Fast oscilloscope front-end circuitry

[allanw]

I'm making a board using this ADC: ADC11DV200. It is a dual 200MS/s, 900 MHz analog bandwidth, 11-bit ADC. At $45 in single quantities from Digikey, it has the best price/performance ratio (next up is a single 500MS/s, 8-bit ADC at $55).

I thought it'd be nice to leave the input analog bandwidth as wide as possible to allow fast equivalent time sampling.

I'm planning on using this PGA: LMH6518

It has a bandwidth of 900MHz, and adjustable gain up to around 40dB. It even has programmable bandwidth limits of 20, 100, 200, 350, 650, 750MHz. I do realize the total system bandwidth is reduced to 600MHz due to the cascade of -3dB 900MHz bandwidths.

The PGA's datasheet has a bunch of oscilloscope input circuits, but they only go up to around 350MHz. I'm planning on taking page 28's JFET LNA, and replacing the JFET and other circuitry with higher bandwidth RF FET's. I'll also need to make a basic input attenuator. Does anyone with experience have some advice or some proven circuits to use? This is going on a 4-layer board with only a single +5V supply.

edit: found some interesting reading here: http://books.google.com/books?id=SPwqg7qpFWUC&pg=PA66&lpg=PA66&dq=oscilloscope+front+end&source=bl&ots=_4gV30o6Gx&sig=6p0k3pgmEJxfnTOmf03HHcKZmew&hl=en&ei=CPMFTajfFMH_lgf0xJz_CA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBwQ6AEwATgo#v=onepage&q=oscilloscope%20front%20end&f=false. Seems difficult

[NiHaoMike]

It's cheaper to just buy a good oscilloscope.

[Mechatrommer]

i want to place an order! can i? 900MHz DSO? hope the price will be the same as ds1052e of even alot less!

[allanw]

I have a good DSO already

The cascaded bandwidth of two 900MHz systems is only about 650MHz. Also, I've learned that the amplifier peaks at 900MHz and only has a good flat frequency response if its bandwidth limit is 650MHz. So that probably limits the bandwidth to around 500MHz. And keep in mind it'll only be 200MS/s, so it'll only be useful at high bandwidths if the signal is repetitive.

The most difficult part is the input circuitry at that bandwidths. And I'm not making a fully featured and protected scope front-end, just a basic one that'll only have one or two attenuation factors.

[NiHaoMike]

Just have a standard 50 ohm input (or 75 ohm) since standard passive oscilloscope probes are not going to work very well at 500MHz. Then it's just a matter of using standard broadband RF design. Ask an experienced amateur radio engineer for that advice.