Old shortwave radios: coherent detection vs non coherent - Page 1
Forum section: electronics / rf-microwave

author: mr ed
author_rank: Regular Contributor
timestamp: January 03, 2025, 04:59:47 am
content: 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?

author: radiolistener
author_rank: Super Contributor
timestamp: January 03, 2025, 05:18:33 am
content: 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.

author: antenna
author_rank: Frequent Contributor
timestamp: January 03, 2025, 05:54:25 am
content: Quote from: radiolistener on January 03, 2025, 05:18:33 amYou'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.

author: radiolistener
author_rank: Super Contributor
timestamp: January 03, 2025, 06:05:53 am
content: Quote from: antenna on January 03, 2025, 05:54:25 amthis is enough to get 1 Hz fading for DSB. Here is example of 1 Hz offset with DSB demodulator.

author: wofritz
author_rank: Contributor
timestamp: January 03, 2025, 08:36:07 am
content: It can help receiving AM with carrier in situations with heavy selective fading by providing a stable carrier when the received carrier drops out.

author: radiogeek381
author_rank: Regular Contributor
timestamp: January 03, 2025, 11:50:15 pm
content: Was this feature used to recover the carrier for a DSBSC (suppressed carrier) signal. (This became a thing in SW broadcasting.)Though one can tune an LO carefully to the nominal carrier frequency, unless the LO is phase locked to the carrier, you'll here some odd audio effects that a friend once referred to as "gas music from Jupiter."To some it may even sound like selective fading, but it isn't. The fading is caused by phase shift between the two sidebands causing cancellation and reinforcement alternately.

author: Dave Wise
author_rank: Regular Contributor
timestamp: January 04, 2025, 06:05:53 pm
content: Quote from: wofritz on January 03, 2025, 08:36:07 amThat's why I used a synchronous detector when I was listening to AM shortwave broadcasts. Without it, every fade lost audio; with it, I barely noticed. I was using a Hammarlund SP600 (with a long AGC time constant) with IF output routed to a Sherwood SE-3.

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2025, 06:25:25 pm
content: Quote from: mr ed on January 03, 2025, 04:59:47 amAre you sure they aren'tcohererdetectors?Verydifferenthttps://en.wikipedia.org/wiki/Cohererhttps://www.geojohn.org/Radios/MyRadios/Coherer/CohererMobile.html

author: mr ed
author_rank: Regular Contributor
timestamp: January 04, 2025, 09:39:23 pm
content: Dsb Suppressed carrier is new to me, I have not been active for decades. I can see the need for a precise carrier injection else one sideband side and goes up in frequency and the other down without it. Donald duck speaks!For regular am, I guess it helps fooling the agc? Why not just turn off the agc then?As a side note, it seems there is a price premium for many 30 to 40 year old sw receivers. No bargains on ebay for sure. I'm used to getting top quality test gear, way better built or much higher spec as better deals.