Converting 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 for 50)
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 be applied.

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.

See below:

                            tcxo pcb

I have a lot of surplus UHF transceiver boards with 1SV229 varicap tuned 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:

            mod_unmoded tcxos1sv229                 
On 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.
tcxo shunt c