SSB & CW Filters

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 is done using a Spectrum Analyser (SA) with a Tracking Generator (Built in or otherwise), a VNA, or a Crystal Tester and a Frequency Counter (Simple Crystal Tester designs can be found online for construction). A Spectrum Analyser or VNA is preferable as the loss of each individual crystal can be measured.
  • Simply connect a crystal between the analyser tracking generator output and the RF input. That he SA input is 50 Ohms is of no matter. The challenge here is to match the correct number of crystals of the same frequency. When they are in the actual HamPi the actual frequency of the crystal filters 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).
  • Choose also crystals that are not only well matched in frequency but also have the lowest loss. < 1dB is good. <0.4dB is excellent.

Testing

  • The constructor might have their own way of testing crystal filters.
  • If not then a 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 following procedure and wording is still under construction… (Any offers of assistance from the experts?)

Crystals

In all 13 crystals are needed for the HamPi and are all fitted on the Radio Board. An exception to this is if the CW filter is not needed, and then only 8 crystals are needed:

  1. 2 crystals are for the wideband pre-filter / Noise Blanker delay. This filter is about 15kHz wide so as long as the crystals are a near match to the SSB and CW filters they well work well.
  2. 6 crystals are for the SSB filter and need to be within a 100HZ match of each other.
  3. 5 crystals are for the CW filter and need to be within a 100HZ match of each other.
  4. Ideally all crystals would be on the same frequency, but in practice this is not the case and that is fine. Even when very similar crystals are used for the SSB and CW filters, due to the different circuit and capacitor values used the loading is different, and therefore the SSB and cW filters end up being on different frequencies. This is absolutely fine as the HamPi is designed to allow for this in software.

SSB filter

The SSB Filter Centre Frequency can now be calculated, which will be in the centre of 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.

  1. Select 6 closely matched crystals and mount them in the test PCB, along with the correct capacitors. Capacitors need to be of good quality low loss type. NPO / C0G are recommended. On the Radio Boards either multi-layer ceramic leaded or SMD capacitors can be used. If poor quality parts are used there will be higher losses (and frustration).
  2. The SSB filter needs to be around 2000 to 2200HZ wide at -3dB from the top of the passband. Any less (narrower) and the audio quality will be reduced making it sound sharp. Some rigs do use wider filters, 2400Hz or more) but while this may sound nicer to the ear on a quiet band, it might not filter out QRM of adjacent stations and interference as well as needed.
  3. You will note some ripple on the top of the passband. These are the peaks of each filter crystal. They should be fairly regularly spaced apart. You an try to swap around the crystals to improve the spacing.

CW filter

  1. The circuit of the CW and the capacitor values are different tot eh SSB Filter.
  2. Select 5 closely matched crystals and mount them in the test PCB, along with the correct capacitors.
  3. The passband of the CW filter will be very narrow, roughly 500Hz. Find and note the side skirts of the passband. Due to the 5 crystals being on much closer frequencies there will be much more loss in this filter (up to 10dB). This is normal.r
  4. Now the maths again for this filter to find the centre of the passband.

Next step

  1. Now the filter crystals are selected and tested in sets as complete filters, they can be soldered onto the Radio Board.
  2. Note each crystal can be ordered to the 0V ground layer of the PCB on either side where there is a small area of no solder mask.
  3. Next the software procedure followed for setting the HamPi IF frequencies for both SSB and CW. This procedure is to be found on the Radio Board page.
  4. Link to Radio Board page.

Loading

Scroll to Top