A6A3
A6A3 Last Converter, Circuit DescriptionThe Last Converter has three main purposes:
1.Converting the 321.4 MHz IF signal from the A6 RF Module down to 21.4 MHz for processing by the
2.Providing the means for correcting for variations in conversion loss versus frequency in the input circuitry.
3.Filtering out 278.6 MHz image response.
The Last Converter consists of 11 basic elements. They are a 321.4 MHz amplifier, a 321.4 MHz bandpass filter, a 321.4 to 21.4 MHz converter a 21.4 MHz preamp, two pin diode attenuators, two pin diode controlled variable gain amplifiers, an output stage, a temperature compensation circuit, and power supply filtering.
Power Supplies @
The power supply filtering consists of series inductors and shunt capacitors (L21 to L24 and C49 to C53). In addition to this, R38 and VRl provide a 6.2 V source used for biasing transistors.
321.4 MHz Amplifier @
The 321.4 MHz amplifier consists of Q8 and its associated circuitry. Q7 provides bias for QS by setting the base current.
321.4 MHz Bandpass Filter @
The 321.4 MHz amplifier drives the 321.4 MHz bandpass filter which consists of L4 to L8 and C8 to C12. This filter is made up of five LC tank circuits coupled magnetically and capacitively. Capacitive coupling occurs through PC board traces on the circuit side of the PC board. Coupling into and out of the filter is done with tapped inductors L4 and L8. The filter is tuned by means of C8 to C12. This filter rejects the 278.6 MHz image response.
321.4 to 21.4 MHz Converter @
The 321.4 MHz filter drives the 321.4 to 21.4 MHz converter. This converter consists of a packaged
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