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In article ,
Ron Hardin wrote: Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660, does anybody else? The R8B hears it except with DSB synch detection, so there's some relation between I and Q channels that cancels the hum. In article Ron Hardin wrote: It's still there at 4:30am EST. It's not audible at all if you're hearing both sidebands but obvious in quiet phone call pauses (no music, not fan noise). It's some difference between the upper sideband and lower sideband that gets modulated by hum; so if you don't hear them equally the hum starts to creep in, and at some point becomes noticeable. (So propagation at night can make the hum come and go if you're hearing both sidebands, by depressing one and raising the other.) WFAN's transmitter is diplexed into a common antenna with WCBS's (880 kHz) transmitter. Part of the diplexing arrangement includes impedance matching networks that are supposed to equalize the antenna load impedance reflected towards the transmitter output over the sideband bandwidth. I suppose they could be slightly out of adjustment, causing asymmetrical sideband response. One side effect of asymmetrical response would be the generation of AM-to-PM. At the joint transmitter site are older Continental transmitters that function as backup for the regular Harris DX50 transmitters. Maybe they have one of the Conti's on occasionally? Most of these high power transmitters run off of 480V 3-phase power. Ripple frequency with a straight 3-phase rectifier would be 180 Hz, not 120 Hz. A popular supply arrangement is the 3-phase 6-pulse rectifier which outputs a 360 Hz ripple frequency. 120 Hz ripple frequency would most likely originate in either an audio or low level RF stage powered from a conventional 2-phase rectifier. I'll take another listen tonight. |
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