QUCS TUTORIAL DEUTSCH SOFTWARE

If I were using this filter with a real transmitter I would ensure that I designed it with a cut-off at least a quarter higher than the transmission frequency.

As expected the cut-off frequency has shifted up a little, but the same curve shape has been preserved without any ripples appearing or it being made shallower. Having replaced my component values with preferred ones I can run the simulation again, and I can see from the resulting graph that I’ve been quite fortunate in not damaging its characteristics too much. The revised curve from the filter with preferred values. In my case I always pick higher values for that reason: my nearest harmonic I wish to filter is at double the frequency so I have quite some headroom to play with. Particularly with the higher-order filters with more components in the network you can observe the effect of individual components on the gradient at different parts of the graph, but as a rule of thumb making values higher reduces the cut-off frequency and making them lower increases it. What effect will that have on the performance of my filter? Changing each value and re-running the simulation shows us the graph changing subtly each time, and it can sometimes be a challenge to adjust them without destroying the filter entirely. So each of the components in the above schematic needs moving up or down a little way to a preferred value. RS, Mouser, Digikey, Farnell et al exist to save me from such pits of electronic doom, why on earth would I do anything else but buy ready-made? My revised filter circuit with off-the-shelf component values. 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.

QUCS TUTORIAL DEUTSCH SOFTWARE

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.

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.

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