Radio Board

The sections of the Radio Board. This one with TCXO Si5351 option. 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 diplexer, 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.

Construction

  • IF Frequency. 8, 9, or 10.7mHz. 9mHz is recommended. Other IF frequencies are easily possible. Contact for details.
  • Crystal CW Filter / SSB Filters. Both SSB and CW filters are catered for, but either / or options is possible. Do you want one or both?
  • SMD Si5351 + TCXO for VFO and BFO or Si5351 sub boards (More warmup drift).
  • Audio Amplifier. Either a TDA2003 (Higher audio output) or a LM380.

Option Choices. First of all decide what options you want / need. You can change or add at a later stage, so starting simple might be a good option. The options are:

  • Get hold of some M3 spacers or the like and attach them to the 4 outside mounting holes. (I use nylon ones about 30mm long). This raises the PCB from the bench so components can be mounted in their hordes(!) so they can be soldered later in batches. Use the HTML IBOM to show where to mount the parts and tick them off when done. This makes construction quite quick and easy.
  • If you have chosen the Si5351 + TCXO SMD option solder these in place first
  • I recommend placing all resistors next as per the IBOM. Then solder them in place soldering them on the top of the PCB. This is easiest. Next when done crop the leads on the back of the board.
  • Next the non-electrolytic capacitors. Then the other parts, starting with the smaller parts so that burning with the soldering iron is avoided.
  • When fitting the Crystal filters, various solder short pads are provided. These serve two functions. a) Do allow for any of the two filters (SSB and CW) to be in either position on the board. For example if a Spectrum Communications SSB filter is used, do not use the SPECTRUM PCB, but fit all parts directly to the Radio PCB in-between the relays. b) The other solder short pads are associated with the large filter area marked ‘J9 XTAL Filter 1’. This area of PCB caters for about 9 different Crystal Filter footprints from various manufacturers (What have you already got or can scrounge?). This area also caters for the Spectrum Communications CW Crystal Filter, but for this the filter IS built on the Spectrum PCB and mounted on top of this RADIO PCB as a sub board. It can be mounted using M2.5 screws and an extra nut to space it off the main PCB. OK, back the solder shorts! They on the back of the board, are there to save having stray and un-needed tracks, so once the filter is mounted choose the shortest path to track the relays through to the filter and apply solder blobs. (Note: Previous versions of the PCB didn’t have this, and the stray tracks were in circuit, without any negative effect. This just seemed more elegant!)
  • Various areas were left unmasked giving provision for screening, but screens were (fortunately) found to be unnecessary.
  • For coil and transformer winding details, circuit diagram, and other tips a separate PDF files are emailed to customers of PCBs.
  • … more following

Construction and mounting of the Radio Board

Any errors found or improvement suggestions please email them in, they will be appreciated.

The VFO Oscillator with TCXO bottom right, Mixer right, Diplexer, RF amplifier and TX Amplifier bottom left

SSB Filter top and CW filter bottom. These can be swapped, and there are many filters catered for on the top filter footprint

Top is the AF SSB & CW Audio filtering. Centre the IF amplifiers and Notch filter. Bottom left the Mic amp/VOGAD, filter, and TX circuit and right the BFO Oscillator

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