23cm Amplifier Development using RFSim99 - Part 1

I have an assortment of MMIC's recovered from a cellular base station. Since an engineer previously selected these as suitable for a device where linearity matters I thought they would be good contenders for UHF and microwave projects. The SXB4089, a 500mW output device, looked interesting for a 23cm transmitter so I decided to document my approach. This post concerns the steps leading up to  generating PCB artwork for a test board.

Step 1 : Set up RFSim99

1. Load the S2P file into a unmatched schematic and press Simulate
2. If not auto-scaled set the lower frequency to 50MHz via the Graph Limit Setup command
3. Repeat for the upper frequency of 6.05GHz
4.  Start with 500 points and reduce until we get something close to 1296Mhz. 497 is just right

Already we note that matching will be needed on input and output where the return loss is too high (S11=-0.84dB, S22= -2.46dB)

Step 2 : Check for Instability

1. Switch to the Smith chart. S11 and S22 plots show the unstatble region is outside the smith chart boundary
2. With S11 sweep the frequency by dragging the slider. Very small possibility of instability at 630MHz noted.
3. Sweep frequency for S22 display, note large range of unstable loads at 473MHz.

Step 3 : Fix instability

1. I try a range of resistors in series with Port 2. Nothing appeals.
2. Repeat for Port 1. I quickly establish that a minimum of 1.5 ohms brings stability on input and output at all frequencies.  
   
3. Let's be a little conservative and use 2.2 ohms going forward.

Step 4 : Matching

1. Drag the slider so the frequency is 1296MHz.
2. Press Auto Match
3.  Be amazed at how clever Stuart Hyde is. I'd like to meet him to say thank-you in person.
4. A conjugate match simultaneously brings input and output to 50 ohms and updates the schematic
5. Sweeping with the slider still shows no instability

Results:

A stable amplifier with a gain of 18dB matched on input and output to 50 ohms.

You could change the values to he nearest standard value, and press Simulate to see what happens to gain, matching and stability. However, in Part 2 I will cover how I plan to replace the matching networks with micro strip.