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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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Old 11th Apr 2024, 11:50 pm   #1
Wendymott
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Default VHF inductor test jig

Hi peeps. My 2 metre TX/RX SSB radio is stalled at the moment, waiting for a RA30H1317 Mitsubishi Power module. I have two on order from Minikits in Aussie, which should arrive in about 10 days. Meanwhile I put together a pcb with Band pass filter at the front and a LP filter at the back.
However I had been given a circuit with the correct values for a 7 piece filter, and the inductors were quoted with nH values. 66/75/66 . However trying to get such values by experiment winding was proving a challenge. I have a Low value inductance test jig with switchable "C" 100pf and 10pf, duly connected to my Siglent Spectrum analyser with T/G. All was well, getting nice series trap dips, but something was wrong. I hit on the idea of a VHF oscillator with a known capacity and using a calculator could measure the frequency pretty accurately. After a couple of tries I finished up with the circuit shown, to guarantee the capacitance for the calculations, I used a range of small inductors to characterise the jig. 3 x 3.3uH, 3 x 2.2uH, 3 x 1.5 uH and 3 x 1.0 uH all giving me an average "C" of 30.05pf.
I realised my error with the passive test jig, in that there would be lots of additional capacitance not taken into account in the calculations..
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Old 14th Apr 2024, 11:59 am   #2
John_BS
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Default Re: VHF inductor test jig

I've never forgotten the day when, as an 'active' RF design engineer, I decided it would be useful to have a ball-park figure for the inductance of every mm of (say) 1mm wire (as in: leads of discrete components).

I'd clearly forgotten my A-level physics, as the text-book answer is that the inductance of a "lone" current-carrying wire is infinite. Here we meet the real world: of course what goes out must come back, so what's the return path for the current?

In most cases, the return is probably an adjacent ground plane or the confines of a screened box, and the model we require to calculate 'inductance' is in fact a short transmission line (I'm assuming VHF frequencies here).
Eg, for our 1mm diameter wire, centred inside a 1" box, Zo turns out to be c. 200 ohms, which yeilds c. 0.7nH/mm.
This is a rather long-winded way of observing that the precise inducance of small inductors at VHF is strongly influenced by adjacent structures, and really it is best to try to measure in-situ if possible, or a least to replicate the circuit environment when carrying out a measurement.
(PS whist we're talking transmission lines, my old boss delighted in telling us young-uns that if we wanted to make 600 ohm coax, we just need to suspend some 40 swg up the middle of a tube-tunnel).
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Old 14th Apr 2024, 1:24 pm   #3
Cruisin Marine
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Default Re: VHF inductor test jig

Those croc clips will change the inductance considerably
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Old 14th Apr 2024, 10:41 pm   #4
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Default Re: VHF inductor test jig

I agree Cruisin. And working with the very small values, it all adds to the mix. Needs a re think. The pcb needs to be mounted upside down, with the contact pins as short as possible. Obviously the crock clips were for convenience, but as you gratefully point out WRONG
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Old 16th Apr 2024, 4:41 pm   #5
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Default Re: VHF inductor test jig

Hi peeps.. An update. Replaced BFR96 osc transistor with a BFR 92 SMD device, Short end all tracks to test pin with SMD caps. Pcb double sided with copper ground plane, PCB mounted upside down as near as possible to top cover. Characterised and is now 6.9pF.
at the terminals . Easily tested the required inductors 66 nH and 72nH.
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Old 16th Apr 2024, 4:50 pm   #6
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Default Re: VHF inductor test jig

Well done. That's a useful bit of kit you've made there.

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Old 16th Apr 2024, 5:59 pm   #7
Cruisin Marine
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Default Re: VHF inductor test jig

Nice modification- well done
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