The AA8V 6x2 Superheterodyne Receiver
by Greg Latta, AA8V

BFO Schematic Diagram and Circuit Description

Click here for a higher resolution (larger) schematic.

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Schematic Diagram and Circuit Descriptions Page

The beat frequency oscillator (BFO) is used for CW and SSB reception. The oscillator generates a signal which is mixed with the output of the IF amplifier in the detector to produce an AM signal that can then be demodulated by the detector. The BFO is activated when the mode switch is placed in the "C" (CW/SSB) position.

The BFO uses 1/2 of a 6CG7 dual triode in a Colpitts oscillator circuit. In a Colpitts oscillator, capacitance is used to provide the feedback required for oscillation. The frequency of operation is controlled by the resonant frequency of coil L8, BFO/Pitch control C8, and the two 330pf capacitors in series. Stability in the BFO is as important as stability in the local oscillator. Any drift cannot be tolerated. Expensive silver mica capacitors (indicated by a "*" on the schematic) must be used throughout to guarantee stability. To protect against frequency changes caused by B+ variations, the oscillator is operated from a regulated 108V source.

The oscillator operates between about 1698kHz and 1702kHz. The frequency is controlled by the BFO/Pitch control on the front panel. For CW reception, the BFO/Pitch control should be adjusted to the high side of zero beat (lower capacitance for C8). Operating on the high side of zero beat takes advantage of the asymmetrical response of the crystal filter to provide the best single signal reception. The BFO should also be operated on the high side of zero beat when receiving lower sideband SSB signals on 40m band. For reception of lower sideband SSB signals on the 80m band, the the BFO/Pitch control should be adjusted to the low side of zero beat (higher capacitance for C8).

Click On A Section of the Schematic
Below for Information on That Part of the Circuit:

BFO Schematic Regulated Plate Voltage Plate Decoupling Capacitor Load Resistor 6CG7 Vacuum Tube Grid Leak Resistor Grid Capacitor Output To Detector BFO Pitch Control Oscillator Tank Circuit And Feedback Capacitors

Or click on one of the links below:

 Oscillator Tank Circuit and Feedback Capacitors
 BFO/Pitch Control
 Grid Capacitor
 Grid Leak Resistor
 6CG7 Vacuum Tube
 Load Resistor
 Plate Decoupling Capacitor
 Regulated Plate Voltage
 Output To Detector

Oscillator Tank Circuit and Feedback Capacitors:
The BFO frequency is determined by the tank circuit, which consists of coil L8 and two 330pf capacitors in series. The two 330pf capacitors effectively form a center-tapped 165pf capacitor, which is in parallel with L8. BFO/Pitch control C8 provides for fine control of the oscillator frequency.

The two 330pf capacitors do double duty. In addition to resonating with coil L8, they also form a capacitive voltage divider which allows some of the output from the plate to be fed back to the grid, sustaining oscillation.

BFO Tank Circuit

BFO/Pitch Control:
Different BFO frequencies are needed for USB, LSB, and CW reception, so the frequency of the BFO must be adjustable. This is accomplished by placing a small variable capacitor across the oscillator tank circuit. The control, mounted on the front panel, allows the BFO to be adjusted from about 1698kHz to 1702kHz.

BFO Pitch Control

Grid Capacitor:
The grid capacitor allows the oscillator tank circuit to be coupled to the grid of the tube, while blocking the plate voltage, which is also present on the tank circuit.

Grid Capacitor

Grid Leak Resistor:
Grid current rectified by the grid of the tube passes through the grid leak resistor and provides grid leak bias for the tube.

Grid Leak Resistor

6CG7 Vacuum Tube:
The 6CG7 is a dual triode perfectly suited for operation as a detector and BFO. A general purpose triode, it is electrically equivalent to the popular 6SN7-GTB octal tube.

One triode functions as the beat frequency oscillator, while the other section functions as the detector.

You can click on the following link for a 6CG7 tube data sheet.

6AG7 Vacuum Tube

Load Resistor:
The 100k resistor provides a load for the oscillator. As plate current goes up, the voltage drop across the resistor increases. As the plate current goes down, the voltage drop across the resistor decreases. The voltage across the resistor is thus in direct proportion to the current through the tube.

Load Resistor

Plate Decoupling Capacitor:
To prevent RF from the BFO from getting into the B+ supply, a 0.01uf capacitor shorts to ground any RF that may have gotten through the 100k load resistor.

Plate Decoupling Capacitor

Regulated Plate Voltage:
The BFO frequency is sensitive to the plate voltage on the tube, so the BFO is operated from a regulated source. The regulated 108V source also supplies the local oscillator, crystal calibrator, and the mixer screen grid.

The regulated voltage is only applied to the BFO when the mode switch is in the "C" (CW/SSB) position.

108V Regulated Plate Voltage

Output To Detector:
The BFO signal at the plate of the tube is too high, so the output of the BFO is taken instead from the grid of the tube. The BFO output is coupled to the detector through the BFO coupling capacitor.

Output To Detector

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