Crystals for IF Filters
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
CR
Plastic
0.4583
0.43723
30362
4.364
315
11.46
1.000
CR
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
CR
Plastic
2.51947
2.34136
5832
0.795
37.4
99
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?
9
Xtal
HC18/U
9.01715
8.99714
22.74
13.76
5.112
10.883
Special high IP3 units supplied by PA3AKE
9
Xtal
HC18/U
9.0172
8.99713
23.05
13.574
5.17
10.9
IQD type A164A
9
Xtal
HC18/u
9.0159
8.99824
18.18
17.207
4.632
9.6
Sample 1, IQ Designs, unmatched
9
Xtal
HC18/u
9.01712
8.99817
20.604
15.184
4.892
10.3
Sample 2, IQ Designs, unmatched
12
Xtal
HC18/U
12.02021
11.99631
24.121
7.297
5.95
13.221
Significant spurious responses noted
12
Xtal
HC18/U
12.01962
11.99666
16.591
10.61
4.33
12.487
12
CR
2 lead
12.41283
11.71744
2199.81
0.084
18.534
378
Useful for wideband
20
CR
2 lead
20.03948
19.93958
122.53
0.52
12.228
54
Note narrower bandwidth
16
Xtal
HC18/U
16.02179
15.99097
26.14
3.79
6.78
16.7
40
Xtal
HC6/U
40.0036
39.99913
1.726
9.171
7.7254
2.43
Measured at the 3rd overtone freq.
BW
Bandwidth
CR
Ceramic resonator
Cs
Series (motional) capacitance
Cp
Parallel Capacitance
fF
Femtofarads (pF/1000)
Fp
Parallel resonant frequency without any external parallel capacitance
Fs
Series resonant frequency
Ls
Series (motional) inductance
mH
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: