Crystal Measurements for IF Filters

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1. A series of measurements was made on a range of crystals and a few ceramic resonators and the results are shown in the table below. These results are useful for:

The measurement set up is shown first.

1

2

3

4

5

6

7

8

9

10



Series Resonant Frequency in MHz with the following value

for C1 in pF

Parallel

Resonance

Freq.

MHz

Type/Pkg

4p7

10p

27p

56p

120p

220p

Short circuit

Short circuit

2.45

CR

2.5022

2.4835

2.4482

2.4182

2.3902

2.3741

2.3506

2.5245

2.4576

X / 3

2.4584

2.45798

2.45764

2.4575

2.45744

2.4574

2.45738

2.45993

2.5

X / 2

2.50091

2.50044

2.50005

2.49989

2.4998

2.49977

2.49972

2.5026

3.39

X / 1

3.39151

3.39078

3.39018

3.3899

3.39876

3.3897

3.38962

3.39415

3.39211

X / 1

3.39347

3.39276

3.39216

3.3919

3.39175

3.39169

3.39163

3.39635

3.39895

X / 1

3.40072

3.3998

3.39903

3.3987

3.39853

3.39844

3.39834

3.4043

3.39895

X / 1

3.40074

3.3998

3.39903

3.39869

3.39851

3.39842

3.39832

3.40443

3.57954

X / 1

3.5817

3.5804

3.57925

3.57877

3.57852

3.57838

3.57825

3.58613

7.055

X / 1

7.06077

7.05813

7.05558

7.05438

7.05364

7.05336

7.05039

7.0677

9

X /3 a

9.00682

9.00362

9.00078

8.9996

8.99885

8.99865

8.99824

9.0159

9

X / 3 a

9.00726

9.0038

9.00094

8.99953

8.99887

8.99861

8.99817

9.01712

9

X / 5 b

9.00757

9.00407

9.00083

8.9994

8.99858

8.99821

8.99776

9.01696

10

X / 4

10.00812

10.00424

10.00084

9.99938

9.99848

9.99816

9.99789

10.01909

12

X / 4

12.00825

12.00437

12.00076

11.99896

11.99794

11.99755

11.99707

12.01772

12

CR

12.26818

12.16535

11.999

11.8937

11.81071

11.7739

11.71744

12.41283

14

X / 3 *

14.01561

14.01035

14.0057

14.0037

14.00242

14.00196

14.00113

14.02909

20

X / 4 *

20.01398

20.00707

20.0014

19.99847

19.99724

19.99675

19.99586

20.0326

20

CR

20.00876

19.9901

19.96921

19.9562

19.94966

19.94618

19.93958

20.03948

37.5

X/SMT *

N/A

37.501

37.492

N/A

N/A

N/A

37.484

37.547

Type/Package Keys:

CR

Ceramic Resonator

X

Quartz crystal

*

High side spurious noted

1

HC6/u solder seal

3

HC18/u solder seal

5

HC18/u size resistance weld

2

HC6/u size cold weld

4

HC18/u size cold weld

a

IQ Designs non matched series

b

PA3AKE low dust high IP3 type

N/A

Not available

SMT

Surface mount

Notes:


2. If you build a number of different bandwidth ladder filters using the same frequency crystals you will find that the centre frequencies are different - wider bandwidth filters will have higher centre frequencies. This is due to the low frequency edge of each filter being primarily dependant on the series resonant frequency of the crystals. So to increase the centre frequency of a narrower bandwidth filter to match that of a wider bandwidth filter you need to increase the series resonant frequency of the crystals in the narrow bandwidth filter.

The problem with that scenario is that placing a capacitor in series with a crystal will modify the equivalent motional component values of the crystal and capacitor combination - the lower the value of the additional series capacitor the higher will be the changes in motional values resulting in increasingly unpredictable filter results.

One solution that I have tried is to design and construct the required series of filters and measure the resulting frequency responses including the centre frequency. This will provide a good indication of the increase in frequency required to match the centre frequencies of each filter. From these tests you can then try a series of fixed capacitors in series with the chosen crystals to determine the value required to achieve the increase in frequency for each type of filter. Having determined the capacitor value required for each type of filter you can then measure the new motional values with the series capacitor in place.

As an example, I designed and constructed three different eight section filters with bandwidths of 7.5KHz, 2.1KHz and 1KHz and 0.5dB maximum passband ripple using a collection of low cost HC18/u crystals marked 8.863256MHz selected to have their series resonant frequencies within 100Hz of each other. Measuring the filters showed the centre frequencies to be 8.864792MHz, 8.860506MHz and 8.859488MHz respectively so the 2.1KHz filter centre frequency was 4.286KHz low and the 1KHz filter was 5.304KHz low compared to the 7.5KHz filter.

Tests determined that a series capacitor of 18pF was required for the 2.1KHz filter and 15pF for the 1KHz filter to make their centre frequencies the same as the 7.5KHz filter. As this significantly modified the equivalent motional component values, I redesigned the two filters using these new values. The test results show that the new centre frequencies are now within a maximum of 200Hz of each other and the 2:1 shape factors are at least -70dB below the passband which is a good result for low cost crystals.

The switch is shown in the short circuit position.