The article was a bit outdated since a couple of the used semiconductors went out of production already so I ran into this site here by DK1RM, Roland, who created an Eagle layout and updated the circuit design with more current semiconductors.
His site publishes the schematics too.
What the generator does is to use a standard TTL oscillator, attenuate its output signal with a FET, pass it through a filter to achieve a sinus signal. The sinus signal is now fed into a peak detector and the peak voltage is compared to a DC voltage out of a voltage regulator. The output of the comparing opamp is then used to control the FET attenuation, creating a control loop, delivering a sine signal with fairly constant and defined amplitude. This is then passed through fixed attenuators which in turn generate output levels at 0 dbm and -60 dbm.