Stage 4: The control electronics

he control electronics have to perform the following tasks:

  1. Decode the band data from the Lima SDR
  2. Switch the relays for the low pass filters for the various bands
  3. Measure the output power and VSWR, and protect against high VSWR
  4. Measure the temperature of the PA and control the fan
  5. Provide bias currents for the PA
  6. Show relevant information on the TFT screen

Everything, except point 5 (Bias) is handled by the Arduino Micro microcontroller. I started building it up on my experimenting unit (a power supply with +&- 15V and 5V outputs, and a breadboard on top)

The Lima SDR has a number of bandfilters which are used on receive. I believe the original idea was to use them also on transmit, but Bernd apparently changed his mind when he designed the TX board. Hence, the TX board does not have any filtering, and so this should be added. I decided to add LPF’s after the PA. The switching of the bandfilter on the Lima SDR is done by means of a de-multiplexer IC (IC13, 74HC138N). This is controlled with a 3 bit code from the PIC controller IC11. I decided to branch off the signals, and feed them into the Arduino. For this, I mounted a small connector in the unit, and soldered some resistors on IC13 to pick up the signals. The Arduino checks the code, and activates the corresponding output, which controls two relays by means of a switching transistor. The LPF’s are organized as follows:

  • 160m (2MHz)
  • 80m (4MHz)
  • 40m (8MHz)
  • 30&20m (15MHz)
  • 15m (22MHz)
  • 12&10m (30MHz)

Note: the receive board also features filters for the ranges 0-500 and 500-1000kHz, but since I have no plans to transmit on these frequencies, there are no transmit filters made for these ranges. The Arduino gets the information about the RX or TX state from the relay reserved for that, connected to jack X8. A VSWR/Power bridge will be mounted inside the LPF case. An NTC resistor will be mounted near the final power FETs to measure the temperature. A power transistor controls the fan. All relevant information is shown on the TFT screen: