Assembly Guide for the 1KW 222MHz LDMOS Amplifier Kit

Update as of 12/17/2011:

After building 4 of these amplifiers without any issues, the 5th exhibited a low-frequency oscillation which caused an electrolytic bypass capacitor to detonate. One additional component (L3, a 5nh inductor) should be added across the gate pads as shown at the end of this document. In fact, this component is shown in one of Freescale's sample designs, and it's purpose is now clear. My guess is that there are variations in devices from lot to lot, and I finally used one with a bit more low-frequency gain than the others. However, this inductor reduces the low frequency gain of the circuit without affecting the performance of the amplifier at 222 MHz, and eliminated the instability completely.

Trim the coax as shown here. Tin the shields with solder.

Make the 230nh RF Chokes

1. Using the two pieces of #14 magnet wire supplied, make the chokes by winding the wire on a 3/8 inch form (the shaft of a 3/8 drill bit works well).
2. Wind one of the inductors in a clockwise direction, and the other counter-clockwise. This will form them in a way that facilitates mounting, since they will fit best on one side of the board or the other, depending on which way they were wound.
3. Using a file (a nail file works well), scrape 3/8 inch of the enamel from the ends of the coils and tin with solder.

Prepare and mount the 500pf metal mica capacitors. Metal mica capacitors are used in this amplifier in places where the ATC 700B chip capacitors have been known to fail due to overheating from high RF currents. These metal mica parts are very rugged, high Q, and easily withstand the high currents flowing in these spots. In some locations, several are connected in parallel to minimize ESR losses and the associated heating.

1. Locate the four 500pf metal mica capacitors (these may be supplied in any value from 400 to 1000pf). Bend the solder tab on each as shown. These will be paired up as 2 sets of 2, with each paired set comprising a single 1000pf metal mica with lowered ESR. Bend the center tabs as shown in the picture, and solder these first two parts into place on the U trace, which is located next to the ground trace with the rivets in it. The capacitor body should be centered up on the thicker part of this trace, and lined up in the middle of the thinner part.
   
    
2. Bend the tabs on the remaining two 500pf capacitors in the same way, and line them up as shown on the riveted ground trace, right on top of the rivets; solder the body onto the rivets and trace below. The tabs on all capacitors should be touching the body of it's companion capacitor.
   
3. Solder all 4 tabs to the companion capacitor's body.
   

The space between the capacitors is small, be careful to avoid solder shorts that bridge this gap. Verify that the bodies of the capacitor pairs are not shorted to one another.
 

Prepare and mount the 6.8pf matching capacitors

1. Bend the tabs on the 6.8pf capacitors as shown in the first photo.
    
2. Mount one of these capacitors as shown in the second photo. Solder into place on both L traces.
    
3. Place the second capacitor into the position shown, with it's tab on top of it's companion capacitor's body. Solder the tab to the companion's body and the body to the L trace underneath.
   
   Verify that the tabs of this capacitor pair are not shorted to one another (it should measure an open from one L trace to the other).
   

Mount the ATC 700B chip capacitors as shown.
 

Mount the drain bypass capacitors and the 12 ohm coax (TC-12)

1. 1000pf, 220uf, and 2.2uf, one each on each 50v input trace as shown. Note the polarity marking on the electrolytic capacitors

Now connect the 12 ohm coax as shown in the second picture
 

Mount the 15pf matching capacitor.

1. The body of the capacitor must be right up against the coax center conductor as shown, and soldered to it. The other side of the body is soldered to the trace underneath, and the center tab to the opposite trace.
    

Mount the RG-142 coax at the output

1. Form the coax as shown, and place it so it tilts toward the output end of the board a bit. This provides some spacing away from the electrolytic cans.

2. Solder the shield to the left trace, and the center conductor to the right trace as shown.

3. On the other end, solder the shield on top of the rivets in the board, and the center to the output trace
    

Mount the drain RF chokes

1. One end will connect to the shield of the TC-12 coax and the underlying trace, and the other end goes to the 50v input trace below the coax loop.
    

Your completed output board should look like this now.

Check the two 50v power input traces for a short. Your ohmmeter will probably show the charging of the electrolytic capacitors at first, but then the traces should read open. After checking, discharge the 50v traces to ground (the ohmmeter will have charged up the electrolytic capacitors).
 

Moving to the input board now, load the components shown.

1. Trimmer capacitor - after placing the part, align the slot on the trimmer capacitor vertically (this will place it mid-range).

2. Two 1000pf and two 470pf chip capacitors across the U trace to the riveted ground trace

3. a 22pf chip capacitor in parallel with the trimmer capacitor

4. two 4700pf chip capacitors (left of the trimmer and 22pf cap)

5. a 33pf chip cap across the input trace

6. a 10 ohm chip resistor and 1000pf bypass cap
    

7. place the remaining parts of the bias feed:

   1. 100 ohm resistor on the left side of the large square pad

   2. .1uf chip cap (far left, bottom part)

   3. 5k chip resistor (next up)

   4. 5k thermistor (next up)

   5. 200 ohm trimmer resistor

   6. 100 ohm chip resistor (vertical, one end located near a ground rivet)

   7. 6v Zener diode (note polarity markings)

   8. 200 ohm chip resistor (green part shown here)

   9. 500 ohm chip resistor (located under the word "bias")

   10. lastly, a jumper across the pads leading from the "bias in" pad to the 500 ohm resistor. I used a zero ohm chip resistor here, but a jumper of any sort will work. The extra pads are for additional chip resistors in the event that the bias must be fed from a voltage higher than 12v (not used here).

The last step is to connect the 25 ohm coax transformers, and the RG-316 input matching transformer as shown on the completed input board here.

Update: Make and install L3 as shown at the end of the document
Testing:

Once you install the boards, and you are ready to test, here's the recommended method:

1. Connect the output to a wattmeter and a kw dummy load

2. Connect the input to your driver through a wattmeter

3. Turn on the 50v main supply voltage, but not the bias; there should be no current drawn

4. Now turn on the bias and set the bias trimmer for 2.5 amps idling current

5. Drive with 1 watt or less. Tune the input trimmer cap for lowest input SWR or highest drain current

6. Increase drive until you have 1kw out. Input power should be less than 4w, and drain current less than 30A at 50v

The latest changes:

Install a 5nh inductor (2 turns of small gage wire, 1/8 inch ID) across the gate leads of the LDMOS transistor pads in the position shown in this picture. The shape and value of this part is not critical; it's purpose is to ensure stability by reducing the low frequency gain of the circuit.