A primer video on superheterodyne receivers

Deacon

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Superheterodyne receivers are awesome bits of genius. Their applications include not only listening to the radio in your car but also amateur two-way (ham) radio, radar detectors, and many more.

This video does a great job of explaining what a superheterodyne receiver is, why it works, and how it works. It also even accidentally explains why radar detector detectors can exist.

 

STS-134

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Interesting...he used the beat frequency explanation to explain how a mixer works without really explaining how a mixer works. But he didn't mention that there's another step -- converting from IF to audio frequencies and then lowpass filtering the resulting signal to filter out any of the frequencies around the desired signal (although generally the 455 kHz bandpass filter should be able to take care of most of that).

I also wish he had used some spectrum plots of what the carrier and both sidebands look like at RF, IF, and baseband. Seeing this in pictures helps a lot of people understand what's going on. Then once you understand what double sideband amplitude modulation looks like, you can begin to understand how my favorite radio works. I got it when I was in grad school in San Diego and had trouble picking up KNBR 680 AM from San Francisco due to a strong Mexican station on a neighboring frequency (690 AM). This was after MLB started streaming baseball games on the internet, but at the time, I was too cheap to pay for it. On my car radio, the noise from 690's lower sideband splattering all over 680's upper sideband was extremely annoying to listen to especially when 690 would play music -- I'd be able to hear the beat of the songs on 690 overwhelming the signal at times. So no problem, I got a radio that could select either the lower or upper sideband, and since we're dealing with an AM (non suppressed carrier, double sideband) signal, it also has a synchronous detection circuit so I don't even have to fine tune the carrier frequency. So tune to 680, turn on synchronous detection, select lower sideband, and no more noise. As a bonus, I got the ability to tune to the longwave and shortwave bands in addition to that.
 

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I also wish he had used some spectrum plots of what the carrier and both sidebands look like at RF, IF, and baseband. Seeing this in pictures helps a lot of people understand what's going on. Then once you understand what double sideband amplitude modulation looks like, you can begin to understand how my favorite radio works. I got it when I was in grad school in San Diego and had trouble picking up KNBR 680 AM from San Francisco due to a strong Mexican station on a neighboring frequency (690 AM). This was after MLB started streaming baseball games on the internet, but at the time, I was too cheap to pay for it. On my car radio, the noise from 690's lower sideband splattering all over 680's upper sideband was extremely annoying to listen to especially when 690 would play music -- I'd be able to hear the beat of the songs on 690 overwhelming the signal at times. So no problem, I got a radio that could select either the lower or upper sideband, and since we're dealing with an AM (non suppressed carrier, double sideband) signal, it also has a synchronous detection circuit so I don't even have to fine tune the carrier frequency. So tune to 680, turn on synchronous detection, select lower sideband, and no more noise. As a bonus, I got the ability to tune to the longwave and shortwave bands in addition to that.

Something like this?

149210903.E4EvPsft.AM_SWBC_03142013_0048.jpg


T!
 

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Something like this?

View attachment 170773

T!
Kind of like that, although there would have been a strong carrier 10 kHz to the right of the main carrier with its own sidebands, one of which overlaps with the upper sideband of the AM signal you're trying to hear. Since the station often played music, its sidebands would have been a lot more "active" than the desired sideband, and would have made the station hard to hear unless you selected the LSB only.
 

Token

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Kind of like that, although there would have been a strong carrier 10 kHz to the right of the main carrier with its own sidebands, one of which overlaps with the upper sideband of the AM signal you're trying to hear. Since the station often played music, its sidebands would have been a lot more "active" than the desired sideband, and would have made the station hard to hear unless you selected the LSB only.

Got it, something like this:

171361984.edvleZXx.Sideband_Overlap.jpg


Yes, AM Synchronous detection with selectable sideband is a nice thing to have. Of course you can just use SSB also, selecting the sideband you want or that is clear, some receivers have too narrow a filter in SSB though.

T!
 

Disco47

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Am I the only one who thinks this looks like the shroud of turin?
Screenshot_20210123-180440_Chrome.jpg
 
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