The PTS-160 is all BJTs and digital ICs.
Gerhard_dk4xp, in reply #50 to this thread, mentioned a multiplier using a JFET.
An advantage to the JFET over the BJT in this doubler application is that if you apply a decent sine wave to the gate, strong enough to get a non-linear response, but not so strong as to switch the device ON/OFF, in theory the output of the FET (quadratic response for ideal JFET) will contain only the fundamental and second harmonic spectral components, while the BJT (exponential response) will contain the fundamental and all integer harmonics.
Since neither a pure sine wave nor an ideal square wave contains anything at the second harmonic, something must be done to distort the waveform before extracting the harmonic. Applying a sine wave to a device with even-harmonic distortion gives an asymmetry between the "goes up" and "goes down" parts of the waveform, shifting the zero crossing away from the center of the period, and thereby generating even harmonics. If the distortion is odd, then the "goes up" and "goes down" both get distorted from their ideal sinusoidal shape, but the zero crossing stays centered, thereby generating only odd harmonics.