Aug 132011
 

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Well, today I did some more work on the 2m linear.  Earlier this week I ordered some SMA connectors and some 1N5711 diodes for the project.

Two 1N5711 diodes will be used to make a voltage peak detector, to detect when the amplifier is subjected to power above 60mW.  The SMA connectors will be the interconnects between the modules.  This afternoon’s effort was spent soldering the SMA connectors onto two of the boards, and mounting the 2m amplifier module onto the heatsink.

The EME157B2 preamplifier kit was originally intended to be mounted in a masthead box, with BNC connectors soldered to the PCB, and stuck up a pole near the antenna.  In my application, I wanted it to be in the same enclosure (with suitable shielding) as the power amp, so that I could use its RF detection to automatically switch the power amplifier on.  I will also be using different relays, mounted on a separate board.

Instead of mounting the SPDT relays for the kit on the EME157B2 board, I’ve instead left these off.  I also omitted the 2N7000 MOSFET which turns on the relays, and L4, an RF-blocking choke which permits the preamp to run from a 12V source supplied up the coax.  I instead will power the preamp directly.

Since the relays will be on a separate board, the plan is to run wires from the gate and source connections where the 2N7000 belongs, and run those out to a controller board.  With the relays gone, the RF detection and the preamp are essentially two distinct circuits.  So 3 SMA connectors will be needed.  Here is the completed board with the SMA connectors fitted, and suitable jumpers installed, ready for tuning.

Completed 2m preamplifier

Completed 2m preamplifier. Connectors going left-to-right: Antenna input, Amplifier output, RF detector input.

Next, I finished off the power amplifier board, mounting it to the heatsink.  I have left one EMC filter disconnected for now, as the instructions say to power it up first with it disconnected to set the trim pot for 4.5V bias.  Rather than mounting the board flat on the heatsink, I have instead opted to mount the PCB at 90? to the module.  I had to make the supplied eyelets a fraction longer to accommodate this.  I also mounted SMA connectors on this board.

Completed 2m power amplifier

Completed 2m power amplifier. RF input is on the left.

The plan is, I’ll route RG195 coax on the left side to a small module which will contain the overload detection circuit and two SMAs for an external attenuator module.  On the right, a low-pass filter will be connected.  I also had a stab at tapping holes into the sides of the heatsink for mounting a bracket.  This bracket would hold the fans on top, and would bolt onto the main enclosure.  In doing this, I managed to bugger up two of the 8 holes, and thus what I’ll probably do, is buy a M4 tap tomorrow, drill them all out to 3mm and tap them to 4mm.  These are structural holes, so bigger is probably better anyway.

Much of today though was spent designing the controller.  I’m still finalising the design, but a rough schematic is below.

2m amplfier controller

2m amplfier controller

So, not yet going, but big parts are built now.