How it works
- The system takes a carrier frequency and splits it into two identical signals.
- The signals are first phase shifted 135 degrees from each other (to provide a base power output with zero modulation from the transmitter).
- Each signal is then phase modulated by the audio signal, one signal is positively phase modulated while the other is negatively phase modulated.
- The two signals are then amplified to a desired power.
- Finally, the two signals are summed in the final output filter stage of the transmitter.
The result is that when the signals are closer in phase, the output amplitude is larger and when the signals are more out of phase, the output is lower. A complication is the necessity for a "drive regulator", which implementation is quite simple at 10 kW or lower levels, but is more complicated at higher levels. "Drive regulation" is most effective when the instantaneous power output approaches zero.
The Ampliphase system was not developed by RCA, but by McClatchy Broadcasting (a former group owner of AM, FM and TV stations, also a California publisher of newspapers, not to be confused with the present-day McClatchey Broadcasting LLC), first at KFBK, Sacramento, CA (50,000 watts full-time), and at KOH, Reno, NV (5,000 watts days/1,000 watts nights). Other McClatchy AM stations (KBEE, Modesto, and KMJ, Fresno, both of CA) employed conventional transmitters.
The Ampliphase design, originally proposed by H. Chireix in 1935 and termed "outphasing" by him, was later sold by McClatchy to RCA, which turned it into a mass-produced product, first at the 50,000 watt level, and, later, at the 10,000 and 5,000 watt levels. Unlike most other commercial designs of AM broadcast transmitters Ampliphase units do not require modulation transformers nor modulation reactors, thereby saving initial cost. The down-side is the units require more maintenance. Essentially, the Ampliphase concept trades lower "capital" cost for higher "expense" cost, while achieving a modest improvement in efficiency. KFBK still maintains an RCA BTA-50H (the "last gasp" of the Ampliphase concept) as an auxiliary transmitter, but its main transmitter is a solid-state Harris unit, a prototype for which later became the DX-50. KOH has long since scrapped its home-built "outphasing" transmitter for conventional units.
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