Figure 1 - parameters of crystal for a crystal filter
LAST MODIFIED:
The LC and crystal filter software was written by Neil Heckt of Almost All Digital Electronics as an aid for filter designers to simply plug in various filter parameters and hey presto! there is your finished design. Shortly I will present you with the design example of a crystal filter of the crystal ladder filters variety as designed on my evaluation copy of the downloaded software.
DISCLAIMER: I have no connection with Neil or www.aade.com apart from being friends. I receive no remuneration whatsoever for this topic or any registrations of this software.
This downloadable software is available from my friend Neil Heckt www.aade.com. Neil also provided us with his famous and very popular inductance and capacitance meter project kit featured on one of my other sites.
I am quite certain that this LC and crystal filter software will prove equally popular, especially due to the modest cost of $24.95 but for evaluation purposes you may use the software ten times for free.
I was quite thrilled to see a lot of mathematical drudge taken out of the design of a crystal filter by this unique software. Unfortunately you can never get away from from the fact that you must first of all measure the parameters of the low cost crystals you are using.
The usual design procedure proceeds as follows:
Obtain a good selection of the same frequency surplus crystals. Let's say we can obtain very cheaply some 5 Mhz crystals. We need to determine the crystal parameters in figure 1 below.
Figure 1 - parameters of crystal for a crystal filter
Now your crystal parameters can be measured relatively easy, albeit indirectly. The crystal is placed between an accurate and stable signal source of known impedance and a load of known impedance, usually a 50 ohm detector. The signal source needs to be at least 10 mW, stable, preferably 50 ohms, capable of fine tuning and monitored by a quality frequency counter with 1Hz resolution.
The crystal parameters starting with Cp can now be measured. Cp can be easily determined with a capacitance meter operating at a frequency far removed from the crystal frequency, it's as simple as that.
The signal source is slowly adjusted until a peak response is noted, this is series resonance, Fs where both the inductive reactance and capacitive reactance of the crystal cancel one another. The crystal is then replaced by a small value variable resistor and adjusted for a similar response in the detector. The value determined by the variable resistor is Rs in figure 1 above.
Next the crystal is reinserted and swept both sides of centre frequency to determine the 3 dB points which gives us the loaded bandwidth Bw or indirectly QL.
Figure 2 - crystal loaded bandwidth
As the crystal is slowly swept slightly higher in frequency, maybe only a 1,000 Hz or more a pronounced dip will be noted, this is the anti resonance or parallel resonant, Fp frequency where the effect of Cp effectively in series with Cm, resonates with Lm.
Now I could lay some heavy maths on you on how to determine Lm and Cm BUT with Neil's crystal ladder filter software that is not necessary at all.
Open up your software and, for evaluation users, you get to do this ten times only. This is perfectly reasonable and ample for evaluation. Considering the overall capability of the filter software, and we're only scratching the surface here, it must be well worth the $24.95 registration fee.
Click on design and in the drop down menu select crystal ladder. Then for this illustrative example we'll select Chebyschev. Another menu is displayed where we can select the parameters known to us. Note that where I use Lm for motional inductance, Neil chose to refer to it as Ls for series inductance, similarly my Cm is referred to as Cs. The available known parameters are:
Cp, Ls, Cs, and "Q" are known
Cp, Ls, Cs, and Rs are known
Cp, Fs, Fp, and Rs are known
Cp, Fs, Fp, and Bw are known
Cp, Fs, Rs, and "Q" are known
Now from our test set up above we were able to determine the following paramaters in the order I mentioned: Cp, Fs, Rs, Bw and Fp. Armed with this information we'll elect to use Neil's third option of Cp, Fs, Fp and Rs known.
To better illustrate the example of using this software I'm going to accept the defaults the program offers for illustration purposes. Here the defaults were Cp = 4.5 pF; Fs = 5.723998 Mhz; Fp = 5.736073 Mhz and; Rs = 14.63 ohms. In accepting those figures (we could of course have entered our own actual values) we proceed to another screen where I accepted a 5th order filter with a 2 Khz bandwidth. I then elected to go for an untuned filter with capacitive coupling. Lo and behold in a matter of moments we have a schematic of a crystal ladder filter which I have screen captured and reproduced in figure 3 below.
Figure 3 - schematic of a crystal ladder filter
Notice the weird capacitor and source / load values in the crystal ladder filter which result? Firstly, your source and load values of 232 ohms must be correctly matched to earlier and later stages for the filter to operate correctly. This rule applies to ANY filter. Secondly the capacitor values could be obtained by series / parallel combinations or even with trimmers if you have a swept oscillator and CRO to complete the tuning process.
You need to go to Neil' site and this dowload page - http://www.aade.com/download.htm and follow the instructions. Don't forget to pay the registration fee!
I would recommend anyone seriously interested in crystal filters to look up or obtain a copy of a very good article "A unified approach to the design of crystal ladder filters" written by Wes Haywood and appearing in the ham radio magazine QST in May 1982 (page 21).
Feedback to me about this page on Filter software.
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Updated 27th July, 2000