Fig, 3. Diode current, noise figure, and conversion loss of tuner.*
condition the diode generates harmonics: either the sum or the difference of LO and rf signals is extracted from the output of the mixer diode and is called the if signal. In UHF TV tuners, the LO signal frequency is above the rf signal. Until 1966 point contact diodes were used as mixer diodes in UHF rv tuners but Hot Carrier Diodes are seeing increased usage because of better reliability, better V-l characteristics, low noise figure amd low conversion loss.
The inductance L2 and capacitance C4 provide a low-pass filter for the //signal and provide a matching network between the diode output and the // amplifier. L2 and (4 are self resonant at 80 MHz for image suppression. The mixer diode current is adjusted by changing the size of the pick-up loop in the oscillator cavity. The magnitude of the diode current depends on loop area and its proximity to the line Lg. The location of the loop was chosen to prevent overloading the oscillator. Since the location of the loop would change the load reflected to the transistor, it could cause the oscillator to stop. Fig. 8 shows the diode current, noise figure and conversion loss variations across the UHF TV band for one diode loop setting. A smooth current output is desired since excessive variations imnair overall reception. Forward biasing the mixer diode will affect the noise figure and output current; however, an optimum bias setting is difficult to establish because the oscillator output is not constant across the UHF band. Conclusion
The tuner described here performs with an output that was not appreciably affected by variations in transistor parameters. Since mass production was not the aim, life expectancy or durability of the cavity under use was not analyzed,
^Copyright, Fairchild Semiconductor, 1968/Printed in U.S.A./BR-03-0003-38/5M
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