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tuning an LC

One possibility is to hand select and pre-kit a set of Ls and Cs that hit the frequency. That's inelegant, but I could have an intern spend a day selecting a lifetime supply... we'll probably only build 10 or 20 systems ever.

The drive is: +30 DC on one end of the tank. One the other end has 100 ohms to the drain of a mosfet. The source is grounded and the gate is pulsed at 107 KHz,

25% duty cycle. So we get 60 volts p-p across the tank. It's sort of like a class-C RF amp. We want peak amplitude and minimum phase shift. These are all system specs, not mine.

Whatever tuning mechanism I find, I have to tune it. The peak amplitude point will be hard to find. Maybe I can use a DDS to tweak the frequency and find the two -3 dB points, which would be more sensitive than looking for the peak.

That inductor would need a big percentage tuning range. Messy math.

I sort of hate inductors. They are the least ideal, least accurate, hardest to measure, hardest to buy parts. Somebody could set up a garage shop custom inductor business, especially stuff like tunable pot cores, and do OK.

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

Precision electronic instrumentation 
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Reply to
John Larkin
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With a 60Vp-p signal applied?

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

So how about using a switch in series with the LC and opening it every cycle when the L is empty and the C full for a short time period, then closing it. A bit like momentarily pausing a pendulum just as it's reversing direction.

Cheers

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Syd
Reply to
Syd Rumpo

You don't say how close you need to hit (or, for that matter, if you can trim subsequent stages to match the tank, instead of the other way around).

If you design the board for two caps in parallel and only populate one during build, then you should be able to select values such that at the highest inductance possible the frequency is spot-on. Then at calibration time you should be able to measure the resonant frequency, put in just one cap, and adjust the frequency to within about 4% or so (assuming you have a range of caps in the E12 value series). If that's not close enough, then put in spots for two "tune" caps and do it twice, which ought to get you to within 0.8%.

If _that_'s not close enough, then you should have told us how close "close" is.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

The system spec is +-2% frequency. Stock inductors are usually 20%, and caps are available at 2% and sometimes 1%.

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John Larkin Highland Technology Inc

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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

Reply to
John Larkin

Tune using the PHASE, not amplitude.

Reply to
RobertMacy

I could do that, although I'd rather switch capacitors than inductors. That might be the least ugly solution, a "capacitive DAC" across the main LC.

VLF? Big antenna? Transmitting to what receiver?

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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 
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Reply to
John Larkin

That's nice. We stock a right-angle hex rotary switch, looks like a trimpot.

1-2-4-8 capacitors, hex 0 to F.

Cool.

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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 
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Reply to
John Larkin

That's why you put a big cap in series with the 15-nF tank cap, to reduce the swing across the variable element. I'm guessing that the THD spec isn't the most important thing in the mix, but even if it is, it's easy to knock the swing down by using more caps in series, with the DC applied + - + - etc.

Just using two will fix most of it, because one will go up while the other goes down, just as with normal varactors.

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
Reply to
Phil Hobbs

I'd be interested if you can come up with an actual PWM scheme for synthesizing a varactor. Could come in handy elsewhere.

The suggestion was to use a high switching frequency.

If you switch it, cycle

Unlike a crystal, an LC circuit has no memory--it follows a differential equation. Change the parameters and the frequency follows instantaneously--not a cycle later, not 1/Q cycles later, but right now.

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
Reply to
Phil Hobbs

If it's just the frequency and not the waveform that matters, that might be a possibility. The current waveform will show a lot of crossover distortion, of course.

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
Reply to
Phil Hobbs

That's interesting and would be fun to play with. Maybe use two caps, like the dual varicap thing, to keep harmonic distortion down.

Are there any Spice models for hi-K caps? Or maybe I could just breadboard something.

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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
Reply to
John Larkin

OK. That makes sense. You'd need up to a 20% swing in the cap value, but it may be do-able.

(And it's microprocessor trimmable!!).

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

The vari-caps would only see a fraction of the 60 volts if they were in series with the main C, and much bigger. The numbers don't look too bad.

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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
Reply to
John Larkin

John Larkin schrieb:

Hello,

thats why people developed a lot of active filters using only resistors, capacitors and opamps.

Bye

Reply to
Uwe Hercksen

Maybe. I could also wigwag the frequency, alternately above and below 107K, and look at the two amplitudes. They would be equal when the thing is correctly tuned. That would be easy to see on a scope.

OK, combine that with the hex rotary switch 4-bit capacitive DAC.

--

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
Reply to
John Larkin

My requirement is +-2% frequency, so I should shoot for 1%.

Maybe plugin cap(s) for coarse and rotary hex c-dac for fine?

What a nuisance. I'm doing serious manly FPGA I/Q signal processing, and agonizing over one silly LC.

--

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
Reply to
John Larkin

We considered an active way to do this, but it's messy for a floating tank at 60 volts p-p.

--

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
Reply to
John Larkin

Which is why you can start an LC oscillator instantly, and it runs as if it had been doing that forever. It's harder to stop one quickly.

--

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
Reply to
John Larkin

That's kinda interesting! Maybe do it every half cycle.. a bit more symmetrical.

George H.

Reply to
George Herold

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