August 2014 Archives

AM Transmitter Efficiency Improvement

A carrier control system is sweeping AM radio stations now as AM stations rush to save power, Craig Kopcho says this on his experience:

This is a power saving gizmo that I installed in WOKV's night-time transmitter. It is set for a minus 3 dB dip in carrier and modulation power during periods of peak modulation. You can actually see the power drop with modulation. Now this goes against everything that I have been taught but I notice no degradation in coverage or loudness with the carrier control system working. The advantage of doing this is an overall reduction in power consumption. Again I have been where the signal is almost non-existing and remotely switched this system on and off and I cannot hear the difference. It should be noted that we have been granted permission from the FCC to not conform to traditional carrier shift requirements. The video was shot with the transmitter looking into the dummy load. https://www.youtube.com/watch?v=FARdE7_8bQM
Tom Ray corrects:

Your paragraph... ...is a little misleading. The power in the sidebands doesn't change from full carrier operation - so the modulation power is the same. It's the carrier that drops. And, be honest. When you put it on the air, and first saw the common point meter drop, you thought you had to fix something. That was my reaction.

"Everything old is new again," except there is nothing new here, save for the implementation. The technique originates in AM-compatible SSB and/or DSB with a suppressed carrier (DSB-SC). Dynamic Carrier Control has been around for a while, but real-time conversion from pure AM (even with DCC enabled) to DSB-SC is resulting in lowered power bills significantly (Tom Ray reported consistently measuring a 34% reduction in power consumption with a Harris 3DX50 transmitter, equating to about 23% savings for the entire site after including HVAC savings).

The short explanation of this phenomenon is that the carrier itself is being amplitude modulated prior to mixing or modulation when modulation is maximized. In this way, the implementation is more like AM times AM, or AM squared. It effectively behaves as if one placed a variable attenuator in the RF line and turned down the RF drive level as the modulation level approached maximum modulation. The drive level is returned to normal as the transmitter recovers from the modulation peak.

Historical AM transmitters, like WLW's 500kW station, requires large amounts of power to modulate the plate supply. The modulation power at audio frequencies is fed into a transformer installed between the DC power supply and the RF amplifier tubes and the resulting added or subtracted power effectively turns the carrier on and off on a continuously variable basis up to the limits of transformer core saturation, producing the modulation sidebands around the carrier as the constant carrier is modulated in amplitude by the combination of the two powers. As the transformer approaches saturation, it acts like a reactor or a variable choke, limiting the transfer of power to the final amplifier. This is how the 90-100% modulation is achieved; by pinching off the RF PA's power supply. Old fashioned, but it works.

The above technique is being rediscovered as the Class G amplifier, where the power supply bus rails and transistor biases are switched from several operating voltages, i.e.: -50, -25, -15, -7, -3.5 V and 3.5, 7, 15, 25, 50 V. The amplifier is kept operating as a Class-C amplifier at the different power levels and switched to the next range as more power is needed. Basically, a 150 W transistor (i.e.: 2N3055) is being used as a 100 mW (2N2222 or 2N3904 for the new kids), 1 W, 5 W, or 25W transistor when necessary.  

A two-way FM radio rated for 110W was sometimes rated by the manufacturer for 55W to 110W continuously variable -- that is, the power may be set to any level in that range and left there. The efficiency of the final (Class C) amplifier varies considerably through that range, and at power levels below 80%, may be less efficient than at higher power levels due to the fixed transistor bias levels. Those fixed bias levels cause the transistor to conduct longer at lower power levels, effectively reducing the "pulse like" behavior of a Class C amplifier -- conducting less than 50% of the time, approximately 120 degrees of the 360 degrees in a sine wave (33%) --  into a Class B amplifier, conducting 180 or less degrees (50%) and if bias is set high enough, into a Class AB amplifier, conducting 360 to 180 degrees of the cycle. Here I express conduction angle backwards, since we are moving from more efficient to less efficient as transistor bias voltage goes up and the transistor conducts more and more of the time it takes the sine wave to go through the cycle. That conduction time is being expressed in degrees of a 360 degree circle -- a sine wave may also be expressed as a circle, but that's calculus, vectors, and a whole lot of math.

What DSB-SC is doing is amplitude modulating the RF carrier drive to the final amplifier so that as peak modulation is approached, the RF drive is minimized. The resulting total peak power demand on the final amplifier goes down but the effective signal does not because the carrier is suppressed, reducing the amount of power required to amplify the combined lower and upper sidebands and the carrier.

From http://www.w8ji.com/amplitude_modulation.htm:

(If) Unmodulated carrier = 100 watts (PEP or) average carrier power.

Average is the same as PEP because, absent amplitude modulation, the carrier level is unchanging over time.

100% steady modulated 100 w carrier = 400 watts PEP or 150 watts average or "heating" power. Of this 150 watts average or "heating" power, 100 watts is in the carrier, and 25 watts average power is in each of the two AM signal's sidebands.

Carrier average power = 2/3 of the total 100% modulated average power

Total of both sidebands, average power = 1/3 of total average power under 100% modulation

Average power one sideband = 1/6th average power with 100% modulation

Peak Envelope Power 100% symmetrical modulation = Four times carrier power
As the carrier power is dynamically adjusted lower as modulation power increases, the total power required will flatten out, thus allowing an amplifier of a given size to effectively produce a signal as well as a conventional "pure AM" transmitter. One would think the CB radio guys would have figured this out a decade or two ago to get "more power" from a 4W AM or 12W SSB radio. DSB-SC, one would find the power level somewhere between those two numbers, but that's another story entirely.

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