The AA8V 6x2C Crystal Controlled Converter
by Greg Latta, AA8V

Local Oscillator Schematic Diagram and Circuit Description

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Schematic Diagram and Circuit Descriptions Page

Introduction:
The local oscillator is the heart of the converter. It is the most critical circuit in the converter because any drift or instability in the local oscillator will translate into drift and instability in the output signal. The use of a crystal controlled local oscillator guarantees the oscillator will be absolutely stable.

In the 6x2C converter the local oscillator operates at either 3.146 MHz or 10.24 MHz.

With the 3.146 MHz crystal, signals in the 30m band, 10,100 kHz to 10,150 kHz, are converted to 6,954 kHz to 7,004 kHz in the 40m band.

With the 10.240 MHz crystal signals in the 20m band, 14,000 kHz to 14,090 kHz, are converted to 3,760 kHz to 3,850 kHz in the 80m band.

The unusual crystal and output frequencies are caused by the crystals I had to use. They were the only ones I could find.

Local Oscillator Circuit
Click On Any Part Of The Picture Below For Information On That Part Of The Circuit


Click On Any Part Of The Picture Above For Information On That Part Of The Circuit

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Local Oscillator Circuit:

B+ Plate Voltage: The B+ voltage for the oscillator is obtained from the converter power supply. This is approximately 122V. Since the oscillator is crystal controlled, it is not necessary to regulate the B+.

Plate Dropping Resistor: A 1000 ohm resistor is used to isolate the oscillator plate circuit from the B+ bus. The voltage drops from 122V to 118V across the resistor. This 4V drop indicates that the oscillator draws about 4V / 1000 ohm = 4 mA in normal operation.

Plate Bypass Capacitor: The plate bypass capacitor assures that the power supply side of L3 and L4 is at RF ground.

Band Switch: The band switch is a double pole two position switch. Pole S2A (shown here) selects the correct plate coil and capacitor.. The other section, pole S2B, selects the correct crystal.

Plate Coils and Capacitors: Coil L3 and capacitor C13 are used on the 30m band and resonate at 3.146 MHz. Coil L4 and capacitor C14 are used on 20 m and resonate at 10.24 MHz. Coil L3 consists of 38 turns of #29 magnet wire close wound on a 3/8" diameter powdered iron core coil form. It has an inductance of 10.6 uH with the slug fully out and resonates at 3.21 MHz with a 240 pf capacitor across it. Coil L4 is a North Hills 120-8 coil. It is rated at 3 uH -5 uH. With the slug in the middle of the windings it measures 5.54 uH at 10.24 MHz and resonates at 10.24 MHz with a 43 pF capacitor across it.

Output Coupling Capacitor: The output of the local oscillator is coupled to the grid of the mixer through a 10 pf capacitor.

6GH8 Vacuum Tube: The 6GH8 was originally designed for use in the horizontal deflection circuitry of television receivers. Previous experience with it has shown that it is ideal in this application. The triode is used as the local oscillator, and the pentode is used as the mixer. The two sections are internally shielded from each other. Click here for a 6GH8A data sheet . The 6U8 can be used in this application as well. It is a direct plug in replacement. No circuit modifications are needed. The 6GH8 may provide slightly better gain and improved resistance from pulling on strong signals, but the 6U8 should still work fine. Click here for a .6U8A data sheet.

Grid Leak: When the oscillator is operating, the grid is driven positive and rectifies some of the signal. This grid current develops a voltage drop across the 100k resistor that biases the tube. This method of obtaining bias for the tube is known as grid leak bias.

Crystals: The band switch is a double pole two position switch. Pole S2B (shown here) selects the correct crystal. The other section, pole S2A, selects the correct plate coil and capacitor. With the 3.146 MHz crystal, signals in the 30m band, 10,100 kHz to 10,150 kHz, are converted to 6954 kHz to 7004 kHz in the 40m band. With the 10.24 MHz crystal signals in the 20m band, 14,000 kHz to 14,090 kHz, are converted to 3760 kHz to 3850 kHz in the 80m band. The unusual crystal and output frequencies are caused by the crystals I had to use. They were the only ones I could find.

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