External 10 MHz reference option  - second batch.....

External 10 MHz reference option

This option is not part of the standard build. However, it is possible to operate the board from an external 10 MHz reference source. If the following components are fitted, the on-board TCXO can be made to switch off automatically when the external source is fitted. For this to work, the external source must present a low (less than 1k) dc resistance to ground.

R1  0R   Remove

R22   12k      0805
R23   100k    0805
R24   12k      0805
R25   100R*  1206

C45   10n      0805
C46   10n      0805

Q4    FDN338P
Q5    2N7002

                                             ect ref cct

*The value of R25 may have to be increased if too great an input signal from the external reference is present (or attenuators added to ext ref).

Nominal input: 0 – 5 dBm

It is important to ensure that R1 (0R), which is part of the ‘standard’ build is removed.

These components are shown towards the top left hand corner in the following picture (see also the layout drawing at the beginning of this section):