tuning an LC

If you put the two caps in parallel, then how do you adjust the voltage on the variable cap? The signal is going to be across the adjustable caps just as well as the fixed cap.

But running them in series, the DC voltage on the adjustable cap can be set separately. Because the value of the variable is so much larger than the fixed cap, the signal voltage is attenuated on the variable and so less distortion results.

Is this what you were missing?

BTW, have you noticed the nutcase who seems to be following you around? I'm guessing you have a kill filter on him?

--

Rick

On a sunny day (Thu, 14 Nov 2013 13:21:54 -0800 (PST)) it happened Klaus Kragelund wrote in :

My xperience with inductors is not so bad, been using potcores for ages in resonant circuits, and the same for small adjustable coils, every old radio has them. Larkin's circuit reminds me of a piezo transducer driver for Doppler I once build.

Old remote controls that used acoustics had the same circuit, with a step up winding sometimes. Stayed pretty much on frequency even with user abuse (dropping etc). Think I still have some of those driver transformers (adjustable IIRC).

"rickman"

** FFS - you blind as a bat, stupid retard !!!!!!!!!!!!!!

The stinking TROLL called "miso" is following *me* around !!!!!!!!!!!!

I just want all trolls gone.

Firstly, the anonymous fuckwits using false names.

Then jerkoffs like you.

.... Phil

l

Yes, that's true if it's gapped. Maybe I jumped into the thread, but did no t see that Larkins inductor was gapped, thought he would go for the minimum space solution. Gapping it makes it look more like an aircore which I stat ed was the way to avoid the drift.

Cheers

Klaus

f

The permeability of the core, among other things, depends on the strength of the magnetic field within the core. So, instead of a varactor, you could wind a magnetizing coil and use it to tune your circuit. But IMHO it would require a custom-wound coil and some experiments to select an appropriate ferrite material, so probably you won't like this solution. But for completeness... :-)

Best regards, Piotr

"Piotr Wyderski"

** Or, there could be some black faced lackey with a trimming tool who peaks the coil every half hour or so.

Probably the customer won't like this solution. But for completeness... :-)

... Phil

:-)

Once you have an electrically controllable tuning element, you can close the control loop and let the device self-adjust the resonance point at the desired frequency.

Best regards, Piotr

** Or, there could be some black faced lackey with a trimming tool who peaks the coil every half hour or so.

Probably the customer won't like this solution. But for completeness... :-)

My god this cretin is retarded.

... Phil

Try doing the math. Very simple.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Yeah, the little onboard thingies must be gapped too. Or they use the shittiest ferrite known to man... same thing. :)

Neat thing I guess is, if the inductance and tempco are vastly higher than normal, one can reasonably guess the gap is much smaller than usual. Not sure what value that would be, but...

Tim

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

Use a varactor diode as a part of the C. Supply the diode with a tuning voltage from the DAC. And then, as suggested, close the loop in a software trimming/calibration routine.

If the varactor represents a small part of a series C, it will see a smaller part of the 60 V p-p.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com 
------------------------------------------------------------------ 
If Mama Cass had just split that ham sandwich with Karen Carpenter, 
they'd both be alive today.

Biggest still-extant varactor I know of is the BB201, about 100 pF per half. Takes quite a few to replace a 2.2 uF Y5V. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Sure, a synthetic L or C would work. Tunable with a trimpot is easy. But it's 60 volts p-p with half an amp of circulating current, and it's floating.

--

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

50

shielded

trimmable

the caps

steps.

uniform.

in

Here is a wild idea for you: Synthesize the inductor with a power opamp based gyrator. Then you could get controlled tempco and much lower drift and relatively high L accuracy. I bet you could even make it tunable.

?-)

all that easy to get hold of, but I managed it - you should find it easier.

I think i need to see quality dimensioned drawings for this one. I think i can see how to do it but i have this nagging suspicion that i am missing something. Like how to make the adjustment stable.

?-)

ohms. Peak inductor current is half an amp or so.

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

inductors look too wimpy for this frequency and current.

In fact they aren't but John Larkin doesn't know enough about the ferrite cores and adjusters that he could buy to be aware of that

assorted film caps, like those nice WIMA parts.

You wouldn't need a power op amp to feed in enough quadrature current to shift the frequency to exactly 107kHz.

My low distortion oscillator uses this trick - and it seems to work fine in the LTSpice simulation. You do need a good low distortion four-quadrant multiplier to make the frequency shift adjustable.

--
Bill Sloman, Sydney

and relatively high L accuracy. I bet you could even make it tunable.

Drive the tuning quadrature current through an isolated winding with - say - only 20% of the turns of the tank circuit coil.

The tuning current is lot less than half an amp. It would help if the inductor/transformer was wound on a heavily gapped ferrite core, so you could set your base inductance with a +/-3% tolerance, which would make the worst case tuning current a lot lower.

--
Bill Sloman, Sydney

I can see how to do it but i have this nagging suspicion that i am missing something. Like how to make the adjustment stable.

It may be a practicable idea, but it certainly isn't practical.

You can buy heavily gapped EPCOS P26x16 pot cores with a hole through the - gapped - centre, and an adjuster to screw into the gap to bridge the gap in a smoothly variable fashion.

It's a very old-fashioned solution, but Farnell and Newark still stock the adjusters - I've posted the part numbers elsewhere in this thread.

Beats the hell out of grinding ferrite yourself.

--
Bill Sloman, sydney

I've done my share of trim. Trust me. OK, laser trim and fuse trim. I have explained my math.

Your approach doesn't make sense. I guess we will leave it at that.