March 2011 – Page 3 – Engineering Radio

Methods for generating Amplitude Modulation

Amplitude Modulation (AKA AM) was the first modulation type to impress audio on an RF carrier. Prior to this, information was transmitted via on/off keying of a continuous wave transmitter using Morse code or some equivalent.

There are several methods for generating AM in a transmitter.

1. Low-level modulation. The modulation is developed in the first stage RF section, then amplified by subsequent stages to full power. Simple and easy to implement, especially for mobile transmitters and SSB installations. Disadvantages come from the need for linear amplification through all the stages requiring class A or AB amplifiers and do not reproduce wide band AM well.

2. Doherty modulation. William Doherty came up with an ingenious way to use a low-level linear modulator with good to excellent efficiency. Under full carrier, no modulation conditions, the carrier tube is generating the RF carrier, and the peak tube is mostly cut off (very little current). When modulation is applied, the peak tube then begins to conduct, the output of this tube is combined with the output of the carrier tube through a 90° LC network, which is the same as a 1/4 wavelength transmission line. The effect of this is to lower the output impedance, thus allowing the carrier tube to modulate 100 percent.

Later, Continental Electronics and Jim Weldon somewhat modified this system in their 317C series high-power transmitters.

Continental 317B simplified schematic diagram

3. High level or plate modulation. The RF and Audio sections are developed separately within the transmitter, then combined in the final stage of the transmitter. Older systems used a modulation transformer. The advantages are all the amplifiers can be run class C or greater, which reduced electrical consumption and power supply requirements. Much higher power levels are achievable with this design. These transmitters also reproduce wide-band audio much better than low-level modulated units. They are also extremely rugged. Disadvantages are the system requires large audio sections and they take up a greater area and are not as efficient as later modulation methods.

4. Ampliphase. A phase-modulated system developed by RCA where the transmitter developed two RF signals in the final, 135 degrees apart. To modulate the signal, the phase relationship between the carriers is varied, more toward 180 degrees would be a negative peak, and more toward 90 degrees a positive peak. These transmitters required less space and were more efficient than traditional plate-modulated transmitters. They required careful setup and tune-up to reduce distortion and somewhat unfairly earned the name “amplifuzz” from some engineers.

RCA BTE 20 ampliphase AM exciter — RCA BTE 20 ampliphase AM excit

5. PDM or PWM. This is also a high-level modulation scheme but with some slight variations. The carrier power level and modulation levels are set by a PDM encoder card. In Harris transmitters, the PDM frequency was 75 KHz. The carrier is set by the amplitude of the PDM waveform, and the modulation is determined by the duration of the pulse. PDM transmitters require power supply voltages about twice the voltage of a standard high-level plate-modulated transmitter. They also require a damper diode to conduct the B+ voltage to back to the power supply during negative peaks, otherwise, the PA voltage will attempt to rise to infinity. I have found the damper diode to be the weak link in a tube-type PDM transmitter.

Solid state transmitters also use this design with either MOSFETs or BJT, which are then combined in parallel to generate the required output power. This is most often called “Class E” or something similar. In that system, each pair of modulator MOSFETs has its own fast-acting damper diode, usually protected by a fuse.

6. Direct Digital Synthesis. This is a patented design from Harris Broadcast used in their DX series transmitters. The incoming audio is sampled at either the carrier frequency or 1/2 the carrier frequency depending on where in the band the station falls. The solid-state PA modules are then switched on and off at the carrier frequency with the audio levels imposed on the carrier information. The explanation is simple, the application is complex:

Harris DX series AM transmitter block diagram

Of all these transmitters, the Harris DX series is the most efficient from a power input (AC) to power output (RF) perspective. There are several methods of reducing electrical use by reducing carrier power levels during lulls in modulation. The Continental 418E and later series transmitters can reduce carriers up to 6 dB using CCM. Harris and Nautel use similar systems on their DX and XL transmitters respectively. The wheatstone corporate blog has an article: Greener AM transmission Methods that details others.

As far as simplicity, serviceability, and rugged design, the high-level plate modulated transmitters cannot be beaten. Many Amateur Radio operators build these units from scratch using old parts, tubes, and other reused equipment readily available, often for free. I have, in fact, donated several 1 KW AM transmitters to ham radio operators over the years.

If I were to design a “transmitter of last resort,” to use in case everything else fails, it would look something like this:

813 Tube type 250 watt transmitter final

813 Tube type AM transmitter modulator section

The disadvantage of that design is it requires a 2KV power supply, which has its own set of safety concerns. I might substitute 833s for 813s and use heavier iron in the modulation transformer. That way the transmitter could develop a 500 to 1,000-watt carrier. The great thing about tube transmitters is, given the right output components, they can be tuned into almost any load. They are also easily adaptable for emergency operation into temporary antennas.

Bell System microwave relay system

This is a map of the AT&T microwave relay system as it was in 1960. It is interesting for several reasons. First of all, before there were communications satellites, this is the way that data was transferred from one location to another. That data would have been digitized and TDM encoded on a T-carrier, then loaded onto a microwave path. TV networks had loops that transversed the country, distributing network video and audio to all the markets in the US. The first transcontinental New York to San Francisco microwave route was established in 1951. Through the fifties and sixties, the network was filled in across the US and Canada.

Radio networks had been using wired TELCO networks for program distribution for years, although they required far less bandwidth than TV. This was during the time when network affiliation was vitally important to a station. Radio networks provided news and other special event programming, as well as some long-form shows which were an important source of information for the listeners. Any network programming prior to 1980 or so would have been carried by this system.

It was not until the use of C and Ku band satellite services that networks could offer multiple channels of programming. Now, entire radio formats could be programmed remotely and beamed into hundreds of stations across the country simultaneously. That would have been far too expensive to implement over TELCO lines, as the line charges were based on the mileage of the circuit.

Click for higher resolution.

This system included thousands of hardened microwave relay sites, each built to exact specifications and fully redundant. At the time, the long-distance telephone system was an integral part of the US defense planning. Sites were spaced 20-40 miles apart, depending on terrain. In congested areas, like the northeast, area mountain tops are dotted with these sites today, mostly empty. Most of these sites went offline in the late 1990s as phone companies switched to fiber optic cables for telephone and data traffic.

American Tower, Inc. purchased most of these sites in bulk from AT&T in the year 2000. Some sites are well positioned for Cellular Telephone, 3G, and 4G wireless data services, plus other things like Media Flow and general use applications like FM broadcast and two-way. Many sites, however, do not meet any specific needs and sit empty. There was a large fire sale by American Tower in 2002 in which they unloaded about 1,900 of these sites as they were redundant.

I wrote a post titled Cold War Relic: ATT long lines site, Kingston,NY detailing one of these sites near me. Keep in mind, there were thousands of these sites throughout the country.

The burned contactor fingers

This is a set of burned contactor fingers on a Harris HS-4P 30 amp RF contactor:

Harris HS-4P RF contactor with burned finger stock

The back story is this:

The contactor in question is at the base of Tower #3 of the WBNR (1260 KHz, Beacon, NY) antenna array. This is the tallest of all the towers, at 405 feet. As such, it gets struck by lightning often. There was at least one occasion where one of the inductors in the ATU got “sucked in” due to the huge magnetic field of a high current strike. It is not at all surprising to me to find other component issues in this ATU. Because of the burned contacts, I’d suspect that the station was switching modes under power, but I didn’t see that happening today.

The problem manifested itself in very high SWR after changing over from day pattern to night pattern. This did not occur every time, in fact, it only occurred once in a great while at first. Then, over the last couple of months, it began occurring more and more often. Since the snow drifts are now down to a manageable six to eight inches, it was a good day to go out and do some exploring.

First of all, I put the station into nighttime mode just to confirm that there is still an issue. The transmitter, a Broadcast Electronics AM1A showed very high SWR and carrier fold back. Left it in night pattern, but turned it off and took a walk, not a drive, to Tower #4 which is all the way at the bottom of a hill, near the old City of Beacon landfill. I figured that I would check that one first, then look at Tower #3 on the way back. When I got to Tower #3, I found the issue right away.

Fortunately, I was able to salvage a set of contact and contactor bar from another relay in the same ATU that was not using them.

The night pattern is only 400 watts, but these are tall towers, 225 degrees, therefore current and voltage are high at the base. In fact, the slightest change at the base of the nighttime towers will greatly upset things.

This is the repaired contactor. I will say, the EF Johnson RF contactors are easier to work on. Those are the ones with the big rocker bar across the top and two solenoids on either side. All of the wiring, status switches and contacts are exposed and easy to get to. This one, not so much. This is the BE AM1A transmitter

It is not a bad unit, compact, sounds good, is reliable, etc. In order to work on the power supply or anything in that top cabinet, the whole thing needs to be removed from the rack and taken down. I suppose that is my only gripe about the thing.

Continental Shortwave Transmitters

I started my radio career working in HF radio, albeit somewhat different than broadcasting. I enjoy the long-distance aspect of HF communications and there is something about the high-power shortwave (HF) rigs that interest me. This is a video of a Continental 418E HF transmitter. The carrier power is 100 KW capable of 100% modulation, which means the peak output power is 400 KW. This particular model has a solid-state modulator, which is in the cage where the guy is walking around. From the video, it would appear they had several blown fuses in the modulator section. The fuses protect the individual IGBTs in the modulator.

This is an older transmitter that is getting upgraded to a 418F. The heavy cable is the connection between the solid-state modulator and the RF final section. Depending on modulation levels, it carries around 33 KV.

From the Continental Electronics website that details the SSM unit:

The modulator consists of 48 series connected modules which are switched on or off to provide the high voltage DC and the superimposed high level audio voltage. The switching is accomplished with Insulated Gate Bipolar Transistors (IGBT). A low pass filter follows the series connected modules which removes the switching signals and allows the DC and audio signals to pass to the RF amplifier. Because each of the modules is either in full conduction with very low loss, or turned off, again with very low loss, the overall modulator efficiency is in excess of 97%.

A full description of the SSM is on the Continental Electronics SSM website. It is an interesting read, including the description of the 12-phase transformer setup.

Finally, a video of the VOA transmitter site in Greenville, NC.

This is part 4 of 5, if one wanted to, one could click through to Youtube and watch the rest of them. The VOA stuff is, as the transmitter engineer notes, 1950s technology. No solid state modulators in these rigs. Those are some old transmitters, still in service and likely to remain that way until the VOA closes that site down, at some point in the future.

Like their FM counterparts, Continental HF transmitters are the gold standard when it comes to high-power tube transmitters. Sadly, they no longer make transmitters for Standard Broadcast (AM MW).