Q of chineese iron powder cores - Page 1
Forum section: electronics / rf-microwave

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 08:19:56 am
content: 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

author: RoGeorge
author_rank: Super Contributor
timestamp: May 29, 2020, 09:29:55 am
content: What Q did you expect, and what Q did you measured?

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 09:39:30 am
content: 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.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 12:18:25 pm
content: can you share S1P file with core measurement? And details about coil and capacitor used for measurementAccording to your pictures for 16 MHz, Q is about 3.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 12:33:05 pm
content: Quote from: paul002 on May 29, 2020, 08:19:56 amI think you got bad Q due to loss in your transformers. Just try to measure it with no transformers

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 12:46:46 pm
content: 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

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 01:09:38 pm
content: 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.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 01:20:58 pm
content: Quote from: paul002 on May 29, 2020, 12:46:46 pmI 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.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 01:35:54 pm
content: Quote from: paul002 on May 29, 2020, 01:09:38 pmYour circuit has:BW = 5.2794 / 15.91703 MHz = 0.33168Q = 1 / 0.33168 = 3.015Try 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

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 01:59:16 pm
content: 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.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 02:02:13 pm
content: Quote from: paul002 on May 29, 2020, 01:59:16 pmCan you give S1P file with coil of 20-25 turns on your T68-6 directly connected to VNA port?

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 02:14:30 pm
content: here it is. sorry is 6 turns, will do a new oneAdd a background artikel I was using. But did not try the swr method yet.

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 02:33:46 pm
content: again now 20 turns

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 03:32:48 pm
content: 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.

author: paul002
author_rank: Regular Contributor
timestamp: May 29, 2020, 03:40:35 pm
content: 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 !

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 03:43:39 pm
content: it looks like remarked low frequency core. For example 100 kHz or something like that.

author: paul002
author_rank: Regular Contributor
timestamp: May 30, 2020, 12:56:08 am
content: 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

author: vk4ffab
author_rank: Frequent Contributor
timestamp: May 30, 2020, 08:18:53 am
content: 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

author: paul002
author_rank: Regular Contributor
timestamp: May 30, 2020, 09:39:10 am
content: 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

author: vk4ffab
author_rank: Frequent Contributor
timestamp: May 30, 2020, 10:08:30 am
content: Quote from: paul002 on May 30, 2020, 09:39:10 am3db of what, the plot is in Z not dB

author: radiolistener
author_rank: Super Contributor
timestamp: May 31, 2020, 01:15:21 am
content: Quote from: vk4ffab on May 30, 2020, 08:18:53 amcan you show Rs/Xs chart from 0.1 to 30 MHz?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: May 31, 2020, 01:34:17 am
content: Quote from: radiolistener on May 31, 2020, 01:15:21 amThere is no such option, there is R+jX or R/w @ X/w

author: radiolistener
author_rank: Super Contributor
timestamp: May 31, 2020, 11:50:12 am
content: Quote from: vk4ffab on May 31, 2020, 01:34:17 amR+jX chart from 0.1 to 30 MHz

author: vk4ffab
author_rank: Frequent Contributor
timestamp: June 01, 2020, 03:52:30 am
content: Quote from: radiolistener on May 31, 2020, 11:50:12 amOk whats the math I need to work out Q from here?

author: paul002
author_rank: Regular Contributor
timestamp: June 01, 2020, 10:51:19 am
content: Q = |X| / Rs or Rp / |X| see article of Jacques Audet, VE2AZX

author: vk4ffab
author_rank: Frequent Contributor
timestamp: June 01, 2020, 11:26:00 am
content: Quote from: paul002 on June 01, 2020, 10:51:19 amYeah I think there is something wrong with my first plot. I will re-calibrate the vna tomorrow morning and make up a new test jig. 1EDIT: did the calibration and got out a quality hardline patch lead and this is what i got. Q of about 150 at 14mhz. Will redo it in the morning when I am less tired and make a new test jig to eliminate any other error. A value somewhere between 50 and 200 would be typically expected. And this value is within that range.

author: paul002
author_rank: Regular Contributor
timestamp: June 08, 2020, 09:00:30 pm
content: Just received new cores, but to my supprise they are also fake. Unbelievable will return it. I did not imagine that german company sell stuff like this. 20 trurns give 2.4 uH instead of 2.0 that already made me suspicious. Now to find a good source of cores.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 08, 2020, 09:12:17 pm
content: A long time ago Micrometals published a warning to it's customers saying that Chinese equivalents were not the same. Buyer beware.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: June 09, 2020, 12:01:49 am
content: Quote from: paul002 on June 08, 2020, 09:00:30 pmDiz from kits and parts in the usa or mark from minikits in australia

author: paul002
author_rank: Regular Contributor
timestamp: June 09, 2020, 10:27:33 am
content: Thanks will go there.Quote from: vk4ffab on June 09, 2020, 12:01:49 am

author: paul002
author_rank: Regular Contributor
timestamp: June 09, 2020, 08:01:38 pm
content: It looks like ow mr Reichelt an apology. Somehow I was not happy with the measurement of the cores I received yesterday. Did some research and found a methode described by W7ZOI in his evaluation of the nanovna. He describes that measuring high Q will not always work on a (nano)vna. So I tried that methode. You create a pass thrue connector with two sma female connectors and connect the core in serie resonance to ground. Measure the S21 gain and calculate the q described in the book Experimental Methodes in RF design. Now the Chinese core still measures a Q of 31 and the new core from Reichelt a Q of 200.Somehow the methode of connecting the core direct to the S11 port does not work for me.