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Build a low loss 4uH Inductor

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aht2000

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Hi,
I am trying to match the output of an SA612 mixer (1500 ohm) to 50 ohm load. The mixer output has F1-F2 of 10.7MHz (which the one I care about), and RF in and LO will be around the FM band range. So, I think I need my impedance matching components to hold their characteristics up to may be 200MHz.

The 50 ohm load is representing the NanoSA input impedance for now, and eventually will be replaced by a Murata 10.7 Ceramic filter with input impedance of 330 ohm.

Using one of the available online impedance matching calculator for the LC values, I got around 4uH inductor to build (see attached photo). I understood that building such relatively high value with air core would be big in size and would need a large number of turns (increasing resistance, getting worst with skin effect, and capacitance reducing its resonance frequency). So, I tried to use an FT50-43 ferrite torroid I have. I wound 27 turns and measured its S21 using NanoVNA. From the attached chart, it seems to me that the core is acting as an RF choke at the desired frequency. I tried a yellow core (I think it is not suitable for HF anyway) and seems that these cores just add resistivity impedance and does not really increase the inductance. I was under the impression that the role of the magnetic core is to focus the magnetic flux and allow building large inductor values with less number of turns. Does such core exist in reality at such frequency?

Note: I tried the idea of wideband transformer using the same FT50-43 core (tried FT50-61) as well (15 turns on primary and 3 turns on secondary) but again measuring the S11 from the primary side while the secondary is terminated into 50 ohm should much lower inductance than it should be for such core, and high resistivity component.

How to I build a 4-10uH inductor with below 1ohm resistive impedance up to 10-15MHz, and its self resonance is above 200MHz. Or such thing is not doable in real life and there are other ways to tackle this problem?

Thank you.
 

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vfone

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Is practically impossible to get a 10uH inductor with SRF above 200MHz.
 

FvM

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You are designing impedance matching for 10.7 MHz IF. How comes 200 MHz SRF into play? Inductor winding capacitance will be absorbed into the LC circuit.
 

aht2000

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You are designing impedance matching for 10.7 MHz IF. How comes 200 MHz SRF into play? Inductor winding capacitance will be absorbed into the LC circuit.
The output of the mixer contains F1-F2 = 10.7MHz, but contains as well F1+F2 which will be in the range of 180-200MHz, so I want the matching LC (specially the L) to maintain its L characteristics. I was thinking that this matching network (if it holds its characteristics) will help attenuating the F1+F2 as the C part will be low impedance, and the L will be high impedance.

My first challenge in fact is to wind the inductor on the right type of core that would give me the 4uH with the least number of turns but at the same time not increasing the R (lossy) part of the inductor impedance.

If I end up with a very lossy LC network, then I wonder if its power loss will be almost as bad as the power loss I'll be getting if I just connect the mixer with 1500 ohm output impedance to the 50 ohm load.

For your point around the inductor interwinding capacitance being absorbed into the C part of the L shape network, how to do this conversion mathematically. I am aware of ways to convert between Rs+jXs and Rp+jXp, but not L and C.

Thank you.
 

BigBoss

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F1+F2 is natural output of a mixer but you should be interested in 10.7MHz, not this one.
The meaning of matching is to supply max. available energy to other side so your circuit should provide this transfer at 10.7MHz not F1+F2
 

vfone

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At the IF output of the mixer, before the 10.7MHz band pass filter, you can place a diplexer to provide proper impedance over a wide frequency range (for both, F1-F2 and F1+F2).

 

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