MFJ-8100
World Band Receiver
7
with an RF gain pot that has little effect on frequency or
regeneration.
__ Manageable, "tame'' regeneration control circuit.
Regeneration begins smoothly with no pop and has a
comfortable adjustment range.
The result, we think, is a receiver design which bridges the
classic simplicity of regeneration to the performance demands
of the 1990's. Here's how we did it:
In brief, the circuit uses RF regeneration and high levels of
DC feedback. Notice that the antenna is coupled directly to
the source of RF amplifier FET Q3 rather than through the L-C
tuning network. Direct coupling of the drains of Q1 and Q3
isolates the L-C circuit from the antenna input, enhancing
stability and greatly minimizing RF oscillator output to the
antenna. Such RFI has been a serious problem in traditional
regenerative circuits which permitted the oscillating
detector to behave as an unstable but potent QRP transmitter.
R4 reduces the Q of L1 (10 uH) for smoother regeneration.
The SW1 bandswitch selects a combination of simple inductors.
For example, the total inductance for Band A is
L1+L2+L3+L4+L5. The inductance for Band E is only L5. And
so forth.
Air variable C1 uses its 50 pF range and mechanical vernier
reduction to provide smooth "bandspread'' in parallel with C3
and trimmer C5 which perform the traditional "bandset''
function.
Trimmer pot R20 ensures adjustability for smooth regeneration
over all tuning ranges, regardless of individual FET
characteristics.
C17, C9, C10 and R9 form a low pass filter to block RF from
the audio amplifier and provide basic audio filtering.
Volume Control R2 varies OUTPUT rather than low-level input
to the LM386 audio amplifier. This approach further isolates
the RF stages from variations in the audio section.
The LM386 (IC1) circuitry employs all recommended options for
maximum gain and protection from self-oscillation.