The I.F. amplifier is similar to the one used in the 80M receiver project .
The original design has been modified by putting a couple of LED's in the source circuit of each
Mosfet. The voltage drop across the LED's keeps the source voltage at about two volts. This results
in a much greater AGC range. This arrangement was suggested by N6BIU. Thanks Jim.
The I.F. transformer primary has 18 turns, the secondary winding has 4 turns. The capacitors across the IFT primaries
are 82pF. The input/output transformer has 12 turns, tapped at 3 turns from ground. This transformer is wound on
a ferrite core. The mosfets are 3SK45's. The diodes in the product detector are 1N34's. I use a six pole SSB
filter from a scrap CB. The centre frequency is 7.8MHz. The -6db bandwidth is about 2.5KHz.
Update: 8 Feb. 2002.
Since I built this rig, I have replaced the original IF amplifier board
with a completely new unit. I used a dual-gate mosfet as the first IF
amplifier and a Motorola MC1350P as the second IF amplifier. The low
noise of the mosfet combined with the excellent AGC characteristics of
the MC1350P results in a very good IF amp.
The schematics for the new IF and AGC modules are below. I haven't had time to
make a careful check for errors, please let me know if you find any.
New IF amplifier
New AGC circuit
PCB top view
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