136 kHz transmitter vxo
By Claudio Pozzi, IK2PII - August 2001
After some test with my Xtal controlled
I decided that my chances of two ways QSO should be improved by a
VFO. Some useful projects where found in the G3LDO (Peter Dodd) "LOW
EXPERIMENTER'S HANDBOOK", RSGB editor, but I'm not able to copy other's
projects without some modifications (I call them improvements, HI).
My mixture uses easy to find crystals, one
tunable oscillator for the higher frequency crystal, one CMOS IC for
other oscillator, for mixer and as output buffer/amplifier. The best
other projects found in the aforementioned book.
The output frequency is 272 kHz; the transmitter
exciter divides it by two.
2. Circuit description
The transistor Q2 is the tunable oscillator. L2
and L3 are 100 microhenry molded inductors, CV2 is the tuning
Tuning range is sufficient to cover the 137.600 to 137.800 QRSS portion
of the band. The U1A is the buffer/amplifier for driving the U1C mixer.
The U1B port and associated components form the
fixed frequency oscillator. Adjust the trimmer CV1 for a secure start
the oscillations. Don?t care the exact frequency.
The port U1C forms the mixer, at the output are
present all the mixing products and their harmonics products. The low
filter (L1, C5 and C6) clean the signal. The FET Q2 adapts the 272 kHz
sinusoid to the following buffer/amplifier. The output is a square wave
about 12 volts peak to peak.
The circuit was mounted wit the dead bug
into a 30-year-old 144 MHz VFO box, recycling the good variable
capacitor (do you remember amplitude modulation and the QQE03/12?).
3. Performance test
The test setup was entirely built with 136 kHz
The VXO connected to the transmitter exciter (dividing by 2), my direct
conversion Xtal controlled receiver, the PC running ARGO in 30 seconds
dot mode. After some warm up I tuned the VXO to 137699.53 Hz on the
screen. After half hour the frequency was 137699.49 Hz, after another
137699.42 Hz. The measuring chain have two unknown points, the
stability of RX Xtal and of PC audio board Xtal, but for now I can't
my test bed. I think that 0.1 Hz in 90 minutes is enough for QRSS