05-01-2017, 05:57 PM
The original circuit I did was run at 50Khz and acted as a proximity sensor for a boundary fence using a basic OP amp and a parallel tuned circuit shunting out local radio stations and phone signals. The previous design picked up a radio station from overseas and did not work.
All I used could be honed down to two capacitor coupled one transistor amplifier stages. Once you link the output to the input you get a square wave. Add a volume control and turn it down until it stops oscillating. You can vary the gain by using a pot and capacitor in place of the emitter decoupling as well giving a low impedance control of gain.
Adding a chain of resistors and points to connect crystals will them make the feedback through the "volume" control more complex as will adding tuned circuits to the emitter resistors. You get one emitter resistor that is inverting and another that is non-inverting.
For the really cunning constructor there is JFETS and detector diodes that can be used to make an AGC loop with the FET added across one of the emitter resistors.
A test set can be made as complicated as the constructor likes starting with a simple two transistor square wave generator and growing as far as is needed for the task in hand. OP amp chips are expensive for high frequencies but a couple of transistors will be fine.
I have never made a full circuit and it remains a block diagram and no more.
Just remember that parallel tuned crystals are high impedance at resonance and series tuned crystals are low impedance at resonance.
All I used could be honed down to two capacitor coupled one transistor amplifier stages. Once you link the output to the input you get a square wave. Add a volume control and turn it down until it stops oscillating. You can vary the gain by using a pot and capacitor in place of the emitter decoupling as well giving a low impedance control of gain.
Adding a chain of resistors and points to connect crystals will them make the feedback through the "volume" control more complex as will adding tuned circuits to the emitter resistors. You get one emitter resistor that is inverting and another that is non-inverting.
For the really cunning constructor there is JFETS and detector diodes that can be used to make an AGC loop with the FET added across one of the emitter resistors.
A test set can be made as complicated as the constructor likes starting with a simple two transistor square wave generator and growing as far as is needed for the task in hand. OP amp chips are expensive for high frequencies but a couple of transistors will be fine.
I have never made a full circuit and it remains a block diagram and no more.
Just remember that parallel tuned crystals are high impedance at resonance and series tuned crystals are low impedance at resonance.
