Crystals for the Radio Board will be made available already matched and SMD COG capacitors already mounted on the board (Unless otherwise noted due to availability). Some constructors will for whatever reason (a different IF frequency, or for fun and experimentation) decide to provide their own. Details of how to build and test the filters follows:
The HamPi Radio can operate on various IF frequencies and this depends on the available Crystals that are used for the First IF filter (2 Crystals) the SSB Filter (6 Crystals) and the optional CW Filter (5 Crystals). All these crystals need to be matched to within a few hundred Hz of each other. The IF chosen can be in the range of roughly 7.8 to 11Mhz (with conditions). Some IF frequencies will cause in-band harmonics – so the maths must be done! Although 8, 9 and 10.7Mhz IF rigs have been built and tested, the final choice has become 8MHz – or al least within a few kHz of 8MHz.
The reason for this choice is the easily availability of stock crystals. Packs of 10, 50 or 100 crystals marked 8.000 MHz can be purchased at a reasonable price from AliExpress and the like. It is strongly recommended that the taller profile HC-49U are sourced. The low profile type HC-49S profile are available, often entitled ‘Resonator Passive Oscillator’ and thee are most likely to be ceramic resonators and not crystals at all. This is not to say they will not work ok, they can do, but this author has had varying results with ceramic resonators. If SMD type low profile crystals / resonators are to be used the footprints on the PCB cater for this on the reverse side.
Out of a batch of 50 crystals it has proved to easily test and select the necessary number of crystals that work on the same frequency, or at least very close to it.
Capacitor Selection
- It is best to use COG type capacitors as they will be temperature stable. An example is: 50V 39pF C0G ±5% 0805 Multilayer Ceramic Capacitor. The footprint for these on the Radio Board caters for both SMD and THT parts.
Crystal Selection
- This can easily be done using a Spectrum Analyser (SA) with a Tracking Generator (Built in or otherwise), or by constructing a simple transistor Crystal Tester and a Frequency Counter. The Spectrum Analyser is preferable as the loss of a crystal can also be measured.
- Simply connect a crystal between the analyser tracking generator output and the RF input. That he SA input is 50Ohms is of no matter. The challenge here is to match the correct number of crystals on the same frequency. When they are in the actual HamPi the actual frequency of the crystals all operating together will be measured and set in software. (As an example in the latest set of crystals 13 were selected on or very close to 7.997500 MHz. When 6 of these crystals were put in the SSB filter the actual centre frequency was 7.999000 MHz, and 5 used in the CW Filter 7.998275 MHZ. This is perfectly fine).
Testing
- The constructor might have their own way of testing crystal filters.
- If not then some small test boards will be required for this. The first two boards will need to be wired as per the SSB filter and the CW filter on the Radio Board.
- The next board will be a impedance matching and buffer board.
- For more details contact me using the email address on the Contact page and I will email you some PDF files.
