The specifications and the apparent lack of RF clue of the OP are in strong disagreement.
IMHO, if he's honestly willing to do something, I'd suggest first to build a receiver for the 100 MHz FM band, with the required quality. I suspect that it will be pretty difficult to get the IF and detector string work at a HiFi level without proper test instuments.
The transmitter ends be simple as soon as we want HiFi -level stability and distortion specs.
Two reasonably well-matched hyperabrupt-varactor VCOs, the TX one free-running and the RX one phase locked, should get down to the 0.1% level with a bit of care. Of course the OP doesn't have an HP 339A to measure it on.
Cheers
Phil Hobbs
You can say that again. JW's stuff was always interesting and fun, but he had this tendency to use _way_ too many parts.
I'd start with a CPH3910 JFET Colpitts oscillator using a dual hyperabrupt varactor such as a BB201 as the tank cap / voltage divider. That'll have too much gain, but the tuning will be pretty linear over a good part of the range, and since it's a one-off, you can keep the amplitude reasonable by reducing the drain current.
Control the varactor using a pot connected between the power supply and ground, with a 100k series resistor from there to the varactors. Measure the voltage at the wiper, not on the oscillator side. (Note that varactors work in reverse bias.)
A 470-nH to 1-uH inductor might be about right--it'll make the frequency low enough to look at on an inexpensive scope. You'll want to get a cheap Chinese frequency counter to test it with, and a DVM if you don't have one. If you can beg or borrow a decent oscilloscope, that will be a great help.
Couple the counter to the source of the JFET via a 1k resistor to avoid loading it down.
If you make two of them using the same parts and the same layout, you can measure their frequency vs. voltage curves. The THD will be determined by how closely they coincide.
Cheers
Phil Hobbs
Oh, and the varactors are wired in series opposing (common cathode), with the (positive) bias applied at the midpoint.
Yep the art of electronics has a reasonable section on varactor tuned resonant circuits and they are *much* simpler, to the point where I can look at them and just understand what's going on. I assume that for 915MHz operation I will still need a frequency multiplier of some description but getting just the VCO working properly seems like a good first step here.
I'm not worried about my ability to measure most of this stuff, about the only thing I can't borrow stuff to do is look at time series 915MHz signals.
Yes art of electronics mentions this as a way to cancel the effects of the actual output oscillation from changing the varactor capacitance itself.
Right. Building a well-behaved 900-MHz LC oscillator is too hard for a beginner. With the oscillator as described there will be no problem whatever getting enough deviation, so you'll probably want to frequency shift instead of multiplying. And I wouldn't worry about getting up to
900 MHz anyway--just stay out of the aircraft and public safety bands, keep the radiated power way down (1 mW maybe), and mobody will know or care.Right. Otherwise you can get some very strange parametric effects, including subharmonic generation (parametric frequency division).
Cheers
Phil Hobbs
A million different FM receivers have been made and sold. Were many of them designed by someone who derived the FM spectrum first?
It doesn't make sense that something that's been done for 100 years should be less approachable than other hobby projects. There are several methods, so the answer should be how to do one of each. The transmitter can come later, being easier but more dangerous for interference.
Did you count them? ;)
I look forward to reading your explanations of each one, and their respective pros and cons for the OP's application.
The OP said it was an educational hobby project, so what's wrong with a bit of education? Knowing the functional form of the coefficients makes it easy to estimate the effects of sideband cutting and phase nonlinearity, which is pretty useful in the detailed design of the IF filters, for instance.
Also the math is just really pretty, in a small way.
I have no idea how far the OP will get with all this, but I'm all for encouraging folks who show an interest.
Cheers
Phil Hobbs
What was done nearly 100 years ago did not have HiFi quality, as the original post wants. For broadcast quality FM reception, a bandwidth around 300 kHz is needed, with as linear phase as possible. I doubt that this is achievable without either ready-made modules or good measuring equipment.
The suggested 900 MHz band poses a larger challenge to the hobbyist, as the stray inductances and capacitances together with transmission- line effects play a much larger role than on e.g. 100 MHz band.
Leaving that aside for a moment, what could be used to 'recieve' the modulated beam? Would a photodiode do it?
Eh???
You of all people would know. Couldn't resist a gratuitous put-down as usual. Never miss up on an opportunity to throw in a supercillious slur, eh, Bill?
I agree. 900Mhz is not for entry-level experimenters/hobbyists for the reasons you state. I suspect if the OP doesn't come down by a factor of 10 on that he will be frustrated, disappointed and eventually give up.
Hyperabrupt and linear and two words that don't belong in the same sentence. ;-)
You, Flyguy and John Larkin remind me of it more or less nonstop. Happily all of you are remarkably pathetic in your malice. Jim Thompson once claimed to have denounced me to the FBI which struck me as excessive.
That put-down was well-earned, rather than gratuitous.
It's hard to avoid being supercilious when confronted by people who offer a lot to be supercillious about. You'd be a prime example of somebody who must get that reaction frequently.
Oh really? THAT struck you as excessive, but your wish to see John and I hanged for disagreeing with you is perfectly reasonable??
No it wasn't. The OP's question - from a technical standpoint at any rate - was 100% on topic for this group.
As ever, you demonstrate a grandiose sense of self-worth. I believe I've already informed you of what that trait is symptomatic of.
Not so. To get linear tuning, you need a varactor whose C(V) curve goes as 1/(V-V0)**2 in the range of interest.
There are fancy hyperabrupts whose doping profile is minutely adjusted to make them follow that curve accurately, but generally a dual hyperabrupt will exhibit a range of a few volts someplace where that's a pretty good approximation.
Using the dual varactor with no pad caps won't make for the most stable tuning, but it'll have a wide range, for sure. That way you can find the sweet spot.
Not what you'd do in production, at least not without external AFC, but just the ticket for a one-off, I think.
Cheers
Phil Hobbs
Sure. Lowish-performance audio is simple. Making it work reliably in real life would involve avoiding mode hopping, for one thing, and that doesn't happen by accident.
Current-modulating a diode laser produces both AM and FM, but a photodiode detects only AM. You can detect the FM, but it needs an interferometer, which makes the pointing problem much, much worse.
Cheers
Phil Hobbs
I don't recall that I've ever expressed a desire to see either of you hanged. Disagreeing with me doesn't strike me as a qualifying as a capital crime.
And your ideas about technical matters are worth expressing? In fact the problem was that he changed the specification as he went along.
If I ever had a grandiose sense of self-worth, it would have been drained away by working with people who were just as smart as I am - and I worked at EMI Central Research for a couple of years, where they were thick on the ground. Grow up.
Time has passed, and the circuit has been built:
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