Direct conversion receiver - Page 1
Forum section: electronics / beginners

author: Kudesunik
author_rank: Newbie
timestamp: March 21, 2023, 06:39:17 pm
content: Hello! I am essentially a beginner who has picked up general principles, so I apologize in advance for the terms, I'm not an expert in terminology. So, I'm trying to make a direct conversion receiver with basic modules and with anti-parallel diodes as mixer that could receive a frequency modulated signal. As far as I understand, it is possible only (excluding very exotic designs) if it has phase locked loop (PLL). But at the same time I can not understand a few things.First question: in my example attached circuit approximately what happens in the real circuit, after mixing the FM signal with the local oscillator signal and passing it through a LPF, I essentially get some display of the FM frequency deviation, completely removing the carrier frequency. But I fundamentally do not understand what it gives me. If I try to somehow send this signal back to the voltage controlled oscillator, thus creating a loopback loop, then the mixer will be multiplying the carrier frequency with deviation with the same carrier frequency with same deviation and the output will only be twice the carrier frequency and all. How then do other direct conversion receivers get modulating (sound) signal?Second question: in the second image attached the circuit of the direct conversion receiver with a PLL, I built it and it works. But what is happening inside, I did not fully understand. Firstly, there is a bias voltage on the anti-parallel diodes, but how then will the diodes work as a mixer if they need a signal from the local oscillator equal to +- their opening voltage. And what kind of signal is sent back to the local oscillator?Third question: related to the first question and second circuit. It's more theoretical in general, how bias voltage across varicaps calculated? These are non-linear elements and I don’t see at least voltage divider here, only current limiters.I hope I have clearly expressed the questions, thanks in advance for any answers or advice on reading the right literature

author: wasedadoc
author_rank: Super Contributor
timestamp: March 21, 2023, 06:50:57 pm
content: Quote from: Kudesunik on March 21, 2023, 06:39:17 pmThe voltage controlling the VCO is the modulating (sound) signal.

author: Kudesunik
author_rank: Newbie
timestamp: March 22, 2023, 04:31:05 pm
content: QuoteWhere does it come from if there is no FM detector? In the FM signal itself, it is not there either, there is only a carrier +- deviation. The mixer does not issue it either, as far as I understand.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 22, 2023, 05:36:04 pm
content: The Phase Locked Loop contains a voltage controlled oscillator. The PLL has a phase detector which generates the voltage to make that oscillator track the received FM signal. That control voltage is the recovered audio.

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2023, 05:54:55 pm
content: With a VCO that has a linear voltage-frequency conversion function, a properly-designed PLL is considered the optimal detector for FM transmission.Simplified, the loop filter "around" the tracking VCO frequency can have a narrower bandwidth than the overall bandwidth of the FM signal, thus reducing the noise.These bandwidth considerations for FM are complicated, and involve Bessel functions: the standard textbooks should be consulted.