49% * this filter is good for narrow band filters of SSB receivers.
	Quadruple filter (fig.6) with even larger 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	character * 40 ohm impedance (min at the center of the passband) * bandwidth f2=0.32kHz=320Hz * spicy factor 26% * this filter is good for narrowband CW receivers.
	Quadruple filter (fig.7) with 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	character * impedance 82ohm (min at the center of the bandwidth) * bandwidth f2=4.13kHz * pungency factor - 52% * this filter is good for AM and narrowband FM receivers.

My findings:

1. A large number of crystals improves the shape of the filter's frequency response (increases squareness).
2. Large capacitors make for a narrower bandwidth.
3. The input impedance of this type of crystal filter is about 10 ohms.
4. The impedance varies within the bandwidth.
5. The minimum impedance is at the center of the passband.

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

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