![]() ![]() ![]() The filter should be implemented in particular hardware or softwareĪn important parameter is the required frequency response.The computational complexity of the filter should be low.The filter should be localized (pulse or step inputs should result in finite time outputs).The filter should have a specific impulse response.The filter should have a specific phase shift or group delay.The filter should have a specific frequency response.Typical requirements which are considered in the design process are: 3.1 Simultaneous optimization in both domains.2.3 Discontinuities versus asymptotic behaviour.Although filters are easily understood and calculated, the practical challenges of their design and implementation are significant and are the subject of advanced research. The design of digital filters is a deceptively complex topic. Certain parts of the design process can be automated, but normally an experienced electrical engineer is needed to get a good result. The filter design process can be described as an optimization problem where each requirement contributes to an error function that should be minimized. The purpose is to find a realization of the filter that meets each of the requirements to a sufficient degree to make it useful. ( December 2012) ( Learn how and when to remove this template message)įilter design is the process of designing a signal processing filter that satisfies a set of requirements, some of which are contradictory. Please help to improve this article by introducing more precise citations. When I analyzed this 9 Mhz design, I used the motional parameters from a 6 Mhz crystal I had data for.This article includes a list of general references, but it remains largely unverified because it lacks sufficient corresponding inline citations. Here’s a simple 4-pole, 2 kHz BW design. Note: If you build this design, use your own measured crystal parameters. Add xtal/cap sections one at a time until you get the filter complexity and performance you’re looking for. This matching process using variable resistor terminations will have large filter loss during development (25-30 dB), but you’ll get it all back when you match the filter impedance to the circuit load using either a transformer or LC networks. so start off with a bandwidth about 1.5 x what you want to end up with. Be aware that the filter bandwidth will shrink as you add xtal/cap sections. Lower or raise the cap values for wider or narrower bandwidth respectively, re-adjusting the termination resistor values as you go. It will be higher for an SSB filter and lower for a CW filter. This is your desired termination impedance. Adjust the pots for minimum ripple, or better yet, best match in the passband, then measure the pot values. #Crystal filter design seriesFor development purposes, put a 500 ohm pot in series with each of your 50 ohm test equipment outputs/inputs and initially set them to mid-way. Since all caps are equal, you can start off with arbitrary cap values, say 50-80 pf fo an SSB filter, proportionally higher for CW. ![]() This is a good rule of thumb and isn’t peculiar to this filter topology.Ī simple way to build up a filter is to design a 2-pole unit (no shunt crystals, 1 shunt cap) first and get it working. I find that 6 poles are really necessary for good sideband rejection. Note that the extra paralleled crystals on the end sections don’t count as filter poles, so a 4 pole filter has 6 crystals. All capacitor values are equal and it isn’t as termination sensitive as some other filter typologies. The QER filter is a special case of ladder filter that has a very smooth passband response and is very scalable just by adding additional center sections. ![]()
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