A Modulator.

At power levels from about 5 watts to 5000 watts the modulation is done in the last, highest power, stage. A specially made transformer couples the audio from an audio power amplifier into the plate circuit of the RF power amplifier as shown in Figure 6.2 below.

Suppose we want to modulate a 100 kc carrier with a 6 kc audio wave. The modulator will pass the original frequencies through but will create two new frequencies, 94 kc and 106 kc. For 100 percent modulation, the condition shown in Figure 6.1, the new frequencies will be one half the amplitude of the carrier. These new frequencies are called sidebands.

Although the audio, 6 kc, may be present, the tuned circuits in the antenna matching network, the antenna its self, and the nature of propagation of radio waves will not allow the 6 kc audio to be picked up by a distant receiver so we will neglect that frequency from now on.

What are sidebands?

It should be noted that few AM stations actually succeed in transmitting audio frequencies higher than 10,000 cycles. Even if they did who would be capable of hearing them? AM receiver designers have narrowed down the IF bandwidth to minimize interference and thereby have cut the audio response to 5,000 cycles at best. There were very few hi-fi AM tuners ever made and few sold. Although I am probably not the only one still using such a tuner I'll bet that you could count them on your bodily digits and not have to take off your shoes. But I digress.

In practice the example of WSM places the individual cycles so close together as to turn into a solidly colored in area on your screen. For this next part you need to be able to see the individual cycles of the carrier and sidebands so we will return to our carrier frequency of 100 kc. There are radio stations that operate in that part of the spectrum but their programs are not very interesting.

As already said a 100 kc transmitter modulated 100 percent by a 6 kc tone will produce the carrier of 100 kc and two sidebands at 94 kc and 106 kc each having half the amplitude of the carrier. Let's tune across that signal with a high class receiver that has a switch that changes the IF bandwidth. If we set the bandwidth to 100 cycles and tune across the signal we will verify that the 100 kc carrier is there and the two sidebands at 94 and 106 kc have half the amplitude of the carrier. We will also see that their amplitudes are constant as shown in Figure 6.3 below.

The bottom wave is in deed the sum of the top three. Colored vertical lines have been used to mark off points of particular significance. The magenta (some people would call it purple) lines mark the points in time where the two sidebands cancel each other out leaving only the carrier. Green lines mark the points where all three are in phase and a sum of twice the carrier amplitude results. Red lines point out where the two sidebands add together but together they cancel out the carrier leaving no output.

If the modulation was speech or music the sidebands would not be regular sine waves but the carrier WOULD be. That's very important. The nature of the carrier, frequency, phase, and amplitude, does NOT change regardless of the type or percent of modulation.

Where Does All the Power Go?

Saving power, Suppressed Carrier - Double Sideband (DSB).

Put the Carrier Beck.

So we have suppressed the 1000 watt carrier and saved a bunch of power but there is an additional saving that can be made. The information in one sideband is a duplicate of the information in the other. So we need to transmit only one sideband giving another 50% power saving. So our original transmitter that sent out 1500 watts under full modulation is now down to 250 watts for the same signal in the receiver on the other end.

There is actually an additional gain because of the reduction of bandwidth. The original AM, or even the double sideband, signal required a receiver with an IF bandwidth of twice the highest modulating frequency. The single sideband signal requires a bandwidth which is the same as the highest modulating frequency. For telephone quality communication, about 200 to 4000 cycles would require an AM bandwidth of 8 kc where as a single sideband receiver need only have a bandwidth of 3800 cycles.

Notice we are able to lop off the 200 cycles at the bottom. With a double sideband signal the 200 cycle lower limit gives a 400 cycle gap right in the middle of the transmitter's spectrum. There's nothing we can do with it. But in single sideband there is nothing beyond the 200 cycle limit so we can chop it off. The narrower the bandwidth the less noise is picked up and the better the intelligibility.

In practice hams, the military, and commercial communications that uses the HF band, 3 Mc to 30 Mc, use an audio frequency response of 300 to 2400 cycles which requires an IF bandwidth of 2.1 kc. That became the standard because the Collins Radio Company made the best SSB equipment for hams and military when SSB was replacing AM in the late 50s and 60s. Collins arrived at what they felt was the best compromise between bandwidth and intelligibility. They used a 2.1 kc mechanical filter in the receiver's and transmitter's IF section. A 2400 cycle upper limit may be pushing the boundary of intelligibility but if distortion is kept low a trained ear has no trouble understanding what is said. Collins knew how to keep the distortion low. The Collins rigs were, and are, easy to recognize on the air. They just sound better.

No, I don't own any stock in Collins Radio. I wish I had bought some in 1960. I also wish I had bought stock in IBM in 1955, and in Microsoft in 1980, and so on and so on. There I go again.

Generating Single Sideband

The filter method starts out with a single balanced modulator that produces a double sideband suppressed carrier signal. Then a crystal or mechanical filter removes the other sideband. Such a filter can't be tuned so the signal is generated at a fixed frequency such as 455 kc and converted to the desired frequency then increased in power to be fed to an antenna. If that sounds like a backward receiver, that's exactly what it is. Another similar filter was used in the receiver.

Transcevers.

Frequency Modulation (FM).

At the top of this page I mentioned that early AM transmitters probably had a lot of FM. In those earliest transmitters they were modulating the oscillator. If you change the DC voltage applied to an oscillator you do change the amplitude but you also change the frequency. A gadget that was very popular in the early days of transistors was a miniature AM transmitter. It was a little module with 4 wires coming out, two for the 9 volt battery, one for audio, and one for antenna. The battery negative served as the ground for audio and antenna. These produced very poor quality because of the large amount of FM that accompanied the AM.

If you are up on current technology you may be thinking of a variable capacitance diode (varicap). Even in transistor oscillators you have to be careful not to apply too much oscillator signal so as not to drive them into forward conduction or reverse breakdown. I have never used one of these devices in a tube circuit but I'm not confident it could be made to work. The higher RF voltages found in tube circuits would likely preclude their use unless you took very special precautions to keep the RF voltage small.

Any incidental AM that may occur can be eliminated by passing the oscillator through a tube which is being driven from cutoff to saturation. Such an amplifier might be called a limiter or it might be known as a class C amplifier. The latter are always used in radio transmitters.

FM Detection

After limiting it is the job of the detector to recover the audio from the modulated signal. The detector starts with a special coil called a discriminator transformer. The primary and secondary are tuned as in any IF transformer but there is a third winding called a tertiary pronounced tershiary winding.

This word tertiary comes from Greek or Latin or something like that that means 3. It's all Greek to me.

So at the resonant frequency of the secondary the tertiary voltage leads the secondary voltage by 90 degrees. Below the resonant frequency of the secondary the tertiary voltage leads the secondary voltage by less than 90 degrees. Above resonant frequency of the secondary the tertiary voltage leads the secondary voltage by more than 90 degrees. This little transformer has converted the frequency change into a phase difference change.

The Phase Detector.

Is that enough?