14.4 MHz TCXO mods for 3cm beacon use

A tray of fifty 14.4 MHz TCXO's has allowed a fair bit of modification work to be carried out. For 3cm beacon band use, the frequency has to be increased by just over 1 khz.
 meiden tcxo mods
These particular units did not have a voltage tune pin fitted, but the pcb was layed out for both a varicap and a feed resistor. When frequency multiplying up to 3cm the trimmer capacitor is very course and difficult to set accurately so it was removed right from the start. Likewise, the lowest swing varicap diode that could be found (BB837 - 6p6/1v, 0.55p/25v) was fitted. It was hoped that by using a single select-on-test added series capacitor that the frequency of the tcxo could be moved up reliably well to allow the varicap control voltage to fully control the tcxo frequency and so far, that has been the case, resulting in the following circuit:

tcxo mods circuit

Modifications and associated measurements

All measurements were carried out at 20C unless otherwise stated..

Five tcxo modules were modified and measured to give give an indication of spread. Firstly, the trimmer was removed and the varicap and its feed resistor added. The frequency was measured with control volts of 1.23 and 3.75, since this was the range available on the test board potentiometer: At 3.75v control voltage, this showed the following frequency spread between units:

Tcxo module
 Frequency with all medications except the added series C
and Vc set to 3.75v
1
 14.40006 MHz  (10368.0256 MHz)
2
 14.400181 MHz  (10368.130 MHz)
3
   14.4001135 MHz (10368.0817 MHz)
4
   14.4001435 MHz (10368.1033 MHz)
5
 14.400192 MHz (10368.1382 MHz)

These figures may give a reasonable indication of what the likely value of series C needs to be to predict operation in a particular part of the beacon band and (in retrospect, this was found to be a reasonable approximation over the five units).

A 33pf capacitor was then added in series with the transistor side leg of the quartz resonator:

Tcxo module
 Frequency at 1.23v varicap bias
 Frequency at 3.75v varicap bias
1
 14.4011170 MHz
(10368.8424 MHz)
 14.4012355 MHz
(10368.8896 MHz)
2
 14.4012465 MHz
(10368.8432 MHz)
 14.401310 MHz
(10368.8975 MHz)
3
 14.4011196 MHz
(10368.8611 MHz)
 14.401267 MHz
(10368.9122 MHz)
4
 14.4012015 MHz
(10368.9116 MHz)
 14.401268 MHz
(10368.913 MHz)
5
 14.401265 MHz
(10368.912 MHz)
 14.4013855 MHz
(10368.962 MHz)

Thus, the tuning range was 47, 46, 51, 48 and 50 khz respectively.

Module 5 then had its 33pF series capacitor replaced with a 39pF one. The new tuning range was then:

At 1.23v: 14.401031 MHz  (10368.742 MHz)
At 3.75v: 14.401102 MHz  (10368.793 MHz)

This was about right for a current project, so module 5 was used to assess the frequency stability at ambient temperature changes and also when fitted to a 45C oven (described elsewhere).

More tcxo's will be modified as the need arises.








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