Engineering Radio – Page 144 – One Hundred percent Human Generated Content for your Reading Enjoyment

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.

A broadcast console makers perspective

I received a great email from Michael “Catfish” Dosch, console designer for Telos / Axia Audio Systems. The email was sent in response to a comment I posted on the WEBE WICC Studio Build Out post. I thought the email was very interesting and informative, presenting a perspective that most broadcast engineers do not often see or appreciate. I asked Mike if I could use it as the basis for a blog post and he agreed. I am not going to blockquote the entire thing, but here are the unedited email and pictures.

Quote:

“Ken said you had a concern about the ruggedness of our consoles as compared to the old PR&E boards. You might not know this, but I was with PR&E before joining Telos. In fact, I designed many of those old PR&E boards. I guess that makes me an old console designer. Ahem.

The Element design is more modern in construction and styling, but it is no less rugged than those old PR&E boards. In fact, you could stand on it if you wish. The top is a 1/4-inch machined aluminum plate supported by structural aluminum ribs on the backside. The chassis itself is made of custom extruded aluminum structural pieces and machined aluminum side panels. The flat sheet metal on the bottom is not structural, it’s only a cosmetic cover. You’ll see a lot of folded sheet metal in other consoles because it’s cheap and easy. But it’s not as rugged as the Element approach which is why we chose to go with a more complex and expensive mechanical design.

Telos Axia console cross section — Telos Axia console cross-section, Courtesy of Axia Audio / TLS corp

One very visible difference between Element and PR&E consoles is the use of Lexan on the front panels (PR&E would use aluminum or steel on the top panel). This might seem less rugged, but it is actually chosen because it is a more durable surface than painted and silkscreened metal. It is more scratch resistant and it is rear-printed so that the markings never wear out. Silkscreens would wear off under heavy use — particularly next to faders and monitor controls — and look horrible over time. These Lexan panels will look just as good after 15 years as they do now.

But Lexan for all of its durability has its own limitations. The edges can crack under abuse. This is why you see many older Wheatstone consoles (they have used Lexan overlays for many years) with cracks and tears at the very edges of the plastic. This is particularly troublesome in the fader slot. A frayed edge on a faders slot can cut your fingers. That is mighty unpleasant! So when we decided to use Lexan, we wanted to have all the benefits and none of the drawbacks.

So we designed a machined recess on each channel that allows the Lexan insert to have its outside edges protected by the aluminum. More obvious are the bezels around each button and even the fader slot. Look carefully and you will notice that all of the control bezel edges are above the lexan. The edges of the lexan are not exposed and therefore not prone to cracking, chipping, or splintering.

Axia Audio console control surface, courtesy of Axia Audio / TLS corp — Axia Audio console control surface, Courtesy of Axia Audio / TLS corp

In this drawing, you can see the panel without the lexan. The machined pocket to protect the outer edges of the Lexan, plus the raised edges of the button and fader bezels to protect the edges around the holes. These button guards are also designed to prevent accidental actuation of the buttons. And while the guards are designed to protect accidental actuation, they never hinder deliberate activation. Notice the guards at the sides of the ON/OFF buttons and not on the top and bottom. Even operators with long fingernails will have no problems with these controls. The small round keys are engaged with a light touch of the fleshy pad of the fingertip.

Yes, I think we built great consoles at PR&E. But Axia was a fresh start, a chance to raise the bar even higher, by retaining many of PR&E’s better attributes and improving upon some of the weaker areas. DIPswitch configuration has been replaced with the convenience of the web browser. Spill-prone motherboards and electronics have been eliminated from the control surface. Unreliable monitor pots have been replaced with optical rotary encoders rated for 5,000,000 rotations.

And you asked about the faders. This is a particularly important component in a broadcast console. PR&E used Penny & Giles faders for many years. We used their Series-4000 faders in the X-Class consoles (BMXIII, AMX, ABX and STX). This was their top-of-the-line fader at the time and performed beautifully… for a year or two. Then our clients started experiencing field failures at a very high rate. We worked with P&G on a return/rework/replace program that took years to clean up. Our clients were disappointed and we spent a fortune making things right. It was that experience that caused us to begin searching for alternatives.

The market for high-end faders is quite small. There are tons of consoles out there for live sound, home recording, etc., but these products are sensitive to costs and generally use very cheap faders. There just aren’t enough high-end recording consoles or broadcast consoles being built to attract a lot of fader vendors. After a lengthy search, I disqualified all but two fader companies: P&G and a Japanese firm by the name of Tokyo Ko-on Denpa (TKD). I assigned one of our engineers to create a set of environmental and life-cycle tests to see if the TKD faders could keep up with the P&G faders. We were all shocked by the results.

Out of 100 of each type tested in various environmental conditions and physically cycled for the accelerated equivalent of 10 years of heavy use, we had only one TKD fader failure, compared to more than half of the P&G Series 4000 faders! We defined “failure” as any deterioration to specifications or any discontinuities. All the failed units had discontinuities (audio dropouts). We were able to clean the failed TKD fader and it passed the retest. About half of the failed P&G units were cleaned and passed the retest. So in the end, the practical results were TKD 100% good and P&G 75% good. Not what I expected at all.

We then designed a TKD fader into the Radiomixer. We watched the customer support logs carefully for problems. Out of the first 1,000 console channels shipped, we saw one TKD fader failure during the first year. Warranty replacement of course. The failure rate did not increase with use as you would normally expect. We were seeing consoles with 3 or 4 or 5 years of heavy use with no fader problems at all. I have heard of 20 year old Radiomixers with original faders still working great.

One particularly elegant feature of the TKD fader used in Element is a side loaded wiper arm. This prevents liquids or other foreign matter from spilling into the fader slot and directly into the fader element. This feature alone is probably responsible for extending the useful life of the faders by a considerable amount. Of course, these can be disassembled and cleaned just like a professional fader from P&G, they just don’t need it so often.

Axia Audio TKD faders, courtesy of Axia Audio / TLS Corp

Some have the misconception that if a fader is not P&G, it must be cheap. Actually, these are very expensive faders, about the same cost as P&G. But they are so well made, I think they’re worth every dollar. I know there are still some folks out there who remember P&G’s glory days when they made bullet-proof faders. I remember fondly those days as well. But in my experience, the TKD fader is superior to the equivalent P&G fader. We feel so confident, that we warrant all Axia consoles for 5 years, including all components….”

End Quote

That is a great explanation of what goes into one of these consoles right from the designer. The pictures are courtesy of Axia Audio / Telos Corporation and special thanks to Mike for taking time out to give us a glimpse into the mind of a console manufacturer.