For a tuned circuit, if it's a parallel LC, I'd say you all you have to do is just to take the parasitic C into account by subtracting it from the C you are adding in parallel. (I assume you know how to solve for C given L and the SRF.)
In a series LC ... not so obvious.
If it's in a LPF, I'd say either make sure the SRF is well above the highest frequency you want to stop, or else if the SRF is too close, simulate the filter and see how you may need to tweak other components in order to compensate for the effect of the parasitic C.
Eamon VE2EGN AB1NK
| 15154
| 2018-10-23 15:35:50
| kb1gmx
| Re: inuductor wisdom needed: selection vs srf
|
If you can get the thing to tune above the SRF you will be against the fact that ts a parasitic tune and its Q will be low.
SRF is self resonant frequency, its going to be hard to use that coil at a higher frequency as its already resonant a a lower one.
IF anything you want the SRF much higher than the desired operating frequency for good Q.
Allison |
| 15155
| 2018-10-24 22:12:27
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
SRF shouldn't usually be a problem in these cases; the C values used in a tuned circuit or a filter would normally be much greater than the few pF of parallel C that determines SRF.
For interest, I swept a couple of the coils from my home-made GDO; they are tuned with a 100pF variable capacitor. This coil covers 0.4 - 0.8 MHz (it has some hundreds of turns so might be expected to have high stray C);
This one, covering 1.8 - 4.2 MHz, has somewhat fewer turns;
In use, the few pF of stray C is "swamped" by the tuning capacitor; it may have some effect at capacitor minimum which might be 5 - 10 pF but not a great deal.
A similar effect should be found in filters.
SRF is very important in RF chokes which can be rendered useless by the effect.
An ideal choke would be operated at its SRF where impedance is a maximum but this implies single-frequency operation whereas our useage is usually over a range of frequencies.
One rule-of-thumb is that a choke should be operated at no more than 10% of its SRF; that ideal is often difficult to achieve.
Kerry VK2TIL.
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| 15156
| 2018-10-25 04:06:40
| Steve Dick
| Re: inuductor wisdom needed: selection vs srf
|
Hi Kerry. Now that you mention it, I am using an rf choke for a particular circuit in a crystal radio. A commonly used part which I’ve seen in many crystal radio circuits is a 27 millihenry RF choke, which is supposed to provide high impedance at RF but a d.c. return path. However, its self-resonant frequency is way below the AM broadcast band. The best I’ve found is 470 KHz. So I backed off to an 18 millihenry choke with a self resonant frequency of 720 KHz, hoping that its impedance would not drop too much at the high end of the broadcast band. I also toyed with putting two rf chokes in series, one large one, and one small one, with self-resonant frequencies spread out across the broadcast band. This seems to be overkill though, for a relatively simple crystal radio. Any thoughts? Thanks.
-Steve K1RF
| 15157
| 2018-10-25 07:49:16
| kb1gmx
| Re: inuductor wisdom needed: selection vs srf
|
The use of multiple series chokes in wideband circutis is not uncommon and most commercial bias tees do this.
That said.... If a choke is self resonant below a desired frequency then consider a different construction. Most molded and otherwise very compact chokes are also high capacitance. Look at old school radios for things like multi section chokes using Pi and fan wound. This is one place where the experimenter can do it better.
Allison |
| 15158
| 2018-10-25 09:23:51
| Rod
| Re: inuductor wisdom needed: selection vs srf
|
Kerry, Thank you for the thought and energy you have put into this endeavour.
I do not understand the charts.
Were these measuring the L in series with the transmission path? The ref is 48k and the peak is 50k... what does this mean? Could you plot transmission loss, either linear or in dB? Any clarification would be beneficial. Thanks, Rod KM6SN
| 15159
| 2018-10-25 09:24:37
| Rob
| Re: inuductor wisdom needed: selection vs srf
|
RE: old school radios Is there a good instruction how toi wind a FAN WOUND choke for 1mHz AM broadcast band reception?
| 15162
| 2018-10-25 20:59:59
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
Steve; that may well be overkill for a simple crystal set but tinkering and improving (or, sometimes, worsening :) ) things is why we have this Group and is what Wes, in EMRFD & elsewhere, preaches.
Rod; the charts are S11 (reflection) measurements of scalar impedance Z. I think that is the essential parameter of a choke (although not of an inductor in a resonant circuit). The figures are Z in ohms.
I do not claim accuracy of the impedance numbers as measurements this far removed from a VNA's "comfort zone" around 50 ohms are very difficult and this was a "quick & dirty" measurement to see and demonstrate the effect of SRF.
The frequencies will be correct.
Today I did the same measurement, scalar impedance Z, on three "bought" RF chokes, of 1mH, 47uH & 15uH;
The black trace and the black figures in the marker table are for the 1mH choke, the red ditto are for the 47uH choke and the blue ditto are for the 15uH choke.
No great surprises re the SRF; the larger values would have more turns, equating to more inductance and less inter-winding capacitance, than the smaller values and that means lower SRF.
The responses are fairly broad, indicating a "lossy" device, a desirable property of a choke (although not of a tuned inductor); I guess that these little chokes have a ferrite core that is "lossy".
The 1mH device has quite high Q, probably due to its high inductive reactance; since this is the upper value in the Q equation the higher Q is not unexpected.
Kerry VK2TIL.
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| 15163
| 2018-10-26 04:00:38
| Steve Dick
| Re: inuductor wisdom needed: selection vs srf
|
Kerry, this is most interesting and I appreciate the data. It is actually pretty challenging making a broadcast band choke that has impedance, as a goal, on the order of 100K ohm across the broadcast band. I ended up using a cascade of three chokes. Two of them are 3-pie wound.
-Steve K1RF
| 15164
| 2018-10-26 12:25:06
| Rod
| Re: inuductor wisdom needed: selection vs srf
|
Kerry, Thank you again for the considerable energy you have put into this. Yes, S11 makes sense.
I assume the circuit was the choke to ground; but at those impedance levels,
it would make little difference if the choke was in series with a 50 ohm load to
ground. Thanks again, Rod
| 15165
| 2018-10-26 17:07:48
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
Yes; just choke to ground. It doesn't take long; a quick calibration of the VNA and a few minutes to sweep, save the plot and transfer from the bench computer (which never goes on the 'net so is as pure as Parsifal) via a "stick" to this computer.
There are better measurement methods but this was just a demonstration.
I have an RF I-V fixture but, if I wanted really accurate results at such high impedances, I might consider methods other than a VNA.
Kerry VK2TIL.
|
| 15166
| 2018-10-27 07:31:21
| iq_rx
| Re: inuductor wisdom needed: selection vs srf
|
Hi Kerry, very nice work, thanks. If you still have that measurement setup handy, I'd like to see a sweep of pairs of those inductors in series to ground. I expect some interesting behavior with the capacitance of the inductor above self resonance in series with the inductance of the one below self resonance. Rick KK7B |
| 15167
| 2018-10-27 07:43:12
| iq_rx
| Re: inuductor wisdom needed: selection vs srf
|
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| 15168
| 2018-10-27 16:12:26
| kb1gmx
| Re: inuductor wisdom needed: selection vs srf
|
Also Coilcraft... Been aware of both from my time designing antenna systems. I also have a roller inductor that is conical, works well.
Allison |
| 15169
| 2018-10-27 21:57:07
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
I have done some measurements with the chokes in series as Rick suggested;.
1mH in series with 15uH;
47uH in series with 15uH;
1mH in series with both 47uH & 15uH;
Here is the Z of the three individual chokes that I posted earlier; the vertical & horizontal scales are the same as the above plots to assist comparisons;
It's difficult to draw any quantitative conclusions but the series arrangement has slightly altered the SRFs and the peak Zs have been greatly altered.
Kerry VK2TIL. |
| 15171
| 2018-10-28 04:10:19
| Steve Dick
| Re: inuductor wisdom needed: selection vs srf
|
Kerry, this data is gold. Thanks for posting!
-Steve K1RF
| 15172
| 2018-10-28 06:19:28
| arfghans
| Re: inuductor wisdom needed: selection vs srf
|
For SRF measurements, S11 (reflection) is the easiest but also the worst method. Reason: Capacitance of the instrument plus the test cable is directly in parallel with the device under test. The preferred method is S21 (transmission) where the DUT is in series between the generator and detector. This was discussed at length on the VNWA and N2PK VNA groups, and also is in the work of K9YC in the characterization of common-mode chokes, where even a sub-pF amount of stray C will drastically affect results. My own work shows that SRF is typically low by a factor of 2 when using the reflection method. I also found the S21 method superior to RF-IV for SRF measurements.
If you want exact transmission impedance from S21 data, the required formula is Z = 100 * (1-S21)/S21, with all values being complex numbers.
But you don't need a VNA to make accurate SRF measurements. All you need is a signal generator and some kind of detector. That detector can be a spectrum analyzer, a sensitive power meter, or even a receiver, perhaps an SDR. Apply a terminator to the end of the generator's output cable. Place the DUT between the generator and the detector, and connect the grounds. Sweep the freq until you see the SRF peak.
-Gary NA6O |
| 15174
| 2018-10-28 17:25:16
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
Yes; precise & accurate measurement of SRF is a major challenge and S1,1 is certainly not the best measurement method.
The Agilent Impedance Measurement Handbook is an excellent reference.
I did say, in post #9, that;
I do not claim accuracy of the impedance numbers as measurements this far removed from a VNA's "comfort zone" around 50 ohms are very difficult and this was a "quick & dirty" measurement to see and demonstrate the effect of SRF.
I also said, in post #12, that;
There are better measurement methods but this was just a demonstration.
I have an RF I-V fixture but, if I wanted really accurate results at such high impedances, I might consider methods other than a VNA.
The contribution of the test fixture to stray L & C is always a problem; I have several fixtures such as these;
and these;
that can be calibrated with standards such as these;
If DsUT are mounted in this fashion;
reasonable measurements can be done.
An associated problem is that the circuit in which a choke is to be used will have its own stray reactances and these are very difficult to account-for.
Kerry VK2TIL.
. |
| 15175
| 2018-10-29 09:09:01
| Rod
| Re: inuductor wisdom needed: fixture
|
Kerry, who makes those test fixtures? Rod
The contribution of the test fixture to stray L & C is always a problem; I have several fixtures such as these;
|
| 15176
| 2018-10-29 14:00:29
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
Rod; I do, they are home-made.
Jack K8ZOA once made a limited run of PCBs for crystal testing (but they can be used for other series measurements) but that was quite some time ago and Jack has sold Clifton Laboratories.
I have one mounted on a stand and have made similar fixtures; the ones in my photo are my "nice" ones.
This is from Jack's notes on his fixture;
It's just a 50-ohm microstrip with a gap to accommodate the DUT; his attenuation is the standard 50:12.5 ohms used in crystal testing but any desired value (or none at all) could be used.
A similar layout, without the gap, can be used for shunt measurements; the lower fixture in my photo is set up for either series or shunt.
Fixtures like this are easy to make with PCB material, the copper is cut/scored and peeled using heat from a soldering iron where removal is desired.
Kerry VK2TIL.
|
| 15177
| 2018-10-29 16:36:44
| kerrypwr
| Re: inuductor wisdom needed: selection vs srf
|
I remembered that Kurt OZ7OU designed a fixture similar to the K8ZOA one; see here;
Ivan Makarov VE3IM sells a suitable PCB;
Kerry VK2TIL.
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