Fast Level Translator Driving into 50 ohms.

nsaspook

Joined Aug 27, 2009
13,079
As I mentioned in the original post I am trying to drive a 5ns wide pulse of variable frequency up to 100MHz into 50ohms.

A rise time of 1ns would be nice but I can live with 2ns.

I have attached a an oscilloscope plot which shows the output (green trace) that I currently have. This was measured using a 1.5GHz active probe. The repetition frequency was set to 80MHz for this measurement but the rising edge exhibits the same characteristics at 100MHz. The yellow trace shows the voltage at the Vcc pin but this was measured using a standard 500MHz scope probe.

The slope of the rising edge is fast enough until it reaches 1V but then it slows down and exhibits a pronounced 'bump'. If the initial slope was continued until the output reached its maximum level (> 3V) the overall rise time would be approx. 1.5ns which would be acceptable. The 'bump' corresponds to dip in the voltage measured at one of the 5V Vcc pins on the 74F3037.

The second plot shows the Vcc pin measured with an Active FET probe, the dip at the Vcc pin corresponds to the 'bump' in the output.

I am now thinking that the 74F3037 could give me the output that I am looking for & that my power supply (layout) is the issue.
It simply looks like your under-driving the transmission line.
http://www.google.com/url?sa=t&sour...8rmtCg&usg=AFQjCNFGq7BlWZ-mUrRkE_OXjdGPJr4FSw
 

Papabravo

Joined Feb 24, 2006
21,159
Seems like we are back to:
Rich (BB code):
3.5V/50 Ohms ≈ 70 mA
That might be a good place to start. It seems like the part should be capable of making that
http://www.nxp.com/documents/data_sheet/74F3037.pdf
So what are we missing here?

The VOH is only 2.0V at IOH of 67 mA. Appropriate for a TTL load, but unsuitable for your purposes. It is a fact of life that it is very difficult to get a TTL totem pole driver to supply substantial current at anything approaching VCC. The more current you require it to source the lower the output voltage goes.

CMOS does not suffer from this problem since the output stage is symmetrical. It will sink and source approximately the same current at either rail.

Now question, can you find a CMOS output stage that has the capability that you want? Perhaps a 74AC family part. I think your almost at the limit of that families capability as well.
 
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