1. 4/28/2017
Crystal Filters
Company confidential
Company Confidential

2. Contents What is Quartz Crystal? What is Filter?
4/28/2017
Contents
What is Quartz Crystal?
What is Filter?
What is crystal Filter?
Why Crystal Filter?
Advantages and Disadvantages
Types of Crystal filters
Quartz crystal design parameters
Crystal filters applications
Typical BOM of crystal filter
Crystal filter Specifications
Crystal filter parameters
Present Design status
Action plan
Company confidential
Company Confidential

3. Specifications:
Frequency                 : MHz( Freq Range: 100MHz to 140MHz)
Band width at 3 dB :<20kHz
Band width at 1 dB :>6kHz
Ripple                        :<1dB(70% of 3dB BW)
Insertion Loss           :<5dB
Stop Band Attenuation
Fc ± 100kHz               :50dB min
Fc±1 to 20MHz         :70dB min
Center Frequency tolerance :1kHz
Spurious                     :45dB
Package dimensions :40 x20 x15mm3

4. Schematic and Test Results:

5. 1.What is Crystal Filter?
A Crystal filter incorporates crystals as highly selective tuned circuits in the filter network.
Company confidential

6. 2.Why Crystal Filter?
Crystal Filters consume zero DC power, less cost, can have small signal losses while being extremely selective and almost impossible to overload.
Company confidential

7. 3. Advantages and disadvantages of crystal filters?
Crystal resonators have very high Qs and excellent temperature and aging characteristics. These benefits are translated to the filters so that very narrow bandwidths and highly selective filters can be achieved.
The change of frequency with temperature can be as low as ±20 ppm over a full military (-45 to + 85ºC) temperature range.
The aging of narrow and intermediate band crystal filters is almost solely dependent upon the aging of the crystals themselves. Thus, after proper conditioning, it is possible for the filters to age no more than a fraction of a part-per million per year. This makes crystal filters ideally suited for phase matched applications.
Because of the high Q values available from the crystals (Qs of up to a million are possible with Qs of 100k being typical) very narrow and very selective filters can be made.
Company confidential

8. 3. Advantages and disadvantages of crystal filters?
Bandwidths as narrow as 0.001% are routinely produced.
Filters with shape factors (60/3 dB) of as low as 1.015:1 have been built.
Disadvantages of Crystal Filters
There are two basic problems associated with crystal filters: spurious responses and non-linear drive level responses.
The spurious responses are caused by, inharmonically related, modes of vibration in the crystal. They appear in the filter as narrow responses that degrade whatever band they appear in.
For example, if they appear in the stopband, the attenuation is decreased. but if they appear in the passband they will introduce an unwanted notch. Generally, spurious responses are very narrow and don’t cause a problem unless an unwanted signal appears at their frequency. They have a minimal impact on the overall noise bandwidth.
Company confidential

9. 3. Advantages and disadvantages of crystal filters?
The non-linear drive level causes a series of problems.
First it limits the drive level to a maximum of +10 dB with a recommended drive of –10 dBm max.
Unless the crystals are carefully designed and manufactured the Q and frequency can change as a function of drive and the Q could halve as the drive level was changed from –10 to –60 dBm.
Since crystal filters must operate over wide drive level ranges they should be tested over all expected drive conditions.
The non-linear drive condition is the main cause of Intermodulation distortion in crystal filters.
Company confidential

10. 4.Types of Crystal Filter
Band pass, band reject and notch filters are popular in using crystals for designing
There are Three types of crystal filters
Single crystal filter
Half lattice crystal filter
Ladder Crystal Filter
Company confidential

11. 4.Types of Crystal Filter
Single Crystal Filter
This type of RF filter was developed in 1930s
used in early receivers dating from before the 1960s but is rarely used today
Its response is asymmetric
It is too narrow for most applications
Having a bandwidth of a hundred Hz or less
Schematic here is filter using single crystal
Variable capacitor in the circuit is used to compensate
for the parasitic capacitance in the crystal
Company confidential

12. 4.Types of Crystal Filter
Half lattice Crystal Filter
This form of band pass RF filter provided a significant improvement in performance over the single.
parasitic capacitances of each of the crystals cancel each other out and enable the circuit to operate satisfactorily.
By adopting a slightly different frequency for the crystals, a wider bandwidth is obtained.
slope response outside the required pass band falls away quickly, enabling high levels of out of band rejection to be obtained
Typically the parallel resonant frequency of one crystal is designed to be equal to the series resonant frequency of the other.
Despite the fact that the half lattice crystal filter can offer a much flatter in-band response there is still some ripple.
Company confidential

13. 4.Types of Crystal Filter
Half lattice Crystal Filter
The response has a small peak at either side of the centre frequency and a small dip in the middle.
Rough rule of thumb it is found that the 3 dB bandwidth of the RF filter is about 1.5 times the frequency difference between the two resonant frequencies.
For optimum performance the matching of the filter is very important.
To achieve matching, matching resistors are often placed on the input and output.
If the filter is not properly matched then it is found that there will be more in-band ripple and the ultimate rejection may not be as good.
A two pole filter (i.e. one with two crystals) is not normally adequate to meet many requirements.
The shape factor can be greatly improved by adding further sections.
Typically ultimate rejections of 70 dB and more are required in a receiver.
As a rough guide a two pole filter will generally give a rejection of around 20 dB; a four pole filter, 50 dB; a six pole filter, 70 dB; and an eight pole one 90 dB
Company confidential

14. 4.Types of Crystal Filter
Half lattice Crystal Filter
Schematic diagram of half lattice crystal filter
Company confidential

15. 4.Types of Crystal Filter
Crystal Ladder Filter
For many years the half lattice filter was possibly the most popular format used for crystal filters.
More recently the ladder topology has gained considerable acceptance.
In this form of crystal pass band filter all the resonators have the same frequency
Inter-resonator coupling is provided by the capacitors placed between the resonators with the other termination connected to earth.
Schematic diagram of crystal ladder filter
Company confidential

16. 5.Quartz crystal filter design parameters
Input, output impedance, bandwidth, crystal Q, Shape fact and cutoff
These factors are dependent upon the number of poles (equivalent to the number of crystals), their Q value, and their individual frequencies.
Maximum bandwidth that can be achieved is controlled by the filter impedance and also the spurious responses that are present in the individual quartz crystal elements
The location of the important responses for quartz crystal filters can be controlled by the size of the plates deposited onto the crystals. By making them smaller the responses also become less critical.
This means that the quartz crystal filter will need impedance transformers at the input and the output. The down side of this is that the impedance of the overall quartz crystal filter rises. This obviously needs to be avoided if at all possible, but for wide band filters it is often the only option.
Flat, polished, fundamental, overtone, and inverted mesa crystals can be used in band pass crystal filters.
Company confidential

17. 6.Softwares required for crystal design
Wes Hayward’s XLAD and GPLA programs were originally sold commercially, then included in DOS versions with IRFD, then Windows versions in EMRFD.
AADE’s Filter Design program was originally commercial but became free ware recently.
Both perform design and provide analysis (response curves).
Company confidential

18. 8.Crystal Filters Applications
Radio communications receiver for IF selectivity
Transponders
Radars
Carrier telephone equipment
Navigation equipment.
Company confidential

19. 9.Typical BOM of crystal filter
Bill of material for a typical filter might look like this:
Company confidential

20. Few Manufacturers of crystal filters
Rakon
Vectron
Fox
NDK
FILTRONETICS
Company confidential