Crystal Filter Board

The Crystal Filter Board features:

• This is a small sub board that fits onto the top of the Radio Board

• It can be used with other non HamPi projects

• Measuring 42mm x 38 mm

• A 6 pole ladder filter for SSB   ~ 2.3kHz bandwidth at -6dB

• A 5 pole ladder filter for CW   ~500Hz bandwidth at -6dB

• Will be supplied built and tested

The Crystal Filter Board uses custom selected AT cut crystals manufactured by Hy-Q.  There are websites and YouTube claiming that cheap crystals can be used, but after much testing this has been found to not be the case. Cheap crystals are usually designed for 'roughly' 8MHz and for running microprocessors and the like. To get a ladder filter to operate correctly the crystals have to be pulled off frequency using quality  series resonant capacitors. Cheap crystals 'can' work on a spot frequency but when pulled in a ladder filter usually exhibit increasing attenuation, and lumpiness.  This shows as a nasty roll-off at the upper frequency of the filter. Cheap filters also do not give a good filter skirt profile.

A poorly shaped filter shows itself not only in the received audio quality but in the transmit quality.

For this reason and a demand that the HamPi is foremost a quality transceiver, quality Crystal Filters were a demand.  You might be able to buy 50 crystals from China on the cheap,  and make an ok filter but it will not provide quality filtering.

You get what you pay for...