tuning an LC

This is pretty much how ham radio automatic antenna tuners work 150 W - 1.5 KW (a PIC controlling relays..). I would go with it, no risk. Here's a picture of the inside of one:

--
/*  jhallen@world.std.com AB1GO */                        /* Joseph H. Allen */ 
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--;)q=3&(r=time(0) 
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0:p%79-77?1:0:p158?-79:0,q?!a[p+q*2 
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817;)printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

On a sunny day (Wed, 13 Nov 2013 09:11:00 -0800) it happened John Larkin wrote in :

Inductive loop, several channels between about 50 kHz and 150 kHz, used for simultaneous translation in different languages. I think these days they use IR,

power amp12 - R - C - 4 adjustable L - - | - | power amp2 - R - C - 4 adjustable L --| | power amp1 - R - C - 4 adjustable L ------------------------- | inductive loop around hall | ---------------------------------------------------- | ///

In this case the 'inductive loop' was hundreds of meters long, sometimes more than one turn, and had a much higher L than the adjustable. The adjustable Ls together with C each tuned to one frequency, so the amps were not shorting each other. Modulation was narrow band FM.

I had a nice LED bar display that you could switch to each amp output to tune for maximum, one up one down button, 2 select buttons, counter in RAM, early eighties!

Voltages got really really high, like hundreds of volts at the C point in resonance. The amps were class B transistor ones The 'R' in the LC was to reduce Q so there was sufficient bandwidth for each channel.

I had a license to run it....

Now the 12 channel receivers, using ferrite rods, LOL, was an other piece of art.

Inductors, inductors, inductors.

What do you use it for?

Hey, and you can even get that for free in the driver:

(Bonus retro points if you do it with SCRs, John. The world is watching ;-) )

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

What will be the affect on the Q with this scheme? :-)

--
/*  jhallen@world.std.com AB1GO */                        /* Joseph H. Allen */ 
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--;)q=3&(r=time(0) 
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0:p%79-77?1:0:p158?-79:0,q?!a[p+q*2 
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817;)printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

More importantly, what is the object in the first place... magnetic field or what? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Are you sure this doesn't require the trim cap in parallel?

Technically it resonates at the damped frequency, not the natural frequency, due to the R load. It has been way too long since I have had to think of how that effects phase. But if you wanted the natural frequency, the phase shift (or lack thereof) is the way to go.

You could also just hit is with a square wave and watch it ring. The damped oscillation will be at the damped resonant frequency.

The phase has to be compared to something. In Spice, it looks as is the negative dip of the sine wave is centered on the mosfet drive at resonance, and it's pretty sensitive, good enough to eyeball on a scope.

Rob had a cool idea for tuning: use a step-up transformer and a ceramic or film variable cap, which multiplies the effective C by n^2. That might work, if I can get and afford a variable cap of enough capacitance and voltage. I'd be stepping up 60 volts p-p!

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

Linear Tech has an app note on making a high voltage lowpass using their zero DC offset part. The high voltage path is through a resistor, and then the filtering is done AC coupled with a shunting circuit.

You could probably make an active filter in a sense, i.e. get rid of the inductor. You would still need a real capacitor. The idea is the signal is fed across a cap, then you synthesize a shunt circuit that is AC coupled to connect to the non driven side of the cap.

--- Since your 75µH choke is 50 ohms at 107kHz, you'll need a capacitance of 27.5nF to resonate the tank.

Looking at a 27nF +/- 5% part would give you 28.35nF at the high end, and 25.65nF at the low end.

Then, to resonate the tank, you'd need 78.05µH at the high end and

86.26µH at the low end.

Caddell-Burns makes a series of shielded variable and +/- 10% fixed inductors which, configured like this:

+30 | +-------+ | | [LFIX] | | [27p] [LVAR] | | | +-------+ | [100R] | D PWM>----G S | GND

just might tune through 107kHz with judicious choices of LFIX and LVAR.

I don't have the time to to run the numbers right now, but if you'd like to, the inductors are here:

JF

Maybe every quarter cycle - momentarily close a switch in parallel with the LC when the L is full and the C empty.

Cheers

--
Syd

Hi

Do your the inductors have ferrite cores? If so, and especially if they are gapped (or of powder core type with distributed gap), they might be nicely tuneable by a DC current. The tuning works by saturation, so it can only tune the inductance down, not up, but this is easily countered by selecting coils with a slightly high no-bias inductance value and down-tuning as needed).

Arranging 2 coils to be in series for the DC and in parallel for the tank circuit would give you the option of tuning them with a low-voltage sufficient-current type current source. Think inverted version of the classic double-varicap circuit (varicaps become inductors, voltage source becomes current source, high-R series resistor becomes low-Z parallel capacitor). A switching regulator, sufficiently filtered, could provide the current. See .asc file below for details.

With gapped core inductors the core saturation is steady as the area of the highest flux density slowly shifts from the saturating core into the gap as the DC current is increased, so using a gapped core (with a sufficiently small gap for this effect to be usable) could give you relatively good tuning linearity (and of course continuous tuning with no need of switching "taps" or "steps").

Regards Dimitrij

LTSpice file follows (schematic only, no inductor models in there):

Version 4 SHEET 1 880 680 WIRE 160 160 16 160 WIRE 336 160 160 160 WIRE 16 176 16 160 WIRE 160 176 160 160 WIRE -96 192 -224 192 WIRE -32 192 -96 192 WIRE 336 208 336 160 WIRE -224 224 -224 192 WIRE -96 240 -96 192 WIRE -32 256 -32 192 WIRE 16 256 -32 256 WIRE -224 336 -224 304 WIRE -96 336 -96 304 WIRE -96 336 -224 336 WIRE 160 336 160 256 WIRE 160 336 -96 336 WIRE 336 336 336 272 WIRE 336 336 160 336 WIRE -96 384 -96 336 FLAG -96 384 0 SYMBOL ind 0 160 R0 SYMATTR InstName L1 SYMATTR Value L_res*2 SYMBOL ind 144 160 R0 SYMATTR InstName L2 SYMATTR Value L_res*2 SYMBOL cap 320 208 R0 SYMATTR InstName C1 SYMATTR Value C_res SYMBOL cap -112 240 R0 SYMATTR InstName C2 SYMATTR Value C_filter >> C_res SYMBOL current -224 224 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value I_DC

My first guess would be that it would make it bigger. There's no loss with the switch open and so less loss for one cycle.

George H.

Thanks, but near miss. The adjustable Ls that have enough span can't handle the current. I have some similar Coilcraft samples, too much DCR. Coilcraft is great about samples.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

My thing? It's a signal simulator, a transducer simulator I think. I'm all NDAd on it, and I'm not supposed to even know what the actual application is about, but I have some guesses. In general, it's hard to tune a tank in this frequency range.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

mosfet and have about 60 volts p-p across it. L is about 75 uH, so Xl is a round 50 ohms. Peak inductor current is half an amp or so.

ded inductors are mostly +-20%. So, how to tune it?

Actually, if you buy a heavily gapped ferrite core, they can come in at +/-

3%.

I used the 160nH part for my low distortion oscillator. You might prefer a smaller core, but I was interested in minimising harmonic distortion. In LT Spice the Chan model put the third harmonic content more than 100dB down.

Your 78uH part would need about 22 turns on that core - there's plenty of w inding space so the series resistance would be low.

ent so you can combine one higher and one lower than the nominal value to g et a fairly accurate composite 78 uH using two components.

the frequency. That's inelegant, but I could have an intern spend a day se lecting a lifetime supply... we'll probably only build 10 or 20 systems eve r.

Ferrite core permeability does tend to be mildly current dependent. I'd try to measure the inductance as close to the working current as possible.

With a +/-3% inductor and a +/-2% capacitor, you should be able to get with in +/-5% with a single parallel capacitor.

You'd nominally need 28.4nF, and you'd have to start with 27nF capacitor wh ich you can get from Farnell in polyphenylene sulphide, but only for 50V.

You'd then have to add up to 2.7nF of parallel capacitance to get within 2% of 107kHz. The next easily available value down is 2.2nF, for a total capa citance which is just 1.75% lower, which is 0.9% on your resonant frequency , so you wouldn't have any trouble in getting +/-2% frequency with just one extra capacitor.

--
Bill Sloman, Sydney

Do they still make the vane capacitors that used to be used in many radio equipments. The voltage withstand of those were into the hundreds of volts (used in valve circuits) and would have covered the 27nF or so range you need. Might need some gearing if you need fine control of tuning.

--
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sfet and have about 60 volts p-p across it. L is about 75 uH, so Xl is arou nd 50 ohms. Peak inductor current is half an amp or so.

inductors are mostly +-20%. So, how to tune it?

Farnell still stock the EPCOS trimming screws for the EPCOS P11x7 and P26x1

6 gapped pot cores with a central hole.

P11x7 would be a bit wimpy for your application, but P26x16 should be fine.

The P26 adjuster has EPCOS number B65679E2X22 and the Farnell order number is 2098922 - you can also get it out of Newark if nobody else in the US sto cks it.

EPCOS always had good data sheets and application notes.

covers the P26x16 cores and core formers and the adjuster.

It doesn't say how much the adjuster adjusts the inductance, but probably m ore than enough to take the +/-3% tolerance on the heavily gapped core pair s.

This beats the hell out of a +/-20% off the shelf inductor, though you may want to find a coil winding shop to make your coils. Hand coil winding mach ines are pretty cheap, and if you only need 20 parts it wouldn't be too muc h of a chore to do it in house. Most places where I've worked have had a co il winding machine somewhere around the place, but the Nijmegen University workshop threw their's out a few years ago because none of the new hires co uld imagine winding a transformer.

For 107kHz you might want to wind the coil with Litz wire. It's not all tha t easy to get hold of, but I managed it - you should find it easier.

--
Bill Sloman, Sydney

On a sunny day (Thu, 14 Nov 2013 09:53:22 +0000) it happened Paul E Bennett wrote in :

Are you not confusing nF with pF? those caps had no more than 100 pF. The big tuning ones would go to maybe 600 pF, but would flash over here.