Author Topic: Q of chineese iron powder cores  (Read 1029 times)

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Offline paul002

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Q of chineese iron powder cores
« on: May 29, 2020, 08:19:56 am »
I was building some bandpass filters but having troubles with the 20m (14 mc) ones. I was using T68-6 iron-powder cores which should operate at that frequency. After measuring the Q of the coils it looked that the iron powder cores don't match up to the specs. They are from aliexpress in china. I am curious if somebody has measured real amidon T68-6 cores. I think I have to buy some real cores.
To measure the Q I use the same methode as for measuring crystals. That is have 2 tranformes to create low Z (12,5 ohm) have a series resonat tank circuit with well known C (mica) and use the nanovna to measure the loss in db and calculate the ESR. This can than be transformed to Q = Zl/ESR
 
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Online RoGeorge

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Re: Q of chineese iron powder cores
« Reply #1 on: May 29, 2020, 09:29:55 am »
What Q did you expect, and what Q did you measured?

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #2 on: May 29, 2020, 09:39:30 am »
I was expecting something between 200 and 240. But I was measuring only 6 for a chineese T68-6 core on 14 Mhz. I repeated the same measurements at lower frequencies there it would go up to 100. Measuring a air coil at 14 Mhz gave me 651.

 
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Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #3 on: May 29, 2020, 12:18:25 pm »
can you share S1P file with core measurement? And details about coil and capacitor used for measurement

According to your pictures for 16 MHz, Q is about 3.
« Last Edit: May 29, 2020, 12:29:29 pm by radiolistener »
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #4 on: May 29, 2020, 12:33:05 pm »
To measure the Q I use the same methode as for measuring crystals. That is have 2 tranformes to create low Z (12,5 ohm) have a series resonat tank circuit with well known C (mica) and use the nanovna to measure the loss in db and calculate the ESR. This can than be transformed to Q = Zl/ESR

I think you got bad Q due to loss in your transformers. Just try to measure it with no transformers
« Last Edit: May 29, 2020, 12:35:25 pm by radiolistener »
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #5 on: May 29, 2020, 12:46:46 pm »
Just measured an other one, this one is about 200. Looks much better. I don't think the transformers are an issue. I used them for a lot of measurements and they come from a high quality source. I also measured the Q with the scope using a square wave of 1 Khz and then measure how fast the signal drops to 0.707 V if you multiply by 4.53 you got the loaded Q.

The coils is 0.3862 uH (6 turns 1mm wire T68-1) Cap 248,2 pf loaded Q 1.54 Unloaded Q is Zl/ESR = 38,7 / 0,19 = 200.

If I short the measure points the loss is 0.2 db the loss of the series tank is 0.263 db -> ESR of 0,19 Ohm
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #6 on: May 29, 2020, 01:09:38 pm »
Measured an other one. Here the ESR is 0,71 ohm and XL 40,35 give a q of 56 so it goes up and down a bit.
I don't have the first one any more so cannot replicate it. But 56 is also quite low for a T68-6.
Will order some from germany see how these are.
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #7 on: May 29, 2020, 01:20:58 pm »
Just measured an other one, this one is about 200. Looks much better. I don't think the transformers are an issue.

I don't understand for what reason you're using transformer. Because transformer has losses and decrease Q.

For example, just tested Chinese T50-6 with NanoVNA. There is 22 turns with copper wire from network UTP cable. Directly connected to NanoVNA.

It shows Q about 115-120 at 14 MHz. But note, there is no reliable contact, I just put two wires into BNC connector. I think with reliable contact Q will be a little higher.
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #8 on: May 29, 2020, 01:35:54 pm »
Measured an other one. Here the ESR is 0,71 ohm and XL 40,35 give a q of 56 so it goes up and down a bit.

Your circuit has:
BW = 5.2794 / 15.91703 MHz = 0.33168
Q = 1 / 0.33168 =  3.015

Try to measure coil directly with no transformers. And use more turns to increase wire resistance VNA has worse precision when resistance is too low or too high
« Last Edit: May 29, 2020, 01:56:11 pm by radiolistener »
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #9 on: May 29, 2020, 01:59:16 pm »
Just tried without transformers. Does not work. The nanovna does not get -3db points.  So cannot measure the q.
What software for the nanovna do you use?  I don't see this chart in nano-saver.
The point was that bandpass filters with these cores were having high insertion loss. Trying to measure the Q with the scope also indicated low Q.
When measuring a series resonance you will get low Z, so I used the setup to get a low Z (50 Ohm -> 12,5 Ohm) this setup I also use to measure crystals which works quite good.
Anyway, have ordered some amidon cores and will measure them to see if that is any better or I am just measuring the wrong way.

 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #10 on: May 29, 2020, 02:02:13 pm »
Just tried without transformers. Does not work. The nanovna does not get -3db points.  So cannot measure the q.

Can you give S1P file with coil of 20-25 turns on your T68-6 directly connected to VNA port?
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #11 on: May 29, 2020, 02:14:30 pm »
here it is. sorry is 6 turns, will do a new one
Add a background artikel I was using. But did not try the swr method yet.
« Last Edit: May 29, 2020, 02:23:16 pm by paul002 »
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #12 on: May 29, 2020, 02:33:46 pm »
again now 20 turns
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #13 on: May 29, 2020, 03:32:48 pm »
paul002, something strange with your core. Are you sure, this is T68-6?

With 20 turns T68-6 should be about L=2 uH, but your data shows L=5.06 uH and Q=16.36 at 14 MHz.





« Last Edit: May 29, 2020, 06:34:13 pm by radiolistener »
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #14 on: May 29, 2020, 03:40:35 pm »
yes it is T68-6. Or maybe somebody in china painted it that way >:(.
But it is a good tip I have from 2 suppliers will check them with 20 turns and see if they all are like this.
I also wondered all the time why the count of coil64 was not correct but now I know.
Thanks !
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #15 on: May 29, 2020, 03:43:39 pm »
it looks like remarked low frequency core. For example 100 kHz or something like that.
« Last Edit: May 29, 2020, 05:02:17 pm by radiolistener »
 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #16 on: May 30, 2020, 12:56:08 am »
That is possible. I measured the coil on the scope and it gives me 2,4 uH and a low q about 50. Any way. I wait for the new one and make a comparison. I used them in bandpass filter for 40m and 80m there they work reasonable few db insertion loss, but if the frequency goes up the q drops quickly. Thank for your help
 

Offline vk4ffab

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Re: Q of chineese iron powder cores
« Reply #17 on: May 30, 2020, 08:18:53 am »
Hi ya, I hope you find this helpful, this is 19 turns on a T68-6 genuine I miscounted the actual windings and did not have enough wire left for 20 turns LOL

998422-0
998424-1
998426-2
998428-3

« Last Edit: May 30, 2020, 08:21:07 am by vk4ffab »
 
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Offline paul002

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Re: Q of chineese iron powder cores
« Reply #18 on: May 30, 2020, 09:39:10 am »
Can you Measure the q of the cool at 14 MHz? A way to do that is to Measure series resonance and get the -3db points. Using analisys button and choose bandpass filter
 

Offline vk4ffab

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Re: Q of chineese iron powder cores
« Reply #19 on: May 30, 2020, 10:08:30 am »
Can you Measure the q of the cool at 14 MHz? A way to do that is to Measure series resonance and get the -3db points. Using analisys button and choose bandpass filter

3db of what, the plot is in Z not dB
« Last Edit: May 30, 2020, 10:21:02 am by vk4ffab »
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #20 on: May 31, 2020, 01:15:21 am »
this is 19 turns on a T68-6 genuine I miscounted the actual windings and did not have enough wire left for 20 turns LOL

can you show Rs/Xs chart from 0.1 to 30 MHz?
 

Offline vk4ffab

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Re: Q of chineese iron powder cores
« Reply #21 on: May 31, 2020, 01:34:17 am »
this is 19 turns on a T68-6 genuine I miscounted the actual windings and did not have enough wire left for 20 turns LOL

can you show Rs/Xs chart from 0.1 to 30 MHz?

There is no such option, there is R+jX or R/w @ X/w
 

Offline radiolistener

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Re: Q of chineese iron powder cores
« Reply #22 on: May 31, 2020, 11:50:12 am »
There is no such option, there is R+jX or R/w @ X/w

R+jX chart from 0.1 to 30 MHz
 

Offline vk4ffab

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Re: Q of chineese iron powder cores
« Reply #23 on: June 01, 2020, 03:52:30 am »
There is no such option, there is R+jX or R/w @ X/w

R+jX chart from 0.1 to 30 MHz

Ok whats the math I need to work out Q from here?

 

Offline paul002

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Re: Q of chineese iron powder cores
« Reply #24 on: June 01, 2020, 10:51:19 am »
Q = |X| / Rs or  Rp / |X| see article of Jacques Audet, VE2AZX
 
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