Hey
I want to make a test signal generator with this schematic:
I want to use multiple crystals within 1MHz - 15MHz range.
The exact output frequency is not important.
I want to have 5 or 10 crystals and be able to select the active one with microcontroller.
My question is, how do I modify this circuit to handle multiple crystals?
I have two ideas.
IDEA 1 - just use many crystals and many transistors (I mean, repeat the circuit as many times as needed). Then just use microcontroller and extra transistor to power-on the desired sub-circuit when needed.
IDEA 2 - use multiple relays to select the crystal for the single shared transistor. Then just use microcontroller to drive the relays... in this case I am afraid about the noise...
PIN diode switching would be ideal but as losses are not much of a problem in an oscillator, you can probably use normal small signal diodes.
The more practical solution if you want 10 different frequencies is to use one crystal and a PLL or if they are harmonically related, to generate a comb and then filter the harmonic you want.
This is almost exacly what I wanted, but why is there 12V connected directly on one side (crystal selection) and 9V regulator on transistor collectors? Is it correct? I mean... wouldn't it mean that the collector voltage is 9V and base voltage 12V?
They are using the unregulated +12 V to provide switching current to the pin diodes. It is no problem as mentioned because there is a coupling capacitor C10 which block the DC.