Crystals for IF Filters

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The bandwidth of an IF filter in a communications receiver/transceiver depends on the type of signal that is to be received - Signel Sideband (SSB) requires a typical bandwidth of 2.4KHz, Morse (CW) about 1KHz, Amplitude Modulation (AM) about 6KHz and Frequency Modulation (FM) between 7.5 to 15KHz depending on the required channel spacing/ maximum deviation. A multimode rig will therefore require sufficient filters to cater for the required operational modes.

If filters are to be home made then the following table will give constructors an idea of what is possible using both quartz crystals and ceramic resonators in 8 section ladder filters:

Freq. MHz

Type

Case

Fp MHz

Fs MHz

Cs fF

Ls mH

Cp pF

Max BW KHz

Comments

0.455

Plastic

0.4583

0.43723

30362

4.364

315

11.46

1.000

Plastic

1.0191

0.97256

7791

3.437

81.406

25.2

1MHz IF was used in the Squires Sanders SSR-1 receiver

1.000

Xtal

HC6/U

1.00099

0.99988

8.252

3070

3.717

0.604

2.450

Plastic

2.51947

2.34136

5832

0.795

37.4

2.45768

Xtal

HC18/U

2.4597

2.45733

7.71

544

3.87

1.332

Purchased in 2008

2.45768

Xtal

HC6/U

2.46075

2.457

15.9

263.6

4.16

2.556

20+ years old

3.2768

Xtal

HC18/U

3.27949

3.27653

4.683

503.8

2.592

1.61

Note the much reduced bandwidth for this case style compared to the next six units

3.39354

Xtal

HC6/U

3.39985

3.39362

9.574

229.7

2.608

3.388

Used in Heath SB-Line filter 404-200

3.3936

Xtal

HC6/U

3.40051

3.39263

23.813

92.417

5.126

4.068

Heath SB-line CIO crystal

3.395

Xtal

HC6/U

3.40155

3.39387

24.2

90.9

5.35

4.1

Heath SB-line centre of IF2

3.39524

Xtal

HC6/U

3.40179

3.39524

9.774

224.81

2.533

3.562

Used in Heath SB-line filter 404-200

3.3954

Xtal

HC6/U

3.40171

3.39434

27.649

79.515

6.376

4.001

Heath SB-line CIO crystal

3.3964

Xtal

HC6/U

3.40316

3.39542

26.353

83.371

5.78

4.046

Heath SB-line CIO crystal

7.800

Xtal

FT243

7.81819

7.81718

1.822

227.495

7.051

0.54

WWII era?

Xtal

HC18/U

9.01715

8.99714

22.74

13.76

5.112

10.883

Special high IP3 units supplied by PA3AKE

Xtal

HC18/U

9.0172

8.99713

23.05

13.574

5.17

10.9

IQD type A164A

Xtal

HC18/u

9.0159

8.99824

18.18

17.207

4.632

9.6

Sample 1, IQ Designs, unmatched

Xtal

HC18/u

9.01712

8.99817

20.604

15.184

4.892

10.3

Sample 2, IQ Designs, unmatched

Xtal

HC18/U

12.02021

11.99631

24.121

7.297

5.95

13.221

Significant spurious responses noted

Xtal

HC18/U

12.01962

11.99666

16.591

10.61

4.33

12.487

2 lead

12.41283

11.71744

2199.81

0.084

18.534

378

Useful for wideband

2 lead

20.03948

19.93958

122.53

0.52

12.228

Note narrower bandwidth

Xtal

HC18/U

16.02179

15.99097

26.14

3.79

6.78

16.7

Xtal

HC6/U

40.0036

39.99913

1.726

9.171

7.7254

2.43

Measured at the 3rd overtone freq.

Bandwidth

Ceramic resonator

Series (motional) capacitance

Parallel Capacitance

Femtofarads (pF/1000)

Parallel resonant frequency without any external parallel capacitance

Series resonant frequency

Series (motional) inductance

millihenry

Xtal

Quartz crystal

This table comes with a health warning:- mostly just one example of each type was measured so the information is presented as typical of what is possible. The reader should make their own detailed measurements during the design phase once they have chosen their preferred technology.

Modern crystals in the 2-3MHz range in HC18/U (miniature) holders appear to have characteristics that produce significantly narrower pass bands than their older HC6/U equivalents. There is an obvious inverse relationship between the LC ratio of the equivalent motional components Ls and Cs of the crystal and the maximum bandwidth that can be achieved with a ladder filter - particularly clear with the crystals in the 3MHz range. The ratio of Cp to Cs also has a considerable effect on the achievable bandwidth.

Ladder filters have an asymmetric frequency response (slower attenuation curve on the low frequency side of the pass band) which becomes worse as the bandwidth is increased for a given filter. The maximum bandwidth is what is possible for each type of crystal or ceramic resonator is shown in the table but users must ensure that the overall response is satisfactory for their specific requirements.

Where a greater bandwidth is required than can be achieved with ladder filters then half or full lattice filters must be used.

Crystal parameters and maximum bandwidth figures were obtained using the calculators on two adjacent pages on the Giangrandi web site. These calculators allow the user to enter four measured crystal parameters and determine the component values and frequency responses for filters using from two to eight sections - a great time-saver.

Keys: