Dear BITX homebrew-heroes!,
I combined the ideas of Ruud and Alf and, to my surprise, the IRF510
is not self-oscillating anymore (at all DC bias settings) !!
I did following (20 min work):
Added 3 decoupling C's and 2 rf chokes to pa and driver circuitry and
added a ferrite bead (from an old videoplayer) over the gate of the
MOSFET. Also added a cap of 68 pF between drain of mosfet and gnd.
I think the 10 cents ferrite did most of the job.
RF power is still too low (appr. 2 Watts RF), so there is still
something to do. Driver circuit needs an additional RF transisitor I
guess. I found a nice 2SC1384 from an old CB-transceiver.
Furthermore I can gain by further shielding the PA driver and improve
the trifilair mosfet coil (I have one made, but it's behaviour seems
to be similar when compared to the bifilair coil).
Did some RTTY transmissions with 2 Watt yesterday though and the
BITX17 sounds beautifully. Good USB, no remainders of carrierwave
noticeable. I used a full lambda one-fed wire antenna (in the garden)
with an adjustable matching network (pi-filter). No reply/qso yet
(but the band was very dead after 10'o-clock in the evening in
Holland and 2 Watt is QRPPPP).
73 and lots of homebrew fun
Ron
PA2RF
--- In BITX20@..., "alf" <fpdbase@e...> wrote:
Hi to all homebrew BITX20 members,
Hope everybody is having fun building their version of the
BITX radio.
After attending local hamfest at the end of February, collecting
all the relevant parts, started assembly of BITX20. The chassis, L
shape was salvaged from the power supply of a old Fax machine and
it's circuit board was used a source of some of the parts for the
BITX20. Lot's of good parts in old equipment hi hi. A piece of single
sided pcb was fitted to the chassis, copper side up, and ye old ugly
style construction began. The VFO was built first and a compartment
of double sided PCB surrounds it. A red LED is used to fine tune the
frequency and gives about 4khz range. Various coils were tried at L4
to improve frequency stability, ended up using a ferrite bobbin from
the salvaged Fax board. The Receiver front end was built next,
opposite side of VFO partition. T1 mixer transformer is mounted on
the crystal filter side (IF) via a feedthru capacitor. The IF, BFO
and audio stages assembled and tested next. Used 2N2222A
transistors throughout except for the PA amplifier of course, the
audio output device is a TDA2002. The PA amp was built on a small
heat sink salvaged from a old TV chassis, size 8 X 4.5 cm . The
2N2218 driver coupled to IRF510 using TV balun taken from TV tuner,
and some instability occurred when driving a reactive load. This was
cured by fitting a ferrite bead directly onto the gate leg of IRF510
Fet. The receiver audio is very clear and sensitivity better than 0.5
uV. Don't procrastinate, if your thinking it's all too much trouble,
just have a go, you will learn more by doing.
Many Thanks to Asher for allowing me too much fun..
Best regards to all BITX members
Alf VK2YAC
