Sunday, 20 November 2011

Resolution Bandwidth Filter #2

Despite the broken milling machine, I managed to finish two PCBs for the bigger NDK crystal filters I have. Their footprint is huge mainly due to the integrated transformers. This particular filter is an 8-pole 10.7 MHz with a 3 dB bandwidth of 7 kHz.
 I used double-sided copper clad FR4 for the fences. It oxidises really fast so I'd recommend tinning it before you attach the strips on the PCB.

The physical dimensions of the board is 1.2 x 2.4 inches, which translates into a 1 x 2 size SLIM. Although these filters function very well I think I prefer the ones with a smaller footprint.

 Bottom view, ready to have the matching circuit placed. I was recommended to solder the bottom of the crystal can to the top copper plane for shielding purposes. I fear however, it may melt some of the connections inside since the metal can soaks a lot of heat. Instead, I used adhesive copper tape and soldered it to the PCB.

The matching circuit of this particular filter consists of (I think to remember) a 1.8 uH and a 2.2 uH inductor in series, and a 10 pF in parallel with a 6...30 pF trimmer.

I don't have anything but copper tape at the moment to serve as a lid. It should work just fine. I should try to get my hands on some copper or brass sheet but it's another thing that seems difficult to obtain here in Helsinki.

The SMA connectors are mounted and the in and output labelled. This filter is ready for tuning.

I have a printout of the amplitude response of the tuned filter somewhere. I should add it here once I find it.

Picture taken with my iPod. The ripple is quite nice.

I recorded the 2-port touchstone data for this filter with 1600 points. You can download it as a zip file from my Google docs:

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