I can wind my own inductors, but therein lies a whole world of pain of its own and I’d rather not go there. It’s an extremely idealised graph, and experience has taught me that real-world filters using these designs have a lower-frequency cut-off point, but for our purposes here it’s a good enough start.Īs previously mentioned, the component values are not preferred ones from a commercially available series, so I can’t buy them off the shelf. Plug in the required figures and it spits out a circuit diagram, which we can then simulate to show a nice curve with a 3dB point right on 30MHz. The filter we’re designing is simple enough, a 5th-order Bessel filter, and the software is the easy-to-use QUCS package on an Ubuntu Linux machine. ![]() The idealised graph produced by QUCS for our filter. So it’s worth taking a look at the process here, and examining the effect of tweaking component values in this way. The software calculates ideal inductances and capacitances for the desired cut-off and impedance, and in selecting the closest preferred values we modify the characteristics of the result and possibly even ruin our final filter. The results are good, and anyone can become a filter designer, but as is so often the case there remains a snag. Happily as with so many other fields, in recent decades the advent of affordable high-power computing has brought with it the ability to take the hard work out of filter design, Simply tell some software what the characteristics of your desired filter are, and it will do the rest. The 30MHz low-pass filter, as QUCS delivered it. There are tables and formulae, but even after impressive feats of calculation the result can often not match the expectation. It’s a network of capacitors and inductors usually referred to as a pi-network after the rough resemblance of the schematic to a capital Greek letter Pi, and getting them right has traditionally been something of a Black Art. Use the drop-down combo box to select the primary capacitor values for C1 and C2 and observe the cutoff frequency and required inductor for a 50 ohm inductive reactance at the cutoff frequency.If you are in any way connected with radio, you will have encountered the low pass filter as a means to remove unwanted harmonics from the output of your transmitters. Standard value caps are given plus a second capability which allows the user to parallel additional capacitor values to allow a variety of cutoff frequencies to be chosen. This app allows the user to build filters from the capacitors that you have on hand. Two filters may be connected together in series for further lowpass attenuation. Suggested uses include following VFO’s or lowpass filtering transmitter output. This small app is for designing simple 50 ohms input and output impedance pi filters for RF applications. Download Pi Filter Designer 50 Ohm Operational Notes PI Filter Designer: No other impedance scan be calculated with this program. In addition, the user may select an additional capacitor value to put in parallel with both caps C1 and C2. The filter 3 dB cutoff frequency and required L1 inductance are automatically calculated and displayed. ![]() This program allows the user to design simple lowpass filters by selecting from a variety of standard capacitor values either empirically or to suit what you have on hand. PI Filter Designer is a simple 3 element 50 ohm input and output impedance pi filter designing application.
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