SO2R Review

SO2R stands for Single Operator Two Radios. This mode is used by many contesters to increase their scores and in general become more productive during a contest by tending two radios. Most times the radios are on different bands, but they don’t have to be. Sometimes the second radio is just used in ‘monitored hot standby’ mode, ready to spring into action should the primary radio fail.

SO2R is also handy if you’re waiting for a band opening or want to be able to chose the best radio/antenna combination for a given QSO. I discuss these options in detail in the SO2R is Not Just for Contesters Anymore! blog post.

In SO2R mode two Minis are used to control two separate radios. Each Mini provides all its radio control features as if were all alone in the universe and each radio has its own antenna.

The SO2R controller in the center ties the Minis and common devices (mics, keys, headphones, etc.) together and provides audio connections to and from the radios via two small audio interface modules. The antenna port of each radio connects to an optional dual-port antenna switch that is under control of the CTR2-Mini Antenna Switch Controller. This switch allows any radio to be connected to any antenna but blocks connecting two radios to the same antenna (that would be bad).

The SO2R controller’s function is to route the common keys, PTT, microphone, headphone audio, and the function key pad to either Mini-A/Radio-A or Mini-B/Radio-B at the touch of a button. Transformer isolated audio from the selected radio is also routed to an external USB adapter which connects to your PC’s sound card for 3rd party programs such as WSJT-X.

Both Minis and the antenna switch controller are linked to each other via WiFi.

SO2R Functional Diagram

CTR2-Mini SO2R Controller

The CTR2-Mini SO2R Controller is a pretty simple device. There are no processors, therefore, no firmware. Just old school switches, buttons, a few latching relays, and a couple of op amps. Most of the complicated circuits are SMT devices so the board house will do the SMT assembly. The only thing you’ll need to do is mount the through-hole components and assemble it.

The front panel is kind of unique. At least I’ve never seen one done this way before (but then, I don’t get out much!) It’s basically a true single-sided circuit board with no through-hole wire leads sticking through. The front of the board has white silk screened control labels on a black solder mask. Pots and switches mount from the back in holes cut in the the board by the board house. Each pot or switch has large solder pads next to it on the back side where each control’s leads are connected using 1″ jumper wires. All the connections to the main board end up on two 12-pin headers that are jumpered to the main board using DuPont jumpers as shown in the next photo. Everything fits nicely in a PacTec KEU-7 sloped panel enclosure. The front panel replaces the aluminum panel supplied with the enclosure. You will need to trim the enclosure a little so the stacked 3.5mm jacks will clear the housing.

The front panel turned out so well that I wrote an article on the design process and sent it to ARRL to see if they were interested in publishing my method. I’m happy to report that they’ve accepted it for publication in QEX sometime towards the end of the year.

Inside the CTR2-Mini SO2R Controller

A closeup of the front panel in the next photo shows the various controls. They are grouped by function. Starting at the top-left, the Tx Enable buttons select the Mini and radio that will be enabled to transmit. (Both Minis and radios are active all the time but only one transmitter can be enabled.)

  • Pressing the A or B Enable button automatically enables Tx on the Mini connected to that port and disables Tx on the other Mini. When using the CTR2-Mini Antenna Switch Controller you have the option to open the antenna to the unused Rx when you enable Tx on the other radio. This provides additional isolation for the offline Rx.
  • Pressing the [Tx Enable] button (button [A]) on either Mini automatically switches the SO2R controller to the enabled radio the same as pressing the Tx Enable button on the SO2R controller.
  • Paddles, microphone, and the Straight Key/remote PTT/function key pad signals are automatically routed to the selected Mini/Radio.

Below the Tx Enable buttons are the Tx-A and Tx-B Mic Gain pots. Each radio has it’s own mic gain adjustment. The power switch and the mic selector are below the Tx Mic Gain pots. Two microphone inputs are provided. Jumpers permit enabling mic bias and whether the Ring of the mic plug is used as PTT on each mic jack.

In the middle are the Rx volume pots with the Rx audio routing switches below them.The Rx audio switches give you four options:

  • Both ON (as shown in the photo): Rx-A audio routes to the left headphone speaker and Rx-B audio route to the right headphone speaker.
  • Rx-A ON, Rx-B OFF: Rx-A audio routes to both headphone speakers.
  • Rx-A OFF, Rx-B ON: Rx-B audio routes to both headphone speakers.
  • Rx-A OFF, Rx-B OFF: Rx-A and Rx-B are combined and route to both headphone speakers
    • This option allows you to listen to both receivers as if they were one. This is handy when using the Frequency or Tracking options in Link mode as described in this post.
    • In theory combining receivers like this gives you a 3 dB reduction in noise but because each receiver is on its own antenna you probably will hear an increase in noise since every antenna has different noise characteristics.

The function buttons on the right side are programmable and can send CW buffers or open various menus on the Mini. They automatically route to the enabled Mini. The PTT button can be used to key the enabled transmitter.

CTR2-Mini SO2R Controller Front Panel

One question you may have is “Do I need the SO2R controller to operate two Mini’s and two radios?”

No, you don’t! The controller simply provides a convenient method to control the signals to and from the selected Mini/radio. You can use the link modes with just two standalone Minis. The Minis manage Tx Enable switching and the antenna switch without the SO2R controller. You can use a mixer board to manage the audio. You’ll also need a way of routing your keys and PTT to the selected radio which can easily be done with a couple of DPDT toggle switches. I designed the SO2R controller to put all these functions in one box.

Simplified Diagram

The schematic diagram for the controller is spread out over five pages so I created the simplified diagram below to provide a better idea of what’s going on inside the controller box. For clarity I left the antenna switch and it’s controller off.

Common Device Routing

The routing relays are shown in the Enable Mini-A position. These are latching relays. As you can see, the common devices (keypad, USB audio, remote PTT/straight key, paddles, mics, and headphones) route to Mini-A. Pressing the Enable Mini-B push button (not shown on this diagram) latches the routing relays into the Mini-B route which steers the common devices to Mini-B.

Pressing either enable switch automatically enables the Tx Enable option on the selected Mini. Press the same enable switch again to turn Tx Enable off.

Audio Routing

Audio routing is handled by DPDT toggle switches. The Mic Select switch routes the selected mic to the Tx-A and Tx-B Mic amps. Each amp has a level control. Mic bias can be enabled independently on each mic by adding J9 and J10 on the main board.

Receiver audio from each radio goes through buffer amplifiers. The Rx-A and Rx-B Vol controls allow you to set the receive level for each receiver. The Rx audio routing switches have four settings that are described above.

Winding down…

As you can see, there’s not a lot to the SO2R controller. But, when combined with two Minis and the Antenna Switch Controller a very capable SO2R control system can be created that allows you to use just about any two radios you have in your shack. You don’t have to be a contester to take advantage of this setup. Anyone with two radios and two or more antennas will find SO2R adds to their casual operating experience.

Let me know if you’re interested in building a SO2R controller. I can supply the front panel and main PCBs with SMT devices pre-installed for a very reasonable price. I’ll be creating a project file on Mouser with the necessary components you’ll need to buy to complete the controller.

73, Lynn, KU7Q