SSB techniques are not limited to shortwave systems. In fact, some ham radio operators use SSB on the 160-meter band, which lies in the medium-wave spectrum.
There is no technical reason that AM broadcast stations (including "Part 15 AM" systems) could not use SSB.
However there are legal reasons for licensed US AM broadcast stations not to do so, because that transmission mode is not permitted for them by the FCC.
Compliant Part 15 AM operators don't have such a legal restriction, except maybe for metering/understanding the final r-f stage input power of the transmitter during modulation.
But regardless of that, they do have a practical restriction in that almost nobody in the general public receiving such SSB transmissions would be able to demodulate them without very high (and unacceptable) distortion of the program audio.
We need Rev. Robert to come in here and give more details about his transmitter idea. Like prophets from the bible he doesn't give us enough to go on.
I, for one, do not know what I'm talking about.
Rich, for another, does know what he's talking about,
but none of it is necessarily what Robert was talking about.
Radio8Z took the leisure way out.
Many wise people have gone on to other topic postings.
Many wise people have gone on to other topic postings.
Or maybe otherwise?
Taking Robert's points one by one here I speak to the idea of using a crystal oven for direct digital systhesis (DDS) which is a method of producing specific frequencies.
The frequency accuracy of DDS depends, as does the accuracy of conventional oscillators and phase locked loops (PLLs), on the accuracy of the crystal used as a reference. The crystal accuracy can depend on the supply voltage, the temperature, and the aging of the crystal. The aging effects are long term and over the course of a year typically are 1/10 the drift compared to the short term drift caused by temperature changes.
The circuit voltage is easily stabilized and is not an important factor. The aging is related to the quality of the crystal and is not under the control of the user except for periodic calibration. This leaves the temperature which can be controlled. This is the function of the oven which keeps the crystal and many times the associated circuitry at a constant temperature around 75 degrees C with the idea being that if the temperature is stabilized the so will the drift due to temperature changes.
The maximum allowable drift in frequency for a radio transmitter depends on the service in which it is used. For AM broadcast the tolerance is +/- 10 Hz from nominal. A question remains as to whether this can be achieved without an oven but remember there is no frequency accuracy specification for part 15 operation, it is just a good idea to keep it as close as possible to the nominal frequency to reduce co-channel interference to your own signal.
I have recently run a test of frequency vs. temperature for a PLL circuit I designed and constructed and found the following. The circuit uses a 74HC4060 IC as the oscillator with a 10.24 MHz crystal purchased for $1.35 and two silver mica caps. for crystal loading and a glass piston trimmer cap. for frequency adjustment. The ambient temperature was changed from 39 to 110 degrees F and the PLL frequency was measured as it varied with temperature. The data are:
Temp F(Hz)
39 - - 1680002
74 - - 1680000
80 - - 1679999
90 - - 1679998
100- -1679997
110- -1679996
The maximum change in frequency over this temperature range is +2/-4 Hz or 6 Hz which is small enough for use in part 15 AM broadcasting.
Neil
From your posting, Radio8Z, I gather that using a "crystal oven" with a DDS circuit, as proposed by Rev. Robert, is a sensible and quality assuring idea.
However, you did not say whether a crystal oven was used with your temperature experiments with a working system, so I guess it was not, and that you found the tolerances with your circuitry within the range acceptable to quality part 15 operation.
As to Mr. Rich, or Dr. Rich, as it may, I liked the "otherwise" for its play on a word.
BALANCED MODULATOR
Since, as said above, balanced modulators are used to produce DSB and SSB, I believe Robert meant to say "unbalanced modulator" which is a lesser-used name for an "asymmetrical modulator". Without resorting to exotic modulation schemes, asymmetrical modulation can be achieved through audio processing only. The Innovonics 222 audio processor is one example. Two limit levels are implemented. One for positive audio peaks, and a second for negative audio peaks. In this way the AM modulation envelope can be shaped, for example, to 130% positive peaks and 95% negative peaks. All that is required of the transmitter is allowance for enough head room in the modulator and RF output stage to handle the positive peaks without hitting the power rail voltages. This is exactly the same as the oft talked about "head room" of a stereo amplifier.
SHIELDED TOROID
A toroid coil configuration, by nature, already confines the magnetic flux substantially within the "doughnut", so an external shield would add little value. A shield would be detrimental to the tank operation because it will add capacitance between the toroid windings and ground. As Neil said, the added capacitance will shunt across the antenna causing some loss of current to the antenna. Another way to look at it is the capacitance will be in parallel with the coil and, therefore, will lower the coil Q.
check your spam box phil. sent you a email got no reply. interested in another box for my hamilton
Carl asked, and I should have included in my post, about the crystal being in an oven. It was not. The purpose of this test was to determine if the PLL is suitable for outdoor use. I have yet to get it down to 0 degrees F but plan to later. The tests were done using a styrofoam shipping box with a small fan, a thermocouple sensor, a temperature controller, and an ice pack for cooling and a 60 watt light bulb for heating. To get down to 0 requires that I put the circuit in the freezer with the power and monitoring wires attached. I just haven't gotten to that yet.
I have done similar testing on the crystal oven system I designed and posted some months back and it holds frequency from 0 to 110 degrees F to within the resolution of the frequency counter which is +/- 1 LSB.
Neil
Large Trimmer Capacitor
and Capacitive Isolated Stages.
Why, in the first case, and what/why in the second.
Large Trimmer Capacitor
Just guessing here. I think Robert may have intended that a large trimmer capacitor would provide a desirable greater tuning range.
There are two ways to configure a trimmer, parallel and serial.
A parallel trimmer is connected in parallel with the antenna capacitance. In this case the trimmer capacitance should be small relative to the antenna capacitance (30 pF) so that we don't get too much shunting of current away from the antenna. A 10 pF trimmer would allow variation of 30 pF to 40 pF. The range for resonance must be supplied accordingly by the coil taps.
A serial-connected trimmer must be fairly large with respect to the antenna capacitance to be useful for tuning over the best range. A 365 pF variable capacitor connected in series with a 30 pF antenna will provide a range of 0 pF to 27.7 pF. Even with a larger value variable, the max will approach 30 pf. Only the upper range of maybe 20 - 30 pF combined is useful, and the coil taps should be positioned accordingly. My observations show this is not a good way to go. The measured output is significantly lower indicating severe losses through the variable capacitor.
Capacitive Isolated Stages
Again guessing here. I think Robert's intent was to isolate stages in the transmitter so that variations in the output circuit don't propagate back to the PLL or oscillator, which would lead to instabilities that would cause "FMing" of the AM carrier. This has apparently been a problem with some transmitters, but it doesn't need to be a problem when present-day standards of design and PCB layout practices are observed. I agree that it is an absolute requirement!
the larger ATU components mean less loss. appreciatively coupled stages is what is used in the THIIv5. it prevents the different stages from affecting one another.