We found this circuit in Radio Electronics from the 90s. The components are current. The sensor is a common general-purpose transistor. The connection to the sensor is made by twisted pair.
The circuit shown is a documentation on 80's tunnel diodes. The frequency depends on the delay line...
This circuit generates excellent sinusoidal signals and has the frequency adjusted by VR1 which is...
This circuit was adapted from an Italian publication that no longer exists. This is “Onda Quadra”...