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8th Mar 2023, 6:11 pm | #21 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,864
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Re: Identifying toroid cores for RF use
That's an excellent way to find the conjoint of the ranges "Good for resonated inductors" and "still good enough for transformers". Speedily effisient.
You still need to get down and dirty with Q/loss measurements to split those two ranges. Be aware that with a bifilar twisted winding you may get transmission line coupling effects at high frequencies, making the top end look more extended than the core is responsible for. David
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9th Mar 2023, 2:25 am | #22 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
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Re: Identifying toroid cores for RF use
Measurement of unloaded Q of an inductor is a tricky business. You have to ensure that the signal generator and scope are weakly coupled to the LC tank to have a minimum of -20db insertion losses in both ends. Then the measured Q and L are indicated readings that include the distributed capacitance of the coil windings. The actual L has to be corrected by finding the distributed capacitance or self resonance frequency of the inductor.
The series LC dip method is not foolproof. It can catch you out if you trust the measurements 100%. |
9th Mar 2023, 10:34 am | #23 | |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,094
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Re: Identifying toroid cores for RF use
I also use an aerial analyser to test torroids but in a much simpler way. I just wind a turn or two through and see what the analyser thinks it is seeing.
EMC ferrites look like resistors. If it shows actual inductance at HF then it is likely to be dust iron. The point here is that it is about loss and a lossy core makes the winding look like a resistor - simple. Quote:
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9th Mar 2023, 5:54 pm | #24 |
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: Identifying toroid cores for RF use
One of the popular EMC ferrite materials, Fair-Rite number 43 makes good inductors into the hundreds of kHz, is great for transformers to well above 30MHz, and doesn't start getting properly lossy until 100MHz.
Other makers have comparable materials and they often turn up as LOPT cores and deflection yokes (The TV gang are gonna lynch me for revealing that!). And you'll also find most RFI sleeves in plastic moulded onto cables are similar. David
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9th Mar 2023, 7:27 pm | #25 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 992
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Re: Identifying toroid cores for RF use
This is a fine article referring to using a nano VNA
https://pa3a.nl/wp-content/uploads/2...fying-them.pdf
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9th Mar 2023, 7:41 pm | #26 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,996
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Re: Identifying toroid cores for RF use
If you're going to be using them for any significant amount of RF power, you need to determine when they will saturate.
B-H curves ! https://www.electronics-tutorials.ws...ysteresis.html I remember one of my competitors used a toroid in a low-frequency RF amp circuit which, when thrashed, heated the toroid beyond its Curie-point.
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9th Mar 2023, 9:08 pm | #27 |
Hexode
Join Date: Dec 2018
Location: Lugo, Spain
Posts: 483
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Re: Identifying toroid cores for RF use
I do not own an mfj analyzer so i,m going to have to think about another option.
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