I once purchased a cheap FM transmitter that had a USB port for charging your device. Super cheap. $5, I think. It sounded like crap, so I decided to open it up and see how they managed to build it so cheap.
The first indicator that it might be super cheap was the fact that it would transmit on two frequencies; if I recall, those were 88.5MHz and 106.2MHz. "That's strange", I thought, "most North American FM radios won't tune an even-numbered frequency (the .2)."
Once I got inside, I realized the magic. The FM transmitter section was based around a 17.7MHz crystal. 88.5MHz was the 5th harmonic, and 106.2MHz was the 6th harmonic. There was a single transistor that was used as part of the oscillator. "OK", I thought, "that explains the bad sound quality."
Then I started looking closer at the PCB and discovered a few parts that were missing. The scariest was in the USB power section: instead of a 5V regulator (as indicated on the board), there was instead a jumper directly from input to output. That is, it was pumping the noisy, spiky, 13.8V from the car's ignition system directly into the USB port. Luckily, I didn't ever actually plug anything into that port, but it could have gone miserably bad...
The first indicator that it might be super cheap was the fact that it would transmit on two frequencies; if I recall, those were 88.5MHz and 106.2MHz. "That's strange", I thought, "most North American FM radios won't tune an even-numbered frequency (the .2)."
Once I got inside, I realized the magic. The FM transmitter section was based around a 17.7MHz crystal. 88.5MHz was the 5th harmonic, and 106.2MHz was the 6th harmonic. There was a single transistor that was used as part of the oscillator. "OK", I thought, "that explains the bad sound quality."
Then I started looking closer at the PCB and discovered a few parts that were missing. The scariest was in the USB power section: instead of a 5V regulator (as indicated on the board), there was instead a jumper directly from input to output. That is, it was pumping the noisy, spiky, 13.8V from the car's ignition system directly into the USB port. Luckily, I didn't ever actually plug anything into that port, but it could have gone miserably bad...