Farhan believes that by referring the Ubitx users to the
general user community for support, everyone will benefit
from the combined knowledge and experience of the
world’s most competent Bitx users.
On the Technical Side
Ubitx is a state-of-the-art design (Figure 4), with
economy playing a role in each decision. Farhan has
sought throughout Ubitx’s design to balance cost with
The control center for Ubitx is the Arduino Nano
microcomputer which is coupled with an Si5351 frequency
Farhan has named this combination of
devices “the Raduino.” Generic firmware for
the Raduino comes preinstalled, and more
enhanced versions can be found on user
The Silicon Labs’ SI5351 frequency
generator has greatly reduced the complexity
associated with building multiple band
By including just one 25 MHz crystal to
serve as the frequency reference, the SI5351
can digitally generate oscillations in the range
of 8 kHz to 160 MHz. To paraphrase Farhan,
the SI5351 has reduced the complexity of
multiband transceivers to a relatively trivial
Neither Ubitx nor its predecessor (the
Bitx40) would have been possible at their low
price were it not for the Si5351.
The Ubitx uses an up-conversion, double superhet
design (Figure 5) to the first intermediate frequency of 45
MHz, eliminating the need for large band-pass filters. At 45
MHz, the roofing filter is 15 kHz wide.
The signal is down-converted to 12 MHz where an
eight-crystal filter (Figure 6) is used for both CW and SSB.
The eight-pole filter in the second intermediate frequency
is 2. 3 kHz wide.
The receiver front end has a 0-30 MHz low-pass filter
which is used to keep out signals from FM broadcast
stations; refer to Figure 7. This design utilizes a doubly
balanced diode mixer which is made up of two
matched Bat54SL diodes. No amp is required here
because sensitivity at this point is adequate.
Ubitx uses four intermediate frequency
amplifiers which don’t require transformers; each
one has a gain of 16 dB.
Generic 2N3904 transistors are used throughout
the transceiver with four 2N3904s serving as
push/pull drivers to the finals section which uses
inexpensive IRF510 field-effect transistors.
Following the finals are four filters which are in
place to prevent harmonic distortion on the output
5. Up-conversion schematic for the Microbitx transceiver.
6. Eight-pole filter schematic for the Microbitx.
7. Schematic for the Microbitx
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64 September/October 2018