questions about tank circuits - Page 1
Forum section: electronics / rf-microwave

author: greenelephant
author_rank: Newbie
timestamp: March 24, 2024, 10:12:57 pm
content: In reference to the three jpeg files attached, how can 3 tank circuits be modified to allow a lower limit of 100KHz? can the first one with capacitor c20 be modified to allow 100khz by changing c20 to 2500 picofarads? I appreciate everyones feedback?

author: jonpaul
author_rank: Super Contributor
timestamp: March 24, 2024, 10:58:52 pm
content: Check Pi and Tee filter designs.Use filter scaling laws but note effect on Zo/Zin in the applicationJon

author: szoftveres
author_rank: Regular Contributor
timestamp: March 24, 2024, 11:39:33 pm
content: The first circuit is a BPF, the other two are LPF, not sure if your question makes much sense. What are your specs?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 25, 2024, 02:30:23 am
content: Quote from: greenelephant on March 24, 2024, 10:12:57 pmThe quick and dirty method is to open up LT Spice, simulate the filters and play with values till it looks kind of right for your application. The second method is to learn a bunch of math and run the calculations to design the filter, or use a suitable calculator tool for the topology.

author: RFDx
author_rank: Regular Contributor
timestamp: March 25, 2024, 04:13:13 am
content: Quote from: greenelephant on March 24, 2024, 10:12:57 pmNo, it's not enough to adjust only C20. Changing C18/C21 to 33nF, L4 to 820uH and C20 to 2.2nF retains the passband ripple and return loss and extends the lower corner frequency to less than 100kHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2024, 05:17:50 am
content: The characteristic frequency of a network is of the form w_0 = 1/sqrt(L*C), and characteristic impedance Zo = sqrt(L/C). Exact element values vary above or below these, according to the filter function designed, and how the elements are interconnected; but the overall network will average (that is, geometric average) to these values.Assuming the designs are correct, and appropriate for whatever impedance system they're embedded within, frequency involves rescaling everything proportionally. As frequency goes down, inductance and capacitance go up.Impedance matching is paramount. None of the three listed, indicate what impedance they are designed for, so they aren't very useful. You don't mention what impedance you're designing for either, so I'm afraid we cannot draw any meaningful conclusions.The second one, for example, seems to be designed for around 280Ω. But this depends on the desired Q, the peaking in the transition band, and could be meant for other values.Tim

author: p.larner
author_rank: Frequent Contributor
timestamp: March 26, 2024, 08:19:23 am
content: search for tonne filter software,comes with a lot of arrl antenna books.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 26, 2024, 09:45:13 am
content: Quote from: T3sl4co1l on March 25, 2024, 05:17:50 amIf you are using a "lowish" impedance, high values of inductance start to look like wire wound resistors, that is, the resistive components of your inductors become high enough to turn your "LC" network into a LCR network, with a quite different Q & passband.Many years ago, using filter charts & formula I built a notch filter for just above 500kHz at an impedance of 50.The charts & formula were happy, but the real world result was nothing like the prediction.That was how I learnt why many filters were designed for higher impedances, with matching sections at the input & output.