1206 Attenuator PCB - failed

wip_d2

This is a failed experiment. I made a design failure so I tossed them into the bin but have left this page for you to learn from. 

Reason for failure : the cooper poor on the top layer and a few miss-placed via's introduced some strange behavior at higher frequency (adding more capacity and inductive on higher frequencies). 

Introduction

As a hamradio homebrewer I often use attenuators to get a signal down a few dB and sometimes I want very strange values. For those cases I designed these 1206 PI network Attenuator PCB's with SMA Edge connectors using KiCad. 

1206_att_sch

Based on this schematic I designed the PCB shown below. I added area's without solder mask around the edges to a low adding some shielding over the PCB (not visible in image below).

1206_att_pcb

The cool thing from KiCad is that is also has a 3D viewer.  

1206_att_3dv2

For the attenuation values I used this calculator : https://chemandy.com/calculators/matching-pi-attenuator-calculator.htm and  the image below contains a list of "standard" PI network attenuation resistor values. 

1206_att_tbl

It's nice to have it all in theory but we want the real thing. So I ordered 3 PCB's at OSH Park to see if I didn't mess up this simple design. 

The previews look nice already.  


1206att_pcb01  1206att_pcb02

 

Got mail

After 2 weeks I got a letter from OSH Park, 3 shiny gold plated PCB's. 

oshpark

Next step will be building one with a for example 10dB attenuation and measure the response curves. Image below is from a PCB with 10dB attenuation configuration according the above table

1206_att_10dB

The image below is a response curve from a professional 10dB attenuation (span 1MHz - 3GHz).

1206_att_test_001

The image below is a response curve from the PCB with 10dB attenuation configuration according the above table (span 1MHz - 3GHz).

1206_att_test_002

Looking at the graph there is a lot of room for improvement. Because after aprx 940 MHz the attenuator starts to act inductive up to 23.75nH at 3GHz. 
But again as said before

wip_d2

Update : seems that my NanoVNA-V2 is messing with my head. Check out the screenshots I made with my H4.


Screenshot with a assembled PCB configured as 0dB ATT.

0dB_end

Screenshot with a assembled PCB configured as 10dB ATT.

10dB

It looks better but still they indicate some design failures. After cutting away some parts of the cooper-fill and removing some via's they are much better.
So into the bin with these PCB's and back to the drawing table. 

 

End of story


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