This version is a refinement of the circuits used in the Type III and Type IV preselectors.
The main differences are:
The coils selected are designed for specific bands (see LED indicators).
There is control for band overlap (Extend).
There is selectivity (Q) control for bandwidth "Depth" and "Width".
The resonant circuits are designed for maximum Q utilizing minimal capacitance values, and maximum inductance
Overall insertion impedances are kept high, in the 10 KHz and 20+ K Ohm range, with a loop impedance of 12
Ohms. This yields rejection ratios of approx. 1,000:1, or 60dB.
On the bench:
36dB Selectivity @ 3.5kHz (7kHz passband) = <1% selectivity (Actual at 1MHz=0.7%)
42 - 48 dB Selectivity @ ~10 kHz (10 kHz is one AM broadcast channel)
I also incorporated controls
for the depth of the peaking
filter and a Q-spoiler for a
The final version employs different inductor values
which give slightly higher Q values. The utility of
preselection above 10 or 15 MHz (for me) is
questionable, and since this unit uses a 6-position
range switch, I decided to make the mods that are
shown in the schematic. If you need the very
broadband version, consider the 12-position
inductor switch circuit of the Type-3 Preselector.
The original bands are:
The "Multiplier" adds 30% overlap to the band switch.