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IF Transformer Inductance Sanity Check

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Posted: 5/15/2012 9:50:01 PM
gnsmith116
From: Northern Virginia, USA
Joined: 5/10/2012

threads posts

I'm working on an LTSpice simulation of the Theremax and was wondering if anyone has modeled the 42-IF110 in a Hartley oscillator configuration.  From the data sheet the primary inductance is specified to be 360uH+/- 6% with the following turns.  (in another post the inductance value was listed as a lower value maybe with the slug in a more operational position?)

1-2 (105), 2-3 (2), and 4-6 (3).  The primary is 1-3 so I think that 107 turns corresponds to 360uH.  If I assume that the inductance is proportional to the square of the turns that would mean that the inductance of the coil segment between 2 and 3 would be about .13uH and the secondary inductance would be about .29uH.  These numbers seem too low and I think the simulation results create an oscillator output amplitude that is lower than that actually achieved in the circuit.  I'm specifying the mutual inductance close to 1 (K L1 L2 L3 .99).

Also from what I have read, moving the slug modifies the inductance but I have not figured out if it also changes the mutual inductance.

Anyway, I hope that this isn't already on some forum and I missed it.  Any insight or modeling advice would be greately appreciated.

Thanks in advance,

Greg

Posted: 5/15/2012 11:07:40 PM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

I will have a browse over my simulations and see if I can find some I did for the Hartley oscillator with IFT's - but they have probably been deleted.

Yes, in real life, slug position does marginally affect mutual inductance (it also affect capacitive coupling) - but not significantly enough to warrant including this in a simulation, IMO. I would advise that a value of 0.85 is more realistic for K.

IMO, there is little point in using simulation for more than rudimentary evaluation of a circuit, and this is particularly true for oscillator circuits and circuits using coupled inductors - I have wasted a lot of time by building LC circuits which simulated perfectly, but failed to sustain oscillation reliably in the 'real world' - likewise, there are LC circuits which work perfectly in reality, but fail utterly in simulation.

I am now inclined to do minimum simulation on oscillators, and spend the time at the bench instead.. But I will look for my simulations, and if I find them, would be happy to post them on E-14 (where there are some LT-Spice simulations of theremin circuits already)

And if you do get a running simulation, it would be great to have more posted at E-14 ;-)

Fred.

Posted: 5/15/2012 11:16:21 PM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

I will not guarantee that these values are correct - but you may find them a help (or a hinderance if they are wrong! LOL!)

Part:    Colour:Type:             uH       uH       Sec uH            TURNS                                   MAX uH.

            1-2       2-3          4-6                1-3       1-2       2-3    4-6

 

42IF201 Yel    1st IF                227      121        1.34               180      104      76        8           741.5

42IF201 Wh    2nd IF              307       73          1.34               180      121      59        8           741.5

42IF203 BL    3rd IF               302      75.5        6.8                180      120      60        18          822.5

All above have 180pF across pins 1-3

 

Capacitor-less :

 

1-2       2-3          4-6                1-3       1-2       2-3    4-6          MAX uH

 

42IF104 Yel    1st IF                187.8   153        1.36               156      82        74        7            742.1

42IF104 BL    1st IF                308      72.6       21.16             153      103      50        27          940.8

 

 

Primary inductance 1-3 for all above = 680uH

42IF203 turns and values calculated – not given.

Al (nH) value of 20.97 seems to be about right.

 

The MAX uH value is not relevant - it is the maximum inductance one can obtain by connecting windings together to form a single adjustable inductor.. I knocked this table up some years ago when some adjustable inductors I had ordered (from China) were supplied with completely wrong specifications - I was looking for something to get me out of a deep hole - in the end I found suitable parts on Ebay, and never used or tested  this table.

 

Posted: 5/16/2012 1:38:17 AM
gnsmith116
From: Northern Virginia, USA
Joined: 5/10/2012

threads posts

Fred,

I am amazed that you responded so quickly and of course with the fidelity of the data.  I have spent a few minutes trying to make sense of it.

I tried to simply compare the measured inductance values (you measured them directly?) with the turns ratios to see if there was a constant adjustment factor.  I started simply with the primary and secondary.  I had a good thing going with the approximately 0.5X ratio until the blue IF104.  I think I need to do some work to understand what is going on.  I do agree with you by the way that a simulation can only take you so far but the oscillator pretty much behaves as expected (the primary and 100pf cap) in the Theremax configuration that I pasted below.  The problem is the secondary output.

 

Thanks again,

 

Greg

Posted: 5/16/2012 2:12:34 AM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

" I had a good thing going with the approximately 0.5X ratio until the blue IF104"

First, the BL in this case denotes Black (sorry!)

I dont remember exactly how I derived the table - I had a couple of actual IFTs I measured, and a load of (sometimes conflicting) data on turns and inductances on other IFT's - but which were from measurement and which were from data, I cannot say.

I do remember that I computed Al(nH) as 20.79 based on measurements of one IFT, and used this figure for all the other computations where I only had data on turns, but no data on inductance.

Maths is not my strong point, and usually I spend a lot of time checking my calculations - I do remember that in this case I rushed the whole process as I was in a major panic.. To be honest, I do not remember how I determined the values I did - Which is why I gave the warning that they may not be correct.

Your simulation looks good.

Fred.

Posted: 5/16/2012 2:20:15 AM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

You may find this inductance calculator useful.

Posted: 5/16/2012 2:24:04 AM
gnsmith116
From: Northern Virginia, USA
Joined: 5/10/2012

threads posts

Fred,

Thanks again for the data. I think that your data will turn out to be consistent once I really understand what is going on.  My new cheap Chinese scope is due any day now and if it works (2 ch 100MHz 1Gs/s $430 US!) I will measure the actual amplitude across the primary and secondary.  I would try to measure the inductance directly but I don't have any transformers that aren't in the circuit and I don't want to take a chance on ruining the can or the board at this point.

Greg

 

Posted: 5/16/2012 2:49:12 AM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

"My new cheap Chinese scope is due any day now and if it works (2 ch 100MHz 1Gs/s $430 US!) "

That looks like a damn good price!

If its a Hantek, you wont be dissapointed (not saying you should be dissapointed if its not, LOL!)

I bought a cheap Hantek DSO1060 a couple of years ago (2ch 60MHz) and used it for a couple of months before I got sick - It was (is) great! - I have an array of huge bulky test gear, including 4ch 100Mhz DSO, but the trouble is that it is impossible to do work on theremin RF stages when surrounded by piles of test gear and cables - and impossible to move this bulky gear to a free area away from the lab.. So I invested in some portable gear allowing me to set the theremin up on the kitchen table.

I rarely used my bulky equipment after that - the little 'scope does everything needed and is far easier and quicker. 

Oh, just one thing -

Dont forget that 'scope probes can seriously affect what you are measuring! You cannot, for example, clip a 'scope to a theremin antenna and expect it to work! The capacitance will load any oscillator designed to operate with small capacitances.

For observing signals which occur on theremin oscillators or other capacitance sensitive circuits, you need a probe which will probably cost more than your 'scope did! - There are some schematics and construction details on-line for active (FET) probes which may be worth building, but only if you did a LOT of work on these kind of circuits.

Posted: 5/16/2012 6:35:00 PM
FredM
From: Eastleigh, Hampshire, U.K.
Joined: 12/7/2007

threads posts

I think you may be making your error on the primary inductance - this calculator shows it simply.

Resonant frequency (centre) is 455kHz with a 180pF capacitor between pins 1+3 (primary) so I have assumed the inductance of this primary is 680uH.

I cannot confirm that my measurements or calculations are accurate, but I think I did the following:

Given the number of turns on the primary, and an inductance of 680uH, calculating AL(nH) value for each transformer where winding data is given is simple.

For the 421F201 and 421F202 AL= 20.97, I did not have accurate data for For the 421F203 so assumed AL= 20.97

For the 421F104 I took measurements and determined AL= 27.93, I determined AL for the 421F106 as 29.03

Fred.

 

Posted: 5/16/2012 7:30:04 PM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

Now I am not the brightest bulb on this board, but I am not backward either.

I see a lot of talk using 455khz IF transformers when an actual Theremax uses a 796 kHz I believe. Also the sine wave frequency in the graphic above seems closer to the 796 kHz. I have never modeled any circuits so I may be missing something. I use the plug & play method of parts with the minimal necessary  math. Then sniff for smoke!

Fred I need your opinion of this webpage. You will think I am making this up?

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