10.8MHz SAW Insert Loss?
I test a murata's 10.8MHz SAW, with 270ohm serise resistors in input/output to realise about 330ohm impedance(270+50). But the result is 16 dB, far from the datasheet spec., which is 6dB max. Why?
I have tried to understand the reason. I compute the in/output resistance attentuation effect 8dB, this will cause very well 16dB insert loss. But then, where the SAW attentuation? Isn't it 0 dB ?! No impossible? Because we generally know that SAW has a relatively more loss than others.
But later I found the cuure on datasheet seems like 0dB insert loss. I now don't know which is correct, which is right.
The attachment is the datasheet of SAW.
Pls help me? Thks in advance.
If you just add a 270 ohm series resistor at the filter input and output, the filter will see a match (270 + 50 = 330 ohms). But the 50 ohm signal generator will not. It will see a load of 270 + 330 = 600 ohms. Then you cannot use the power output scale of the signal generator because it's calibrated into 50 ohms load. Using the spectrum analyzer for calibration of the signal generator doesn't help.
The best way to test your filter is building a matching transformer wound on a suitable toroide ferrite core. A 1:6 impedance ratio will do for the input and a 6:1 at the output. Or to build active impedance transformers using e.g. 2N2222A transistors. In this later case you can end up with filter gain instead of filter loss which might be desirable in your application.
The 0 dB insertion loss in the data sheet is only fictive. The reference has simply been adjusted by an amount being equal to the insertion loss. This is very common in filter data sheets.
One trick to measure the filter with this set of extra resistors is to connect the resistors together without the filter. Store the trace. Then run it again with the filter in. Subtract the two traces.
A more sophisticated method is to use minimum loss pats from 50 to 300 ohms on each end. Subtract the pad losses.
http://unitmath.com/um/p/Examples/Pu...imumLoss%20Pad
is a link to the calculations for the pad resistor values and the attenuation.
Below is a program for calculations. You have to enter M in the program for a matching pad.