Pyrojoe's 14.4 MHz TCXO's for voltage tuning operation
Pyrojoe has been selling 14.4 MHz TCXO's at a very low price (15 GBP
and by good fortune, this is the frequency that can be used with the
3cm rx converter boards to turn these assemblies into a 3cm marker or
low power tx.
When multiplied up to 3cm, the Pyrojoe TCXO's are quite stable and they
do not suffer from a great deal of jitter.
However! - they do not have a voltage control pin, so no FM or FSK can
Since the TCXO pcb has pads for the voltage tuning components, and
since the board is only soldered to the can at one point, it is an easy
matter to remove the board and add components.
I have a lot of surplus UHF transceiver boards with 1SV229 varicap
front ends, so these were good diodes for me to use. Initially, I added
one such diode where the manufacturer would have put his, together with
a 100K feed resistor to the fourth (unused) module pad, and added a
suitable pin. Again, pads were already present for the series resistor.
With this arrangement, and its extra loading, the max frequency
(translated to 3cm) with the trimmer C on minimum, and 8v varicap bias,
was about 10368.060 MHz. To reduce unnecessary loading, the trimmer cap
was removed, and with this arrangement, the max frequency went up to
10368.220 MHz, so this was a useful thing to do. I did not want to
remove any of the fixed value capacitors in the oscillator circuit.
Having removed the trimmer C, it became more convenient to place the
varicap diode where the trimmer capacitor had been, resulting in the
following 'before' and 'after' pics:
another batch of these TCXOs, it was found that the frequency range
obtained with the mods outlined above applied, was lower, and it was
nececessary to reduce the value of the fixed capacitor (shown opposite)
that is in parallel with the original trimmer - so you may have
to suck-it-and-see, if you obtain the TCXO yourself.
The shunt capacitor is 27pF on the unmodified units. I have been
reducing this down to 18pF
and obtained a final frequency range of 10 367.992 to 10 368.209 MHz on
the last exciter
that I tested.
Interestingly, if you reduce this capacitor down to about 10pF, the
exciter will cover the beacon band, and will still retain fairly good
stability (+- a few KHz between 0 - 30 C). With the sample
that I tried, there was very little flicker FM either.