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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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5th Feb 2023, 4:00 pm | #1 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
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Inductor tap value ?
Hi Peeps. As some of you may know I am trying to build a 2M / 6 M Transceiver. The 2M P/A is already ok. So I transferred to the 6M part. The Dual filter inductors POST Up converter mixer are two coils back to back, with a two turn coupling loop and a coupling cap to suit the bandpass of 2 Mhz.
However the coupling tap of both resonant parts are a mystery. In the FT290 2 M coils there are only 4 turns in each resonant circuit and they are tapped at 50%. But for 6M the number of turns increases to 9 turns. "Where to put the tap" ? Now as you all know I am neither a theoryetician or mathematician, but I decided to find the Inductance of each coil 400nH and calculated the reactance at 133 ohms. Thus I divided 50R into 133 and got 2.66T or 3T in real terms. Is this the correct approach, or am I way off beam. The sch is for the 2M version, but has been upscaled for 6M.
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Should get out more. Regards Wendy G8BZY |
11th Feb 2023, 5:25 am | #2 | |
Pentode
Join Date: Nov 2021
Location: Toronto, Canada
Posts: 131
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Re: Inductor tap value ?
Quote:
Hi Wendy, I don't know much but not sure your approach is right... What I think I know: When that filter was designed, I think the person did not think of the inductor tap initially... He designed it (2 resonant LC) to give a desired response and this response requires a particular end terminating resistance.. The tap is to transform resistances. Do you mean the component values you posted are for the 6M ? You said 400 nH... You can simulate a double resonator bandpass filter with L of 400 nH(no tap)... Then check what input and output resistance (Rc) gives the desired response Where to put the tap depends on Rc and the end resistances you want (50ohms?) |
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11th Feb 2023, 8:10 am | #3 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 983
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Re: Inductor tap value ?
You need to think of it more like an auto transformer that you are using to transform the impedance from a very high value to one you can work with like 50 ohms.
Probably the easiest way (avoiding laborious calculations etc.) to do it quickly would to use a VNA and adjust the taps until you get the best VSWR in and out. My hunch would be a tap at 2-3 turns.
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11th Feb 2023, 5:53 pm | #4 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 983
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Re: Inductor tap value ?
I should have added, I would terminate the output side with a 50 resistor and measure the VSWR on the input side, then reverse and put the resistor on the input side and measure VSWR looking at the output side. When finished reconnect the FET gate etc.
Another way to do it is to use a sig gen/speccy analyser and tap for best energy transfer.
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12th Feb 2023, 12:27 pm | #5 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
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Re: Inductor tap value ?
Hi peeps. What I was trying to avoid was the laborious winding and rewinding, to see a possible marginal improvement. For my application its not worth the effort. I will stick to what I calculated and see if it stands up in future. THanks for the comments
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Should get out more. Regards Wendy G8BZY |
12th Feb 2023, 2:56 pm | #6 |
Heptode
Join Date: Nov 2009
Location: London, Ontario, Canada
Posts: 672
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Re: Inductor tap value ?
Hi Wendy,
My suggestion is to model the input filter and the following amp in spice. There you can do the "winding and re-windig" with ease. You can optimize the design quickly and painlessly. I did model your input filter, both for 2m and 6m, and the DG-FET amp in LTspice. If you are interested I can pass along the simulation files. You can also take a filter design SW, like Elsie, or some other, and make an initial filter design there. Regards, Peter |
12th Feb 2023, 5:29 pm | #7 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 983
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Re: Inductor tap value ?
Hi peeps. What I was trying to avoid was the laborious winding and rewinding, to see a possible marginal improvement. For my application its not worth the effort. I will stick to what I calculated and see if it stands up in future. THanks for the comments
I didn't suggest that you rewind the coils, I merely suggested you moved the tapping point whilst measuring the response on a mini vna etc. To answer your original question fully. Zs = Zp/N squared. N = sq root of Zp/Zs N turns ratio Zp Primary impedance Zs Secondary Impedance.
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12th Feb 2023, 9:48 pm | #8 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 983
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Re: Inductor tap value ?
You can also take a filter design SW, like Elsie, or some other, and make an initial filter design there.
Elsie is a most enjoyable piece of software, makes it all a pleasure.
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"Behind every crowd, there's a silver Moonshine" |