Updating my G3RUH GPSDO
Page 2

LM6181s are used for the 50R drivers. This is a current-feedback amplifier providing very high slew-rate, bandwidth and high output current. I found 20 on Ebay for a mere £15. The fact that these were surface-mount devices was not a problem and besides, I was up for the challenge! A three-stage R-C filter converts the 10MHz TTL into a pseudo-sine-wave. This is then amplified before being fed to the four parallel output stages. The output of each stage can be set independently to deliver 2V p-p into 50 ohms. The mains transformer has two 15V output windings … ideal for a split-rail PSU. However this incurred a temperature problem. The OCXO runs from a single 5V rail and draws at least 500mA at switch-on. This drops when the OCXO has reached optimal temperature and is well within the capability of a 7805 regulator. However the raw DC from the bridge rectifier is a shade over 20V, meaning that the regulator was having to dissipate 7.5W. The heat-sink was getting very hot indeed. There are two ways to deal with this. Use a larger heat-sink, or drop the input voltage. I decided on both! A 2SC3281 and an 11V Zener diode act as a pre-regulator, dropping the voltage at the input of the 7805 down to about 10.5V … effectively halving the power dissipated in the regulator. The same 10.5V line also supplies the GPS RX and the PLL board. The PCB layout is double-sided with some of the DC distribution on the underside.   

August 2016:

Click the above photograph for a pdf of the circuit diagram