Band Pass Filter Board

Sections of the BandPass Filter board

The bandpass filter board is in some regards a simple thing, but don’t be fooled. It is a central and extremely important part of any radio design. See it as a funnel, two directional for RX and TX. If there are any obstructions in this tunnel it will reflect significantly on the performance, receive and transmit. When testing or finding one particular band, say, is not working well (low output power, or poor performance on receive), suspect the BPF first!, and re-test it. For this reason it is worth a lot of effort to get this board right, and indeed there have been a few prototypes and a lot of time spent in perfecting this board.

  • 16 BandPass filters:
    • 1800 & 630 Meters LF Bands
    • 160 through 10 Meters HF Bands
    • 6, 4 & 2Meters VHF Bands (Not all countries are allowed the 4M band)
  • Each selected with 4 bit parallel TTL level logic (0000 – 1111 Binary) and 16 PIN diode switches OR the option to use very small OMRON G6K relays. A choice of a small performance gain vs cost.
    • An option for other radios here is to not fit the two ICs and fit ribbon cable or wires for individual selection. This is simply done by grounding any BPF to select it.
  • An led for each BPF indicates which is selected. It is extremely helpful (and less frustrating) to be tuning the correct coil!
  • 14dB or 18dB Attenuator (or any other, depending on the 3 resistors fitted)
  • 18dB Preamplifier (remember one S unit is 6db below S9, and 10dB above S9)
  • PIN selectable filter strips.
  • 2 x notch filters for tuning out Local Oscillator leakage.
  • 2 x Relays for selection of external LF and VHF modules

Understanding Band Pass Filter and setup in a nutshell

Have a look at the picture above. There are three tuned circuit stages to this filter. Each stage has a transformer and capacitor that resonate together. In this case IFT2 & C3, IFT8 & C15, and IFT14 & C33. The two other capacitors C9 & C27 link, or couple the stages together. This is how each stage work, and what to aim for:

  • Aim to have all capacitors as low in value as possible, so as to achieve the best performance. The larger the capacitors the lower performance. This will look like too much loss, or the individual stages will not tune to the desired frequency.
  • C9 & C27 set the bandwidth of the circuit. The aim is to have the lowest overall loss, while also having a narrow bandwidth. The larger the capacitance the wider the bandwidth, but the loss will be greater. The lower the capacitance the sharper the notch, but then the band may not be adequately covered. The aim again is to cover the particular band (Say 14.000 to 14.350mHz for 20 Meters) with a relatively flat top, and to see a sharp roll-off as the signal gets out of band. (see below)
  • C3, C15 & C33 are the capacitors in the individual tuned circuits. If they are too large there will be a triple peak and the pass-band will be too wide. If the capacitance is too small the circuit will not tune down enough to the desired frequency, and the ferrite slug in the transformer will be out of the top or bottom of the core.
  • See the image below. This is an example on 20 Metres and the loss here is 2.79dBm. Note that at 1mHz each side of the centre frequency the filter has achieved a 10dB attenuation, and at around 1.5mHz it is -20dBm attenuation.
  • Tune all three transformers to achieve the shape below. The centre transformer, IFT8, it will be seen only changes mainly the frequency and not the amplitude. The outer two transformers adjust the amplitude mostly, and not much change in frequency.
  • It is desirable to have a Spectrum Analyser with a tracking generator option to do this. The one below does a splendid job, but costs around NZ$2500. If you do not have one or you have no access to one (another HAM?), a NANO VNA will also do the job, and are much cheaper. It can be done with a signal generator and a good oscilloscope too monitoring the RF and noting the loss, peaking the sign wave.

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