Cursitor Doom is wrong a lot of the time. He's probably not the worst troll here - John Doe is even worse, and Flyguy isn't far behind. Nobody else is in the hunt.
I'm not from Sydney - I might live in Sydney now, but I was born northern Tasmania, went to university in Melbourne, then spent 22 years in the UK and 19 more in the Netherlands. Decadent Linux User Numero Uno is an American - as is obvious to everybody except Cursitor Doom.
I can't say that I've noticed that Sydney's lingua franca is particularly coarse - I do hang around with university professors and the like, so I might not hear the same sort of language that Cursitor Doom would if he visited here to further his criminal interests.
Queensland is worse. Look at Clive Palmer.
formatting link
Admittedly he was born in Victoria (in 1954) and wasn't moved to Queensland until 1963. Peter Dutton is another creep from the Deep North.
Because taking a few hundred millivolts AF and making a few micro-watts VHF/UHF with pretty good to very good quality can be as a simple as a one transistor "bug" like you probably messed about with as a teenager.
However the converse, taking a few nano-watts VHF/UHF and converting into hundreds milli-watts headphone level AF is going to require many transistors. I am not counting a super-regenerative receiver as although their linearity can be good enough to count as HiFi their high noise level will rule them out.
I reckon there is no need for crystal control if tx-rx are in the same room environment experiencing similar temp drifts, a crude AFC in the receiver could cope.
piglet I am planning off powering the transmit from a PC usb port, so I should have 5V/1A to play with. I don't know what the efficiency of a FM radio transmitter is, but even if it's pretty bad I should be able to push a couple hundred milliwatts into the air so I anticipate the sensitivity of this system will not be a big problem. If there's an easy way to multiply up a crystal to the required frequency then I think I'll do that, else I will go with an LC based frequency generator. The transmitter consuming 5 watts will probably make it heat up though, so I will be wary of temperature-dependant frequency shifts in my design.
You seen to be talking about a phase modulation (PM) system rather than FM. That'll work, but it doesn't have FM's SNR advantage.
Armstrong's patent for FM makes a pretty good read actually--his big insight was that in the high-SNR limit, widening the bandwidth _improved_ SNR rather than degrading it as you'd expect. This is because in that limit the noise is linearly downconverted by mixing with the desired signal, so you only get one audio bandwidth's worth of noise regardless of how wide the deviation is.
Once the SNR drops below about 15 dB, that stops being true--the noise intermodulates with itself, so you get contributions from the whole RF passband.
The other thing that reduces noise in FM systems is _capture effect_ in limiters. (Some types of FM detectors have a limited amount of amplitude limiting, so it's sometimes erroneously claimed to be specifically an FM effect, but it happens in any system with a limiter.)
Basically a limiter just preserves the zero crossings of its input signal, so it suppresses amplitude noise. That's good for a 3 dB SNR improvement right there, but less obviously it also suppresses noise and interference, even right on top of your signal. This is because the stronger signal sets the rate of zero crossings--the weaker ones can make them jiggle back and forth a bit, but can't change their frequency. You can convince yourself of this with a simple phasor construction. I'm too lazy to do an ASCII one, but you can see it by searching inside my book on Amazon for "capture effect". It's discussed on p. 547-548.
If you figure out how to make a nice linear VCO, you can use one for TX and a matching one in a PLL for RX. You don't need a terribly linear phase detector for FM, because your output signal is the VCO control voltage and not the phase detector output.
Ham radio folk have indeed made FM transmitters by frequency pulling or phase shifting a crystal and harmonic multiplying up many, many times to transmission frequency. In the 1970s I worked on all-descrete theatrical/Film/TV VHF radio microphones that used that method so it was considered HiFi capable.
If you haven't already then you might find ham radio publications useful. Old ARRL handbooks are a good source of RF lore and I am a fan of Wes Haywood's book Experimental Methods in RF Design. Hundreds of milliwatts is non-trivial both legally and technically. Have fun!
Phil my intuition was that the reason FM got a SNR boost from going to wider bandwidths was the information carrying capacity is bandwidth * log(SNR) and SNR is linear with power, so if you spread out your power across bandwidth you are basically taking it outside the log function, making the information bandwidth linear with power again (minus the small decrease in the log). I don't know how well this maps onto the proper explanation. I hadn't thought to take a look in your book for help with this - when I was considering the IR option I remembered you pointing out the cascode trick you can do on solar panels to get audio frequencies worth of bandwidth out of them, but like I said earlier I wanted to do an RF project.
For optimum audio quality, do you recommend going the PLL demodulator route, or would a quadrature detector as described here:
formatting link
I can't really find much on the linearity of such detectors. Intuitively it would seem to me that they would not work so well with a large modulation index since it would push the phase difference away from the linear region. Discrete PLL's can apparently be a bit tricky to do in discrete components so I think I'll stick with the simpler quadrature scheme as described above unless it will seriously impact the quality of the output.
That's why it's possible, but not why it works. There are a whole lot of ways of making the SNR worse by widening the bandwidth. ;)
In a pure analogue optical link, you also have to cope with the shot noise of the ambient light, which makes life difficult. (Sunlight is very bright.)
Quadrature detectors are sort of OK, but depend very sensitively on the details of the 'quadrature coil'. A nice wideband PLL with a really linear VCO is the ticket. Pulse counting can be OK too if the IF is high, but you're going to need a linear VCO anyway, so you might as well just use two of them.
Depends. With FM, you're using amplitude-limited waveforms, so the multiplier functions just like an XOR gate, which is nice and linear.
A bilinear phase detector such as a Gilbert cell has a sinusoidal V(phi) characteristic if either of the waveforms is sinusoidal. If you overdrive both inputs so that it's not bilinear anymore, you get this nice triangular characteristic.
If you want to do a pure discrete design, a la 1966, you'll certainly learn a lot, but I think you're unlikely to get anything that a modern listener would describe as high fidelity without a great deal of effort and time. It's your project, of course, but I'd suggest allowing op amps and ceramic IF filters and such. (A 1966 design would also be a bit unwieldy for headphones--you'd wind up looking like Bullwinkle the Moose.)
Mini-Circuits will sell you nice diode-bridge mixers for a few bucks, and those make good phase detectors. (You're going to need a phase detector for your quadrature detector anyway.)
A quick IC comparator such as a FIN1002 LVDS line receiver could make a pretty good limiter--its propagation delay is only a few nanoseconds, and its output edges are a bit faster than 200 ps. Plus it costs 27 cents in reels. ;)
I thought FM and PM were basically identical with the difference being the response to the modulation frequency. Audio equalization is used to compensate.
They're both examples of _angle modulation_, but in practice are completely different. The key distinguishing feature of FM over PM is much wider deviation, leading to much better SNR for the same received power.
An angle modulator driven by a pure tone produces
v = sin(2 pi f_0 t + M sin(2 pi f_mod t)).
The modulation index M is the peak phase deviation in radians, and is also the ratio of the RF and AF bandwidths. In the high-SNR limit, you win SNR like 20 log M, so wideband is a big win.
It's a lot more difficult to make a very wide-range, very linear phase shifter than it is to make a good VCO.
Another route that should be easy, is to use any broadcast FM receiver with one of the many available Walkman/iPod FM transmit boxes, usually automobile targeted. There's very low transmit power, and a selector switch for selecting an unused frequency for transmit.
Third branch, is Bluetooth (but that takes some software or a prebuilt solution).
whit3rd the idea of this project was a fun and educational thing, not necessarily a practical one. So an off-the-shelf thing that works is not what I want. wrt bluetooth actually this entire project came about due to my disgust about the absolute trainwreck that is bluetooth, with its ridiculously high latency and poor reliability. You can literally send a signal around the entire planet in less time than it takes a signal to get from my laptop to my headphones.
Hobbs yeah I have no problem using single-purpose block parts, I'm well aware that building something out of bjt's and twisted wire is beyond my current abilities. I was thinking of using something like this for the phase detector as if my thinking is correct I would also be able to use it for the 933MHz->IF downconversion too:
@LM I know about those, but I'm pretty sure they will have way too much distortion for what I want. I would like to have a high quality audio signal. I'm not an audiophile or anything but I don't want to be able to tell the difference between this and an the same signal sent over an audio cable.
Never heard of him. However, even your Wikipedia states:
"In December 2012, on Christmas Day, Palmer hosted a buffet lunch for
650 disadvantaged people, mostly children and their families." Among a list of similar philanthropic activities, so he clearly doesn't deserve your denigration. I suspect you only despise him for being a conservative politician. As a Communist, you would naturally detest anyone you percieve as 'counter-revolutionary' no matter how kind hearted and charitable they may be.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.