Hi group,
I am ready to share some results obtained from the new version of the board made by itead.
TDR TestingA Tektronix SD24 sampling head was used in a Tektronix 11801 Mainframe. The SD24 has specified reflected rise time of 35ps.
Test setup:
The board was connected to the SD24 with a short length of semi-rigid coaxial cable.
The attenuator on the board was shorted. The avalanche transistor was also shorted collector emitter. This allows the transmission line on the board to be measured with the TDR.
TDR Results
The step rise is the start of the pulse. The first aberration is the SMA connector on the SD24. There is a smooth section which is the 100mm length of semi-rigid cable. The next group of aberrations is the SMA connector, R7 and Q1 components. After this is the transmission line on the board. There are small aberrations corresponding to the bends on the transmission line.
It can be seen that the transmission line on the board is a close match to the 50 Ohm coax.
Pulse TestingIn order to get the best pulse shape I had to play around with the attenuator located on the PCB. Originally I had a single 20dB attenuator. This did not work very well. There was a very fast spike on the output corresponding to a fraction of pF in parallel with the resistor. I split the attenuator into two 6dB sections. I also found it was necessary to add a matching network.
Here is a picture of the attenuator section:
Here is a picture of the pulse generated:
The rise time is around 60ps
Jay_Diddy_B