I.F. Amplifier
I.F. AmplifierThe 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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