tuning an LC

With a 60Vp-p signal applied?

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

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.

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

--

John Larkin Highland Technology Inc

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

Tune using the PHASE, not amplitude.

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?

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

Cool.

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

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

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

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.

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!!).

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.

John Larkin schrieb:

Hello,

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

Bye

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.

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.

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

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.

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

George H.