Gerard,
It is important to get the correct termination impedance for a crystal bandpass filter. The resistance and capacitance are in parallel and both have an effect on the filter characteristics. If the load R is too high or too low the filter will shift up or down in frequency. If load C is too high or too low the filter passband characteristics will change and you can get more ripples. You can see all of this in this Murata animated crystal filter video
There are 3 ways to get the proper termination impedance for the crystal filter: RC matching, LC/CL matching or Transformer matching. >> .
The author of that video is using LC matching but he made a mistake in his calculations. He used the wrong calculator and got the wrong value for the matching C. The steps for correct calculation are as follows:
1. The R and C specified by the manufacturer are in the form R//C so a parallel to serial impedance conversion is required at the filter center frequency. This calculator will do the conversion.
In your case 4000 ohms with 1.5 pF in parallel at 45 MHz. equates to 1031 ohms in series with 2.021 pF or Z= 1031 -j1750. We need to match to the complex conjugate of this which is Z = 1031 +j1750.
2. Next we need to design the matching network. This is a typical calculator which gives several matching options
Two solutions are provided: One is L in series and the other is C in series (DC blocking). The calculated values are 1.572uH and 9.358 pF for the former and 1.968uH and 7.957 pf for the latter. Using either of these will results in a termination that looks like 4000 ohms in parallel with 1.5 pf when the matching network is terminated in 50 ohms.
Roger Need