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Re: QMX Band Pass Filter (and Band pass filters in general) question
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Hi Jonathan The analysis is too simplistic. The filters can be designed approximately using some theory (such as your calculation) but then in the real world the results differ considerably. There are interactions between the LPF and the BPF, and other components such as the load on the filter, termination impedances, and a lot of parasitic circuit elements such the capacitances of the BPF MUX switch pins, and of the PCB traces etc. Generally since these parasitic elements are ADDING to the desired circuit values, the resonant frequency?realized in practice is normally lower than the value given?by a simple calculation.? In the QMX 80-20m version the lowest filter is used for 40/60/80, the mid filter for 30 and the highest filter is used for 20m. The use of few filters, and wide (single series resonant circuit) is a design compromise for the targeted small size and cost. The fact that the QSD intrinsically has such high performance mitigates the liberties taken with the BPF so the overall performance is still excellent. Other choices of receiver?architecture would differ. For example if using a much weaker mixer such as a <shudder>SA602</shudder> then you would probably like to use a much narrower and sharper filter to better protect the weak mixer.? > I eventually want to have a 5 band circuit made of SMD? > components so I can shrink my design into a portable package? > so I am avoiding the QRP labs Band pass filters since they? > use transformers. That's also a compromise, size vs performance. Trade-offs, trade-offs. Physically smaller inductors have lower Q and lower saturation levels.?We assume that in a receive application, flux density is not important as we aren't passing watts through the toroids however in reality flux isn't something that is zero until a certain level then suddenly kicks in. Smaller inductors can easily become the limiting factor to performance of the whole radio. That depends on the rest of the radio architecture. But it is worth bearing in mind if you are aiming for a high performance front end.? Martein PA3AKE's pages? are an excellent read, and specifically in this instance, his discussion on IMD in toroids:? But in the end it's all trade-offs, if very small size is critical then some sacrifice has to be made.? 73 Hans G0UPL On Thu, Feb 29, 2024 at 9:07?AM Jonathan Kayne, KM4CFT <jzkmath@...> wrote: Hi, |
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