3 ms·

Superheterodyne receivers (https://en.wikipedia.org/wiki/Superheterodyne_receiver https://en.wikipedia.org/wiki/Superheterodyne_receiver) use a local oscillator

Superheterodyne receivers (https://en.wikipedia.org/wiki/Superheterodyne_receiver https://en.wikipedia.org/wiki/Superheterodyne_receiver) use a local oscillator to downmix the received radio or television signal for processing at a much more sane, lower frequency. That local oscillator emits its own RF radiation, so it is potentially detectable.

BenjiWiebe 5y ago
Correct. However, if using an rtl-sdr by itself to listen to HF, you are utilizing direct conversion which doesn't use an intermediate frequency. I believe the IF is still being generated (?), but it's no longer indicative of what you are listening to.
- formerly_proven 5y ago
  Direct conversion has no IF but still uses a mixer and a LO, and the LO still leaks out of the input port of the mixer. Because you're directly converting the RF to baseband, LO frequency is the tuned frequency. Unlike e.g. a dual or triple conversion receiver (used in e.g. spectrum analyzers), where the first IF is way above the highest tunable frequency.
- mhh__ 5y ago
  There is almost definitely something to detect if you brought a van full of RF gear, but it's not like the good old days where it was obvious.
jasonzemos 5y ago
Great link as it mentions Radar Detector Detectors. This is actively enforced in the US state of Virginia.
- tata71 5y ago
  Which is one of very few places radar detectors are against the law in the US.
  - jaywalk 5y ago
    I would love to see it challenged all the way up to the Supreme Court if necessary. These laws seem blatantly unconstitutional, and it's kind of amazing that they've held up for so long.
- dguest 5y ago
  It gets better than that: there were radar-detector-detector-detectors [1] which were consumer radar detectors that could detect the police radar-detector-detector and shut down. It's tagged as [citation needed] on wikipedia, but the source [2] does mention this as a feature of some radar detectors. I like to hope it's true. [1]: https://en.wikipedia.org/wiki/Radar_detector_detector https://en.wikipedia.org/wiki/Radar_detector_detector [2]: https://web.archive.org/web/20100620150944/http://radar.757.org/VG2.htm https://web.archive.org/web/20100620150944/http://radar.757....
- eloisius 5y ago
  The lengths highway patrol go to enforce speed limits manually is insane to me. I've never understood why, if speeding is such an important thing to prevent, why don't they use camera enforcement. If you travel between cameras A and B in less than the minimum time it would take at the speed limit, somewhere along the way you were speeding. There's no way around it. You could speed and then travel slower than the limit to average it out, but there would be no point, and enforcement is already spotty. Just send the ticket in the mail, or dispatch a patrol car if someone is speeding recklessly fast. It seems like it would calm traffic, because people would travel the speed limit instead of speeding and then abruptly dropping from 90mph to 65 when they see a cop hidden behind a bridge. I've seen demonstrations that traffic waves cause jams even when there is no problem with throughput, just because someone slammed on their brakes. I'm actually not sure what it'd do to ticket revenue. If there was no way around it because your average speed gets measured, I guess people would stop speeding. Maybe the gambling aspect of it is desirable to governments because if there's a small chance of getting a ticket, lots of people will speed and thus get tickets. If tickets are a certainty, no one speeds and revenue dries up. Do police unions lobby against using this kind of enforcement because it'd mean less cops lurking under bridges for an easy payday?
  - charcircuit 5y ago
    There are legitimate reasons why someone would go faster than the speed limit (safety) that your simple 2 camera setup would not be able to understand.
    - Spare_account 5y ago
      Could you elaborate on some of the safety reasons for exceeding the speed limit, to help me understand? We use average speed cameras in the UK to enforce temporarily reduced speed limits during road repairs on the motorways. It seems to work quite well, it certainly feels like most people adhere to the reduced limits. Personally, I am aware that if I speed (for whatever internal justification I convince myself of), I just need to reduce my average speed by the time I pass the next camera (they're big and yellow and deliberately conspicuous so drivers know where they are).
junon 5y ago
Magic. Got it.

BenjiWiebe 5y ago

Correct. However, if using an rtl-sdr by itself to listen to HF, you are utilizing direct conversion which doesn't use an intermediate frequency. I believe the IF is still being generated (?), but it's no longer indicative of what you are listening to.

formerly_proven 5y ago
Direct conversion has no IF but still uses a mixer and a LO, and the LO still leaks out of the input port of the mixer. Because you're directly converting the RF to baseband, LO frequency is the tuned frequency. Unlike e.g. a dual or triple conversion receiver (used in e.g. spectrum analyzers), where the first IF is way above the highest tunable frequency.
mhh__ 5y ago
There is almost definitely something to detect if you brought a van full of RF gear, but it's not like the good old days where it was obvious.

jasonzemos 5y ago

Great link as it mentions Radar Detector Detectors. This is actively enforced in the US state of Virginia.

tata71 5y ago
Which is one of very few places radar detectors are against the law in the US.
- jaywalk 5y ago
  I would love to see it challenged all the way up to the Supreme Court if necessary. These laws seem blatantly unconstitutional, and it's kind of amazing that they've held up for so long.
dguest 5y ago
It gets better than that: there were radar-detector-detector-detectors [1] which were consumer radar detectors that could detect the police radar-detector-detector and shut down. It's tagged as [citation needed] on wikipedia, but the source [2] does mention this as a feature of some radar detectors. I like to hope it's true. [1]: https://en.wikipedia.org/wiki/Radar_detector_detector https://en.wikipedia.org/wiki/Radar_detector_detector [2]: https://web.archive.org/web/20100620150944/http://radar.757.org/VG2.htm https://web.archive.org/web/20100620150944/http://radar.757....
eloisius 5y ago
The lengths highway patrol go to enforce speed limits manually is insane to me. I've never understood why, if speeding is such an important thing to prevent, why don't they use camera enforcement. If you travel between cameras A and B in less than the minimum time it would take at the speed limit, somewhere along the way you were speeding. There's no way around it. You could speed and then travel slower than the limit to average it out, but there would be no point, and enforcement is already spotty. Just send the ticket in the mail, or dispatch a patrol car if someone is speeding recklessly fast. It seems like it would calm traffic, because people would travel the speed limit instead of speeding and then abruptly dropping from 90mph to 65 when they see a cop hidden behind a bridge. I've seen demonstrations that traffic waves cause jams even when there is no problem with throughput, just because someone slammed on their brakes. I'm actually not sure what it'd do to ticket revenue. If there was no way around it because your average speed gets measured, I guess people would stop speeding. Maybe the gambling aspect of it is desirable to governments because if there's a small chance of getting a ticket, lots of people will speed and thus get tickets. If tickets are a certainty, no one speeds and revenue dries up. Do police unions lobby against using this kind of enforcement because it'd mean less cops lurking under bridges for an easy payday?
- charcircuit 5y ago
  There are legitimate reasons why someone would go faster than the speed limit (safety) that your simple 2 camera setup would not be able to understand.
  - Spare_account 5y ago
    Could you elaborate on some of the safety reasons for exceeding the speed limit, to help me understand? We use average speed cameras in the UK to enforce temporarily reduced speed limits during road repairs on the motorways. It seems to work quite well, it certainly feels like most people adhere to the reduced limits. Personally, I am aware that if I speed (for whatever internal justification I convince myself of), I just need to reduce my average speed by the time I pass the next camera (they're big and yellow and deliberately conspicuous so drivers know where they are).

junon 5y ago

Magic. Got it.