455kHz sinus oscillator

[atakan_1907]

Hi,
Im trying to build an AM transmitter-receiver pair for school project. And i have some trouble on transmitter side.

I could not find a way to generate the carrier signal. How can i generate 455kHz sine wave using ceramic resonators and opamp. Opamp is not necessary but it's better to use one.

Please help me on this. I've been searching the net for a long time but found nothing usefull.

Another solution is to use L C pair in parallel but i have only resistor type 1uH inductors and any value of capacitors.

 

[tony_lth]

Maybe this will help you

 

[atakan_1907]

Hi Tony, thanks for help. I know theoric oscillator topologies but can't build one. I saw many collpits oscillators before but dont know if i can build one using a ceramic resonator. How should i connect the resonator?

it will oscillate if i use two external capacitors like this?
**broken link removed**

 

[chuckey]

The filter will oscillate at a frequency where it looks like a coil. The extra capacitors are used to match the output impedance of the chip (low) to the input impedance of the chip (high). Sort of 3:1 should do. The filter itself is needs to be terminated with the correct capacitance and resistance to get optimum power transfer, but I believe that they are fairly lossy - 10dB loss?
Frank

 

[atakan_1907]

Ok. I got it work playing the parallel resistor value. But what about the resonator-opamp oscillator?

I think the other way is to generate square using inverter and integrate it twice.

 

[FvM]

Most people use a ceramic resonator (or a crystal), when they want a stable frequency, not affected by voltage, temperatur and component parameter variations. RC and LC oscillators can't compete with it.

 

[biff44]

in the data sheet for the digikey X921-ND there is a simple test circuit using a digital cmos inverter chip. You might want to start there.

Digi-Key - X921-ND (Manufacturer - ZTB455E)

Since the inverter gate is....an inverter, the phase shift thru the ceramic resonator (with those two shunt capacitors shown), must be around 180 degrees.

So if you replaced the inverter shown with an inverting op amp, set to a gain of perhaps 50 with some resistors, and the same ceramic resonators with the two shunt capacitors shown, it should oscillate also. (i.e. inverting op amp gives you 180 degrees phase shift, and the resonator gives you another ~ 180 degrees of phase shift, so the total round trip is going to be positive feedback and will oscillate).

If you are using a cmos inverter, you will need that resistor in parallel with the resonator. If you are using the op amp, the resistor is probably not necessary.

 

[atakan_1907]

Thank you for detailed answer. I'll try and write here.

 

[atakan_1907]

Ok i succeded to generate sine properly. Now how can i modulate it using opamp?

 

[FvM]

An OP won't work as amplitude modulator circuit. You need a multiplier or at least some kind of nonlinear device.

For regular AM (with carrier), a two-quadrant multiplier would be a straightforward solution, either an OTA or a differential amplifier. Also a 4-quadrant multiplier with a bias at the modulation signal (baseband) input.

 

[biff44]

you can "digitally" modulate it by turning the DC power on and off. That is how most key fobs (car door openers) work.

If you want to analog modulate it, you need to pass the continuously oscillating signal thru another circuit.

Exactly what type of modulation do you need/what are you trying to do?