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Picotest Injector Teardowns
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sixtimesseven:

--- Quote from: Jay_Diddy_B on May 21, 2020, 11:31:20 pm ---Hi group,

been busy with KiCAD and Fusion 360.

Any interest in this project:

(Attachment Link)

(Attachment Link)

This is a blocking capacitor and bias injector.

Regards,
Jay_Diddy_B

--- End quote ---

Yes, very interesting indeed! Just found this thread. Great work! I made my own PSSR tester since the picotest unit was far to expensive for me. Ended up doing pretty much what Ti suggested:
http://www.ti.com/lit/an/slaa414a/slaa414a.pdf?ts=1590175942099
Jay_Diddy_B:
Hi,
Let me share a few measurements made on the prototype:



This is a picture of the prototype. It was milled on double sided FR4 PCB. The board is 100mm x 100mm

0 - 200MHz



A DSA815-TG was used for the measurement. The setup was normalized with a BNC barrel connector.

The Bias tee is almost perfectly flat in the frequency range 0 - 200MHz.
This was confirmed using an HP3577A VNA

0 - 1.5GHz

The frequency range was widened to 1.5 GHz



There is a notch at 922MHz

After a little while I realized what was causing this.



I tested a BNC tee piece with an 8cm length of RG316 coax on one limb.

This is the result:



A notch at 625MHz

This notch is when the open length of the line is 1/4 of wavelength.

3 x 10E8 /625 x 10E6  = 48cm

1/4 wavelength = 12cm

velocity factor = 0.66

It kind of works out.


At 922MHz

wavelength = 3 x 10E8 / 922 x10E6 = 32cm

quarter wavelength = 8cm

Open circuit transmission line



The resistor needs to be moved closer to the capacitors.

Regards,
Jay_Diddy_B


Jay_Diddy_B:
Hi,

The bias line was cut in this location:



And the measurement repeated:



Same vertical scale 5dB/div

Considerable improvement.

Some of the other 'wiggles' will go away if I have more vias between the top and bottom planes.

Regards,
Jay_Diddy_B
Sighound36:
Hi Jay

As with all things RF, some of the smallest movements generator some of the most obvious gains.

Great work  :-+
Jay_Diddy_B:
Hi,

I have continued to look at this.

I have been thinking about the best way to connect the large number of capacitor in series / parallel and maintain a controlled impedance.

PCB Design





Prototype board





The board was made from 0.062 FR4 on my LPKF Protomat c60.

Board Assembly





I inserted some of the 'via wires'

Measurement

I used an HP 8714 for the measurement:



This is 3mHz to 3GHz 2dB/div.

looking good!!

Jay_Diddy_B
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