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Old 5th Feb 2023, 4:00 pm   #1
Wendymott
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Default 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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Old 11th Feb 2023, 5:25 am   #2
dmowziz
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Default Re: Inductor tap value ?

Quote:
Originally Posted by Wendymott View Post
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.

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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Old 11th Feb 2023, 8:10 am   #3
Cruisin Marine
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Default 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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Old 11th Feb 2023, 5:53 pm   #4
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Default 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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Old 12th Feb 2023, 12:27 pm   #5
Wendymott
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Default 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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Old 12th Feb 2023, 2:56 pm   #6
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Default 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
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Old 12th Feb 2023, 5:29 pm   #7
Cruisin Marine
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Default 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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Old 12th Feb 2023, 9:48 pm   #8
Cruisin Marine
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Default 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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