Mrf901 Oscillator

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CQMftÛLLEO

local OSQUAItt*

shown in I ig. 2[a) is representative of the MDS receive system as described m this construction article. This system consists of a mixer, voltage controlled local oscillator and i-f amplifier located at the antenna, and the com bination power supply/tuning unit, located at the TV set. The RG-59/U feed line then performs the dual purpose of carrying the i-f signal down the line to the TV and carrying the power supply/tuning voltage up the line to the converter.

This results in a nice, cheap, and efficient system requiring no expensive feedline and connectors and a minimum of critical circuits. No special test gear is required for tune-up.

The block diagram shown in Fig, 2(b] is one of many configuration alternatives possible and is shown mainly to provoke some thought about other uses for the basic system An example of another use would be for fast-scan amateur television where the usual DX signal would be much weaker than an MDS signal, and the guesswork of tuning is eliminated through the use of a crystal-controlled local oscillator

Circuit Description: The Local Oscillator

Building a satisfactory local oscillator is the next hurdle one must overcome in the construction of a microwave converter You will be thankful to know that the twin-diode mixer helps us here. too, because this mixer requires an LO running at only half the frequency needed for other mixers.

In this system we have done away with all the drudgery ot those crystal-controlled oscillatormulti-plier chains also and settled on a nice simple free-running oscillator.** This oscillator has a tuning range of about 900 1300 MHz and a power output capability of several milliwatts. You might ask, "Is that possible? Won't it drift?" The answer to both questions is "yes/' Not only is it possible, but it's also very simple.

Photo C This is a top view ot the power supply/tuning unit

Mrf901 Downconverter

Photo D. This photo shows a complete MDS receive system. The tripod\ mast, and U-clamps are standard TV items. All connectors and the box containing the downcon-verter should be weatherproofed.

The oscillator consists of a printed circuit board with an etched inductor, a Motorola MRF901 transistor, and about six other parts. This ts about as simple as it could be. As far as drift goes, we have found that it is at a very slow rate, completely due to changes in outside temperature,

A TV set is a very broad receiver and will tolerate considerable dirft. Since this LO is a voltage-controlled oscillator and can be remotely tuned from the control box, frequency adjustments can be made at any time. If this type of stability is not satisfactory for you, you may wish to build a crystal-controlled LO. We might suggest the one used on the interdigital converter3 with a doubler from a Paul Shuch article,*0 Possibly another consideration along this line would be the Paul Wade article in Ham Radio, October, 1978.M Although these alternatives would double the size of the converter, it may be worth it —particularly if you plan to distribute the signal to a number of TV sets. This is because, when tuning the converter for one TV set, the others are affected. This may be necessary once your neighbor finds out what you are watching.

Circuit Description: I-f Amplifier

The i-f amplifier is a two-stage amplifier using dualgate MOSFETs. The function of the i-f amplifier is to boost the signal from the mixer and atso provide enough power to drive the RG-59/U coax line to the control box and TV set. This amplifier has been used to drive up to 600 feet of RG-59/U cable with enough signal at the far end to still have a good picture. Although the center frequency is 63 MHz (channel 3), the amplifier is broad enough to cover channels 2 through 4. The output of the amplifier goes into a 75-Qhm, 3-dB pad. This pad gives the amplifier a constant impedance to look into, and, along with the ferrite beads on gate two of both of the MOSFETs, helps make this amplifier very stable.

Circuit Description: Power Supply/Tuning Unit

The power supply used in the MDS receive system performs two separate functions, First, it supplies the operating voltage for the i-f amplifier and local oscillator, and second, because it is adjustable and the local oscillator is frequency-sensitive to voltage, it provides tuning control for the downcon-verter.

The 24-volt ac transformer, diode bridge, and filter capacitor provide about 34 volts dc to the 12-volt regulator. The regulator is made adjustable by the addition of the fixed resistor, R1, and the variable resistor, R2. With the values shown in Fig. 4, the voltage is adjustable from 12 to 17 volts.

Rf choke RFC1 isolates the rf from the supply, and the dc is carried via the feedline to the downcon-verter DPDT switch SI performs the dual function of switching the TV between the VHF antenna and the MDS downconvert-er and properly terminating the unused lead with a 7552 resistor.

It may seem that the current capacity of the power supply far exceeds the requirements of the down-converter, and it does. The

1979 Ham Radios

Continue reading here: Converter

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