Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

Post by kragen » Sun, Nov 13, 2022, 4:33 AM UTC

Johnythree wrote: post #80

kragen wrote: post #73

Earlier quoted context omitted.

A broad band of spectral components generated by the Raspberry Pi's clock generator because it has a lot of phase noise? Or do you just mean that in general that many circuits that generate clock signals can have phase noise but that you don't have any idea about this one in particular? He doesn't seem to be using a mixer, so I don't think you'll get any spurs caused by mixing products in this case. I'm no expert obv…

A PLL is one of the worst for phase noise. It will have a divider chain which will produce jitter and harmonics of the ref osc, and must absolutely rely on careful tuning of the AFC loop to null out these spurs. And as the filter will be digital rather than analog, there is no chance of optimizing it. But that is before we get to the modulation. That again will be digital (rather than analog) so by definition will be…

Post by anticensor » Sun, Nov 13, 2022, 5:59 AM UTC

Johnythree wrote: post #71

simfree wrote: post #68

Earlier quoted context omitted.

Amplifiers are often protocol agnostic. Whether your amplifying TDMA, Trunked radio, CDMA etc. Aviation needs to modernize, I have zero sympathy for the aviation community getting constant hallpasses on ancient altimeters that lack filters to reject RF from nearby frequencies, leaded avgas, and abysmal working conditions for those stuck in the industry. The FAA dragging their feet in the USA trying to act like they s…

Aircraft use AM on VHF because it is still the most efficient mode. Unlike FM it doesn't suffer from the "Capture Effect" so you can tell if a weaker signal is trying to get through on a busy channel. And unlike FM it doesn't suffer from the Threshold Effect where weak signals suddenly disappear into the noise. And of course Digital modulation is ruled out by both of the above considerations. The only mode which is m…

Post by kragen » Sun, Nov 13, 2022, 7:57 AM UTC

pifm_guy wrote: post #78

kragen wrote: post #73

Earlier quoted context omitted.

A broad band of spectral components generated by the Raspberry Pi's clock generator because it has a lot of phase noise? Or do you just mean that in general that many circuits that generate clock signals can have phase noise but that you don't have any idea about this one in particular? He doesn't seem to be using a mixer, so I don't think you'll get any spurs caused by mixing products in this case. I'm no expert obv…

In the case of PiFM, the clock generator is being operated out of spec by a wide margin - it is rated for operation up to 25 MHz, but the code operates it at 100Mhz. That makes it do some weired things - and I suspect one of those is that when adjusting the clock frequency (by changing the divisors), sometimes it stops oscillating for many microseconds. Not sure what impact that has in the frequency domain, but I can…

Post by anthk » Sun, Nov 13, 2022, 12:52 PM UTC

Johnythree wrote: post #67

anthk wrote: post #57

Earlier quoted context omitted.

I know the reverse of your story. And OLD TV from a grandma jamming an entire town: https://www.popularmechanics.com/science/a34229436/old-tv-br...

Yep, this was surprisingly common. When the picture tube in old B/W TVs lost emission, the EHT voltage increased to the point that it was constantly arcing and flashing over. But because the arcing was synchronised to the TV signal, it didn't cause much interference on the faulty TV set. But it did create a horrid rolling bar on any TV set nearby which was tuned to a different channel. So the old lady had no idea she…

Post by thebruce87m » Sun, Nov 13, 2022, 9:39 PM UTC

Johnythree wrote: post #37

cpsns wrote: post #24

Earlier quoted context omitted.

So? It's a proof of concept. Tinker away, laws be damned.

Until an angry Radio Inspector arrives on your doorstep having spent all day driving around your town with a spectrum analyzer to track you down. When you get to attend court, the judge will award the standard fines (and possible jail time) but also the cost of the time spent by the Radio Inspector in traveling to your town.