Radio Board

The sections of the Radio Board. See also the photo below.

The above rendering shows the various sections of the Radio board. For the home constructor, who does not want to build the complete project or wants to experiment, sections can be left out. For example for a basic receiver, the Noise Blanker, one Crystal filter, the Notch filter, Audio filter and all Transmit parts can be omitted. For a Transmit exciter, just this section can be built. This is then a much simpler build with far fewer components. The other sections can be added as desired.

This board includes the following features:

  • VFO – from a few kilohertz up to 150MHZ. Either standard Si5351 modules (2) can be used, or solder the Si5351 and TCXO directly to the board for a higher quality VFO / BFO.
  • If a better VFO is required provision is made for mounting this off board, and programming is by I2C bus.
  • Double-balanced mixer. Standard SBL-1 or the more modern SMD type can be fitted
  • Front-end diplexer filter
  • Front-end Bi-directional rx-tx
  • First Xtal filter and Noise blanker (Woodpecker, electric fence etc)
  • Different IF Frequencies can be chosen using the Function rotary control. The current design works on either 8, 9 or 10.7MHz. Others can be arranged.
  • Main Crystal filters. There are now the option to select 2 Crystal filters, one for SSB and one for CW. There are two onboard locations for filters, and a connector for an off board Crystal filter if the onboard footprints do not match your filter. For SSB something between 2kHz and 2.4kHz is suitable. For CW this can be much narrower, 600kHz or less. F1 Footprint fits various different filters, there are 8 footprints in one (so something out of your junk box or an obsolete rig should fit). They are: TOKO 10M22D from the original Omega 12 x 23mm, GQRP Club SSB filter 36 x 22mm 9 or 10.7mHz and many others. F2 footprint is for a 6 pole discrete ladder filter either from Spectrum Communication who sell them for 9 or 10.7mHz at a reasonable price and they work well, or you could home brew your own. Others: 36 x 27mm, 60 x 25mm and 55 x 17mm. others.
  • Front end and Intermediate Frequency variable gain Amplifiers (AGC) using dual gate MOSFETs BF998, or similar. Either SMD or leaded can be used.
  • Notch filter
  • Balanced Detector
  • Audio filter 3 stage selectable (2 SSB and 1 CW stage). This cleans up audio hiss very well and narrows the audio bandwidth. It can be a 6 stages, but all poles were necessary and not used.
  • Audio amplifier. Two options of IC catered for, LM380 or TDA2003.
  • Automatic Gain Control AGC.
  • Full break-in CW RX/TX switching and muting
  • CW Keyer (stereo jack and software keying for straight key or paddle)
  • TX VOGAD microphone amplifier / compression
  • TX Audio filtering
  • TX Modulator
  • TX amplifier stages & RF TX Output amplifier
  • Other ancillary functions
  • This board uses traditional components for easier experimentation. (Not that any will be needed if you build the standard set).
  • Many of the component footprints cater for a variety of different devices. SSB filter. All transistors / fets can be wither SMD or Through hole and more.

The photos below show part of the Radio board (An earlier prototype). On the left is the double balanced mixer, part of the Si5351 sub board which supplies the Local Oscillator. This or another VFO can be fitted off-board, and there is an SMA coax connector footprint under the Si5153 board. The two coils by the mixer are a duplexer, a front end filter. Next to this is a bi-directional amplifier. In receive it forms a common gate amp with about 16dB gain, and in TX it forms a TX pathway directing the TX signal at the IF frequency back through the mixer. The picture (not the latest issue) shows the IF amplifiers, then the notch filter having an XTAl at the IF frequency tuned across the IF bandwidth, to tune out a single tone. To the right again is the audio detector and then going back below in a right to left direction is the audio filter. This provides two stages of filtering for SSB, and a narrower filter for CW. The original OMEGA had a similar filter with 8 stages, but not all were useful, and when too narrow there was a hollow tunnel sound, so for this rig it was adapted for just 3 stage. At the bottom of the board is the audio amp, and to the left of that part of the AGC system, controlling the gain of the IF stages.

Below is a 3D view of the very latest Radio Board with smaller relays and the option for a CW Crystal filter. (Note the 3D rendering does not always show the correct part, or shows no part at all. If you note the Audio amplifier seems to overlap, only one option is fitted).

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