SAW resonators

[waldvogelmj]

Hi, question on SAW resonators. Does anyone have any idea why R03073 can't simply be used as a drop in replacement for R2622 in an existing transmitter? I've attached the data sheets, both are the same frequency and look to have similar specs. @betwixt had some helpful answers for me in the past but I can't seem to message him.

Thanks

--- Updated Dec 7, 2020 ---

Better yet comparison (same manufacturer).

RP1239 works. RO3073 doesn't. Do I simple need to adjust capacitor size?

 

[betwixt]

I'm here.

They are not identical in specification but I would have thought so close that they would be compatible.

Just one check - measure the DC resistance from the pins to ground on both devices. It is possible one has a DC current path to ground and the other hasn't. In many circuits it would make no difference but if bias is applied to a pin with leakage to ground it would sink some of it away.

Brian.

 

[FvM]

R03073 has factor 7 lower Q. You didn't tell your application, but the difference is large enough to explain why the part isn't working for you.

 

[waldvogelmj]

Glad you're still here!

I get no reading on my meter measuring from either pin to ground on either component. The meter is fine because when I touch a 180 ohm resister the reading is dead on.

I can't say I completely understand the concept of Q factor apart from higher being better. I also don't understand why Murata would produce a component like this with Q that is so low that it wouldn't be useful in generating a 315 MHz signal. If it is the Q there must be a way to compensate for a lower number?

The application is a 1990's car remote transmitter. I'm trying to convert between US and Euro frequency. When I swap in RO3073 to the otherwise unmodified board the frequency drops to 285 MHz. The original boards use both RP1239 and R2622.

 

[betwixt]

Euro frequency is normally 433.92MHz. How are you measuring the frequency?

Brian.

 

[waldvogelmj]

Euro frequency is normally 433.92MHz. How are you measuring the frequency?

Brian.

Yes, I agree. I have both the 433 and 315 versions of the board. I've narrowed the difference down to the SAW resonator and two capacitors. If I swap the 315 parts into the 433 board it becomes 315, and vice versa. So I need to find a compatible 315MHz saw resonator and I'm testing it in a 315 board first to make sure it's compatible.

I just have an Ebay frequency counter, which is dead on accurate with the original 433 and 315 boards.

 

[waldvogelmj]

The datasheets all show different capacitance values along with a diagram that shows an "equivalent LC model" showing the capacitors and the antenna. Do you think it could that I need to match different sized capacitors with each different filter? Would adjusting the capacitor size alter the frequency transmitted to the antenna?

 

[FvM]

The problem is to design a circuit that oscillates at the SAW resonance and not a different frequency also allowed by external LC components. That's the reason why a circuit that runs stable with R2622 most likely works with RP1239 but not necessarily with R3073.

If you look at the ratio of "motional" LC values and other capacitance values, you'll see that similar to a crystal oscillator, the resonance can be pulled maximal a few 100 ppm by changing external capacitors.

 

[waldvogelmj]

Does this one look to be any closer than R3073?

 

[waldvogelmj]

Does anyone know, of the 4 RF equivalent variables (motional resistance, motional inductance, motional capacitance, shunt static capacitance) which is the most important that I'm trying to match? There are a number of SMD SAW resonators available that I may be able to use if I modify the PCB.