Dual-Band Transmitter X2A Buildout Lessons

[NZ0I]

Buildout of the Ver X2A transmitter boards has begun. The power supplies all appear to be working with no PCB modifications required. Lessons learned so far:

Lesson #1. Little circular test point pads were added liberally to the Ver X2A board, and they seem to be working very nicely. Near each test point is located a GND pad spaced just the right distance for the spring-loaded grounding wire on the O-scope probe. Placing a via at the center of each test pad makes it even more convenient to securely place the ground wire and scope probe - in the future I'll place such a via on test and ground pads wherever practical. Here is a picture showing test point P0:

The short ground return on the scope probe is essential for getting a clean signal displayed on the scope, especially at VHF frequencies. A nice clean 145 MHz signal is shown arriving from the Si5351 clock chip using the short ground clip.

Lesson #2. It is very easy to make an unstable 2nd-order op amp low-pass filter. I added some capacitors to the voltage scaling circuit U207 (https://drive.google.com/file/d/1yIzmMdsmdsufcbxD76IPUqQ8WPr3v-FY/view) creating a low-pass filter to help ensure a nice clean DC output going into the switching supply controller U208 pin 1. It does work nicely, but not if one sets C202 = C216. No matter what value one uses, if the same value is used for both capacitors the circuit will be unstable and will oscillate. Setting C216 > C202 ensures a stable filter. The values now shown in the schematic work fine. Lesson learned: always model the circuit.

 

[Gerald Boyd]

Yes that’s  the best way to make the measurements.

Get rid of the test lead loop inductance. The built in test points are a good idea.

Jerry

Sent from my iPad

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