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The Vision Modulator
The Jerrold modulator from the old WB7FID repeater has been reconfigured for the new frequency and had been burned-in for several weeks on the bench. This modulator was originally a cable television head-end modulator operating in the 390 MHz region. Paul Larsen, WA7PXD, kindly procured this modulator surplus, re-worked its upconverter stages, and carefully aligned its filters for the new frequency.
The output level of this modulator is sufficient to drive a hybrid power amplifier such as the Toshiba SAU-4 into sync compression. At this power level it is still remarkably clean, intermod-wise.
Because this repeater is to be an inband repeater installed on a well-shared site, the audio subcarrier modulator built into the modulator is not used. Not using the built-in modulator eliminates any triple-beat problems and eliminates one major source of low-level spurious signals from the transmitter. See the Aural TX page for more information about the Aural Transmitter.
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The vision modulator has been modified to provide "sound-on-carrier" audio. This technique involves modulating the video carrier itself with narrowband FM audio so that one may listen to the repeater's audio using an ordinary HT. The modulator module (try saying that 15 times real fast...) performs several functions: It provides a balanced audio input to prevent ground loops from introducing hum into the audio system, it has a pre-emphasis circuit to properly shape the audio for reception by an HT, it has a 2.5 KHz 3-pole lowpass filter to limit the audio response to keep the occupied bandwidth in the realms where HT's will work well, and finally, it has an audio compressor to keep the audio level more-or-less constant.
The compressor is necessary because the peak deviation is limited to +-3 KHz: The overtone oscillator in the upconverter module cannot be modulated by more than +-7 KHz without causing the oscillator to stop and sufficient "safety" margin was required to maintain reliable oscillator operation and linearity. Even though the deviation is limited, the compressor makes the audio "sound" as loud as a normal 5 KHz channel. Although the audio processor has been configured to provide about 10db of compression, it actually sounds very clean, crisp and "listenable."
Why have this compressor circuit at all? In the case of NASA audio, it is not uncommon for the audio to vary by over 10 db, depending on the source. This amount of variation is quite objectionable on a small HT, as the audio seems to vary from being too quiet to "blasting."
How it works:
Note: This circuit is designed for operation from a single-ended 24 volt DC supply. This was done because the modulator has such a supply built-in, and it takes about 10 volts of audio to modulate the varactor diode in the vision modulator.
Because the modulation imposed on the video carrier is intended to be received on a typical HT, it has pre-emphasis and frequency response limitations imposed as appropriate.
The audio processor is designed to accept balanced audio input. If that is not available, either input may simply be tied to ground: Because it is balanced, any circulating AC currents will be canceled out and no hum should be present.
The balanced audio goes through a lowpass filter consisting of the 680 pf capacitors and series 1.5k resistors (cutoff frequency of about 300 KHz) to keep any RF from getting in as well as a simple highpass filter consisting of the series 0.047 uf capacitors and the 10k biasing resistor with a cutoff frequency of about 150 Hz. This simple filter prevents low-frequency audio from getting in as this audio would not be reproduced by a typical receiver.
The 3 op-amp sections (U1 sections A, B, and D) comprise a balanced input circuit and the output is unbalanced audio. The gain of this stage is set by the 100k resistor between sections A and B. The 0.005 uf capacitor and the 2.2k resistor comprise a pre-emphasis network. Following this is a 3-pole lowpass filter consisting of the 1k resistor, 0.047 uf capacitor, the U2D and its surrounding components. This lowpass filter has a high-end frequency cutoff of 3 KHz: It's purpose is to limit the modulation frequencies to those appropriate for a communications circuit.
Compression is provided by a gain cell consisting of an LED and CdS photocell optically coupled together. This device was constructed by using clear epoxy to glue a red LED to the face of a CdS cell and then "light-proofing" it with black heat-shrink tubing.
When the output of U2B - the driver for the LED - is sufficient to illuminate the LED, the resistance of the CdS cell across the feedback resistor of U2C goes down, reducing the gain of U2C. The time constant of this circuit is that of the CdS cell: It will respond to audio peaks within a few microseconds, but the "release" time is several milliseconds. The result is a fast-acting compressor that will hold down the maximum excursions of the inputted audio to the adjusted level. Note that, in retrospect, it may have been better to put two LEDs back-to-back in the photocoupler - or construct a "full-wave op amp rectifier" so that both positive and negative peaks would trigger the LED. In reality, audio waveforms (especially ones that are bandpass filters such as these) are symmetrical enough. If a bit "steeper" compression is required, the 1k resistor (on LED) may be lowered: This will turn on the LED more "quickly" with audio peaks. The 100 ohm resistor and the 47 uf capacitor are used to prevent the LED current from "modulating" the mid-supply rail.
The compressed audio is passed to U2A, a buffer amplifier. This stage has a settable gain so that the audio output is appropriate for the modulator stage. The U1C stage is used to provide a mid-supply voltage reference with a very low impedance. The 100 uf capacitor is used to eliminate all traces of power supply ripple from the mid-level supply: It was found to be necessary as the 24 volt supply had several 10's of millivolts of ripple on it, which was more than enough to objectionable without filtering. Note that with the time constant of this circuit, it takes several seconds before this mid-supply references stabilizes at its final voltage.
Finally, there is a "mute" line. This simply clamps the audio line to ground with a transistor, shorting it out before it gets to U2A.
How to adjust:
The amount of compression is adjusted by the amount of input audio: The "threshold" is fixed and the audio on the input must be brought up or down (with respect to the threshold) as appropriate. A higher gain on the input means that compression will occur with lower-level audio and vice-versa.
The output level is set after the input gain is set up. A signal is applied that is being compressed - and that is then that is used to set up the maximum amount of deviation. Note that the pre-emphasized audio is that which is actually keys the compressor so - with a true "FM" modulator, the peak deviation is going to be constant no matter what the modulation frequency. If, for example, your application has a much more sensitive modulator, you'd probably want to go about the gain adjustment in a different way.
The varactor:
You may notice that the varactor portion is not shown. This circuit is very simple: The varactor is connected across the overtone adjustment variable capacitor, coupled through a DC blocking capacitor. It is worth pointing that in a typical overtone oscillator, the "adjustment" capacitor is not intended to set the frequency, but rather to tune the crystal into its appropriate overtone mode. It just so-happens that adjustment of this capacitance will have a relatively slight effect on the frequency of oscillation.
When the voltage across a varactor is varied, the vast majority of the capacitance change per voltage occurs below 2 volts. Above this, the rate of change-per-volt typically decreases. In a typical circuit, it is in this upper-voltage range that the voltage/frequency curve becomes more linear: Attempting to modulate audio in the lower end of the curve would result in rotten linearity and, thus, rather poor quality modulation.
Because the varactor is placed across the adjustment capacitor, one can expect to "tune" the oscillator through overtone resonance. What this means is that when the varactor is at too high capacitance (low voltage) the oscillator can quit. This same thing can also happen when the varactor's capacitance is too high.
A compromise point must be reached between the rate of capacitance change (per volt) and the linearity, so the varactor was biased at about 8 volts or so. With the varactor biased there (using a simple filtered, 15 volt zener-regulated supply and a potentiometer) the oscillator adjustment capacitor was set to be in about "midrange" of reliable oscillator operation.
Once this is done, note the bias voltage and then (using the potentiometer) run it up and down over more than the expected peak-to-peak swing of the audio and make sure that the oscillator's operation is stable over that range. At this point, it is pretty easy to graph the frequency-versus-voltage and make sure that it is reasonable.
Using a coupling capacitor, the audio is injected on the voltage bias line of the varactor. As it turns out, about 8-10 volts peak-to-peak of audio is required for the +- 3 KHz deviation obtained.
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The Vision amplifier chain consists of a Toshiba SAU-4 amplifier module (see a picture of it here) followed by Communications Concepts 100 watt push-pull amplifier (modified so that it would actually run at 100 watts!) pictured above.
There are two fans on the amplifier: One on the heatsink itself, and
a smaller fan that draws air across the circuit board itself. Noting
the
overkill heatsink on the amplifier it is no surprise that when operated
in continuous service there is only a barely detectable temperature
rise
on the heat sink. Even if the fans are not operating the heat
sink
has proven to be adequate for continuous operation even at high
(>120 degrees
F) ambient temperature.
Keep watching this page, as it will be updated as time goes
on...Do
you have any questions/comments about what you have just read? If so,
please
email
me and make an ask of yourself...
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