The local oscillator for 10.7 to 2.5MHz conversion

  
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A 52 MHz third overtone crystal is typically 30 ohms at its series resonance according to manufacturer specifications. The crystals used for the prototypes have nearly ten times lower series resistance but they were very expensive. An oscillator can be seen as an amplifier which has feedback through a filter. The amplifier of a series resonance os
The local oscillator for 10.7 to 2.5MHz conversion - schematic

cillator must have both a low input impedance and a low output impedance, preferrably well below the series resonance impedance of the crystal, to preserve the selectivity associated with the high Q of the crystal itself. Since this oscillator is used at a higher frequency the amplitude of the oscillations is controlled by a limiter to make the transistor run in class AB. This gives a good compromise between the wideband noise floor and the close in phase noise. Class C is good at low frequencies but a bad idea for VHF oscillators, particularly if they are intended to be used to generate microwave frequencies. Class A amplifiers give the best close in performance and should be used for microwave frequency multiplier chains. The output impedance of the selected MPSH10 transistor is stepped down by a factor of about 20 in a LC resonant circuit. The coil, L14, is 5 turns of 1. 2 mm enameld wire. The coil is wound on a 7 mm rod and mounted self supporting on the PCB with a distance from the copper to the PCB of 3 mm. The length of the coil is 11 mm. It is essential to feed the amplifier Q5 from a low impedance to make the noise generated in this transistor low. The MPSH10 transistor generates very little noise because of the large current feedback on the emitter. Except at the frequency of series resonance the impedance seen by the emitter is high and loss-less. The frequency of oscillation is adjusted with a trimmer in series...



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