Measure Your Homebrew Crystal Filter

Cryptic letters on the right side are codes for Japanese/Chinese characters.

The handwritten schematics revived.

What do you do when you want to measure your homebrew crystal filter? If you happen to have a spectrum analyser then a possible shematic diagram could be the following. 自作のクリスタルフィルターの特性を測定したいとき、スペクトラムアナライザーを持っているならば、次の図のようにしてはかることができます。

What if you have no spectrum analyser, just like me? Here is a trick to do it with a PC. I found a free FFT software program which makes your PC function as a spectrum analyser. Yes, it is not for the radio frequency but only for the audio frequency. What shall we do?

Superheterodyne down conversion as shown in the next figure is the solution. Assume you built a ladder filter with sveral pieces of crystals with an identical frequency. If so, take another piece of crystal of the frequency and make a local oscillator with it. The local oscillator frequency is then a bit higher than the upper edge of the filter's pass band. Then the pass band RF frequency is down converted by a mixer to audion frequency band which you can deal with by the PC spectrum analyser.



Here follows a schematic diagram for the circuit. As mentioned above, The crystal X for the local oscillator is the same crystal as is used in the ladder filter. R i and R t are the resistors for the impedance matching. taking

R i = Z i-100ohms


R t = Zo

should be roughly OK, where Z i and Zo are the imput and output impedance of the filter, respectively.

The key of this circuit is the white noise generator. If the level of the white noise is not enough, then the measurement gets not reliable. Two possible white noise generators are shown as examples in the insets, where a zenner diode and a voltage regulator are used, respectively. Maybe 78L02 is a bit hard to find, but you can also use 78L05 if you use higher Vcc. Any other commonly used noise generator could be tried as well. You had better repeat trial and error to get the highest noise level.

To the contrary, Q2 and IC1, being on the output side of the filter, should be low noise devices in order to keep the noise floor as low as possible.

回路図はこんな感じです。上に述べたように局発用の水晶はラダーフィルターに使ったものと同じ物を使います。インピーダンス整合用の抵抗として、フィルターの入出力インピーダンスをそれぞれZ iとZoとして、おおざっぱですが

R i = Z i-100ohms

R t = Zo



You connect the output of this circuit and the line-input of your PC. I use the FFT program called WaveSpectra coded by efu available free on the internet. Unfornunately the program is made for Japanese language only, but I am certain you also have the similar thing out there made for your own language(WaveTools might be the English analogue. I am not quite sure, though). Then you may need to set up your sound card to activate your line-input for the WaveSpectra before you start the measurement. Please refer to the instruction manual of the program for the detail of the measurement.

The following is the result of the pass-band measurement of my homebrew 4-element ladder crystal filter made of 3.575MHz crystals. It is actually an average of 100 FFT measurements.

さて、この回路の出力をウインドウズパソコンのライン入力に接続します。音声周波数帯域用スペアナソフトとしては、efu さんの作られたフリーウェアWaveSpectraを使いました。他にも同様なソフトがあると思います。あとは、サウンドカードの必要な設定などをして、測定開始です。測定法の詳細はWaveSpectraのヘルプを参照してください。


Note that the origin of the scale on the abscissa has no meaning because the frequency has been down converted. However, the pass-band width is correct. In the above case, it is something like 350Hz; it depends on the definition which I do not care this time. Besides the pass-band width , correct is the ripple of the flattop.

How about the ordinate? The pass-band attenuation, about -30dB in this case, has no meaning either. If you want to know the attenuation, then you have to measure the reference level by replacing the filter with the 0-dB jumper wires. The top-bottom ratio, something like 50dB in this case, could be not very correct if your white noise generator is weak or your post-filter circuit has non-negligible noises.

Last but not least is tha fact that USB and LSB are reversed in this measurement because the local oscillator frequency is higher than the pass-band of the filter.

Although there are some shortcomings as mentioned above, I would say it is a nice easy method because the pass-band width and the flattop ripple can be measured reliablly. Are these two parameters what you actually wanted to know in hombrewing your ladder filters?

BTW, you may want to know how I designed the ladder filter. Visit the pages of JA9TTT for it. Again, they are written in only Japanese language(*). Why don't you start with learning Japanese first of all?

(*)JA9TTT now added a machine translation utility in his page for foreign visitors.







MINOWA, Makoto
Last revised 2002-07-09.