#High performance crystal filter design how to#
#High performance crystal filter design software#
#High performance crystal filter design series#
#High performance crystal filter design download#
#High performance crystal filter design free#

From the schematics below 4.44pF(Cp) = Cp1 in parallel with Cc1 & Cc2 in series. This does make a small difference if the crystal case is grounded which is usually done for better stopband attenuation. However part of the capacitance is to the crystal case as shown below when measuring the crystal as a 3 terminal device. The crystal capacitance Cp is generally thought of as being across the crystal terminals. You can do this calculation with a hand calculator, spreadsheet, or there are web calculators to do the L-network matching. This requires a shunt capacitor or 95.57pF and a 1.431uH inductor. By removing the capacitors we now try to match 256.1 + j178 to 50 ohms. The 256.1 ohm impedance includes both C7 and C8 (62.3pF = -j178 14.318MHx) in the above schematic. To match the input and output of the filter to 50 ohms is pretty easy. To use the filters with RF test equipment 50 ohm impedances are needed rather than 256.1 ohms.

#High performance crystal filter design download#

To download the LTSpice schematic click here and right click to save as. Notice how LTSpice treats crystals as capacitors with value Cs and all the other crystal parameters are parasitic properties of the capacitor. The matching networks absorb capacitors C7 and C8 reducing component count. The circuit below is the LTSpice simulation of the original filter with 256.1 ohm terminations and below it the same filter with L-networks matching the filter to 50 ohms.

#High performance crystal filter design free#

The filter was analyzed in LTSpice which is free and available at. The program produced a filter which had the 5 crystals, 7 capacitors, and expected terminal impedances of 256.1 ohms. In the program I chose a crystal ladder filter, 5th order, Butterworth, bandwidth 3800Hz. The AADE Filter Design program expects ALL the crystals to have identical parameters. Let's design a 5 crystal ~14.318MHz bandpass filter using the values in the listing above having *** next to them. I usually buy about 25 crystals and can easily make 2 filters.Д¤.) Simulate the resulting filter and adjust slightly if needed.ДҐ.) Match the filter to the desired input and output impedances using transformers or L-networks.Д¦.) Construct the filter using good RF construction techniques.

#High performance crystal filter design series#

The crystals cut to be used in a parallel resonant circuit will have series resonances below 10.000MHz by a few to several kHz.Дў.) Buy a group of crystals to measure so that you can pick ones with similar parameters. all have different series resonant frequencies. 10.000MHz crystals designed as series mode, 10pF parallel, 18pF parallel etc. Here are the basic steps:ДЎ.) Pick a frequency which is a common crystal frequency but near to the frequency you want. I have built about a dozen filters and can design, construct and test one in a few hours. Also consider some of the special crystal runs by the QRP clubs for crystals in the 7.040 to 7.060, 14.040 - 14.060, and other CW frequencies which are still reasonably priced. I have found that HC-49/U crystals usually have higher Qs than the smaller HC-49/US packages. Crystals (not crystal oscillators) are used with the resulting filter frequency usually a few kHz different than the number stamped on the crystal case. I generally match crystals first on Fs, Fp, and Q.ДЄssuming that you are not trying to design a crystal filter for an exact frequency, the procedure is actually quite easy to do.

Notice that all the relevant crystal parameters are included. The table below shows the file created by the AIM 4170. Other VNA's could have the ability to automatically measure crystals but I do not know of any which have the necessary software. Using a small fixture with socket pins, crystals can be measured with precision at a rate of 2 to 3 per minute.

#High performance crystal filter design software#

The measurement of the crystal parameters has been automated in the software for the AIM 4170 VNA (also in the AIM 4170 UHF and VNA 2180). All the programs I have seen require the crystals to have identical parameters. There are several programs which can help design a crystal filter but I generally use the free AADE Filter Design program at which designs several types of crystal ladder filters. The difficulty always centered around measuring and selecting crystals and then designing the circuit.

#High performance crystal filter design how to#

There have been articles on how to design crystal filters in magazines for years.