Pierce oscillator - RF circuit questions

[Eight]

Hello.

I am trying to learn about oscillators by studying some RF circuits, and I have a few questions about a Pierce oscillator. A simplified circuit consists of a XTAL, two capacitors that provide load capacity, a feedback resistor, a power limiting resistor and an inverting element (see attached pic). I learned that to achieve oscillations the inverter provides a 180° phase lag, and the XTAL with the two caps make a Pi-filter that provides another 180° of phase lag. Together we get 360° phase delay which is one of the requirements for stable oscillation (the other requirement being sufficient gain of the inverting element).

The RF circuit that I'm trying to understand is a 433.92 MHz ISM-band transmitter that is supposedly using a Pierce oscillator to transmit a carrier signal while the data is OOK-modulated. It uses a few resistors/caps, a NPN RF transistor with common emitter that acts as an inverting amplifier and a SAWR (surface acoustic wave resonator) instead of a crystal, (see attached pic).

My questions are:

- How is a SAW resonator different from a crystal in this case?

- In many crystal datasheets manufacturers normally specify a maximum crystal load (drive level i.e. about a hundred uW). A higher drive power may permanently damage the crystal which is why the circuit has a power limiting resistor in place. Does a SAW resonator also need any power limiting resistors to avoid damage? The datasheets don't seem to specify any maximum drive level. The circuits don't normally have one. Is the bias resistor enough?

- Is a DC-blocking ceramic capacitor needed directly in series with the SAW resonator? I've seen some circuits that have it and others don't. What's the deal here?

Thanks in advance

 

[vfone]

Simple search on Google...

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[D.A.(Tony)Stewart]

Xtals tend to have a high Q of about 10k. But are often limited to the HF frequency range (30MHz) due to micromachine limitations at UHF while HF Xtals tend to use harmonics aka. overtones. Thus Xtals are not popular at 433 MHz.

Xtal resonators have different geometric modes for oscillations and the frequency depends on the precise cutting angle in minutes of degrees.

SAW resonators now achieve better stability than crystals in the UHF range, which is why they are used. SAW operation is based on surface wave velocity and wavelengths which are longer than in Xtals.

This is from Digikey which shows active sources are shrinking.

Losses depend on substrate quality e.g. bulk GaN has less loss than GaN film.
These work best in 1 port mode (and the other port to gnd.) in parallel resonance at the input before a transistor amplifier.

--- Updated Oct 20, 2023 ---

- In many crystal datasheets manufacturers normally specify a maximum crystal load (drive level i.e. about a hundred uW). A higher drive power may permanently damage the crystal which is why the circuit has a power limiting resistor in place.

Because of the high Q=10k in Xtals this amplifies the voltage across the internal "motional capacitance" of the crystal lattice to some high kV value which can create a risk of failure from insulation breakdown of the synthetic quartz. Some are very high Q and limit power to 15 uW. This is done with a driver source resistor as a current limiter.

 

[Eight]

vfone: Thanks, those do describe the differences, but still leave some questions unanswered.

So, to conclude my first question, the main reason why a resonator is used here in place of a crystal is because crystals are a no-go at UHF. But as devices they *are* interchangeable. If that's the case, does the second and third question apply to SAW resonators in the same manner?

These work best in 1 port mode (and the other port to gnd.) in parallel resonance at the input before a transistor amplifier.

But that's no longer a Pierce, but a Colpitts, yes?