| Electronics > RF, Microwave, Ham Radio |
| Old shortwave radios: coherent detection vs non coherent |
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| mr ed:
Been looking at some overpriced vintage sw receivers. Some boast coherent detectors. I know what coherent detection is ( phase and amplitude like I Q , QUAM etc. ) So, how does this make mono audio better? Something to do with removing carrier noise and getting better signal to noise? |
| radiolistener:
I don't think that it can improve sound quality, it allows to receive DSB modulated signal with no carrier, which require much less power for transmission, but at the same time it requires very stable LO and very stable carrier of transmitter. As a result, this also requires the transmitter and receiver to remain stationary relative to each other to avoid the Doppler effect. |
| antenna:
--- Quote from: radiolistener on January 03, 2025, 05:18:33 am ---I don't think that it can improve sound quality, it allows to receive DSB modulated signal with no carrier, which require much less power for transmission, but at the same time it requires very stable LO and very stable carrier of transmitter. As a result, this also requires the transmitter and receiver to remain stationary relative to each other to avoid the Doppler effect. --- End quote --- You'd have to be going much faster than any automobile can go for doppler shift to make a difference at shortwave listening frequencies. At 60mph (96.6kph), you will have a 1.16Hz shift at 13MHz. |
| radiolistener:
--- Quote from: antenna on January 03, 2025, 05:54:25 am ---at shortwave listening frequencies. At 60mph (96.6kph), you will have a 1.16Hz shift at 13MHz. --- End quote --- this is enough to get 1 Hz fading for DSB. Here is example of 1 Hz offset with DSB demodulator. |
| wofritz:
It can help receiving AM with carrier in situations with heavy selective fading by providing a stable carrier when the received carrier drops out. |
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