I build for VHF so most of the PA problems have not been noted.
The first model I built was fairly close to basic BITX-20 save for
no power amp. I used Ferrite (FT37-43) torrids as I have them.
this version was built as testbed using dead bug (ugly construction)
with output power sufficient to drive a 20m input for a 6m
transverter. Only a few milliwatts are needed for that. Works very
well in the role of an IF for VHF use. The first significant
modification was a Diode based attenuator and audio derived agc.
Works well enough to preserve my ears without causing undesired
effects. The diode Attenuator is placed after the (board level) RF amp
in the recieve path and before the 1st mixer. The attenuator used is
page 6.18 in EMRFD, it's a proven design that works fairly well with
1n4007 rectifier diodes (at 14mhz). It uses a lot of parts and
works well but as an engineer I'm also interested in fewer parts
so I am looking at alternate circuits. One note: using toroids the
stability of the reciever is better than using plastic washers, the
magnetic field is restrained better on the toroid and fewer turns of
wire are required.
I am now building a second model for 6m use. Instead of the VFO
I am using a tested PLL of my own design at ~40mhz and the IF will be
9.6mhz. The use of 9.6mhz is to avoid WWV bleeding into the IF on
10mhz and also the crystals were available in quantity for matching.
A PLL avoids the problems of trying to build a stable VFO for 40mhz!
I've elected to use a available PC board from Far Circuits but
from looking at the board the parts placement is a mystery (maybe a
contest?). Also I will have to clean up the etch as there are wiskers
and other undesired tracks on the board. A picture of a complete
board (with part in place) or a drawn layout would help. The RF power
chain will not use a IRF powerfet as their perfomance (both IMD and
gain) at 12v is poor at 50mhz. I will use more common bipolar such
as 2sc1969, 2sc1970 or 2sc1971 for power. I have used FETs for power
before but at VHF you need better devices.
Allison
KB1GMX
