This page will be completely reviewed and updates soon.
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.
Ignore the following text for now!
The SSB Filter Centre Frequency can now be calculated and it is half way across the passband. What was noted down? An example will be -2475Hz on one side of the passband at S8 and -375Hz on the other side at S8. Now the maths. 2475HZ – 375HZ = 2100Hz. Next 2100HZ/2 = 1050Hz. Next add this to the lower bandpass so 1050Hz + 375Hx = 1424HZ. Remember the original IFW were negative numbers? So we end up with a 8.000,000HZ Base frequency minus 1425Hz = 7.998575MHz SSB Crystal centre frequency and this will become the HamPi IF Frequency.
- The CW filter (5 xtals) procedure is similar to that above. The CW filter can be a centred on a different frequency, but does need to be in a similar range as the RX signals still have to pass through the Duplexer that is a tunes circuit and the first IF filter which has a wiser but not limitless passband. (for example 7.999800 is fine).
- Press the MODE Button once to change to CWN (Narrow). This changes over from the SSB to the CW Crystal Filter.
- Follow a similar procedure to that above but changing the IFN using the main control.
- The passband this time will be very narrow, roughly 500Hz. Find and note the side skirts of the passband. Due to the extra attenuation of a CW Filter the maximum signal strength on the S Meter will be about S8, so the -6db points on the skirts will be at 1dB less, so look and note when they are at S7.
- Now the maths again. 2200Hz – 1800HZ = 400Hz. Next 1400HZ/2 = 200Hz. Next add this to the lower bandpass so 1800Hz + 200Hx = 2000HZ. Remember the original IFN were also negative numbers? So we end up with a 8.000,000HZ Base frequency minus 2000Hz = 7.998000MHz SSB Crystal centre frequency and this will become the HamPi IF Frequency.
- Next, Press the Set button. This will save this precious CW frequency in memory, but not for long.
