I've been working on impulse radar designs and I've really felt the need for a high speed (>3 GHz) oscilloscope to look at the signals in time domain. We are sometimes dealing with one shot signals so a sub-sampling scope is out of the question.
We have a budget of about $7000 so I'm pretty sure we'll have to buy a second hand one.
Any recommendations?
Yes. As someone who works in a similar field my recommendation would be go with a LeCroy. Not only because they make excellent high end scopes (probably the best ones) but also because they offer the longest support (7 years full support after a Series has ceased production, and on a best effort basis after that; they still repair scopes as old as the 9300 Series which stopped production in 1998). That means if your second hand purchase fails you're not left alone as with some of the other alternatives.
For scopes that are still inside the 7 year support period you can also buy manufacturer warranty when the scope is sent in for calibration and it passes, so you can get the same protection as with a brand new one.
Considering your requirements and budget I'd say go with a LeCroy WavePro 7300A. This is a 3Ghz 20GSa/s high end scope with up to 100Mpts sample memory and a massive list of supported software options (i.e. Jitter Analysis, Serial Decode of stuff like MIL-STD1553, ARINC429 and 8B/10B, Signal Data Analysis and so on) which can be invaluable.
Here's the full spec sheet:
http://www3.imperial.ac.uk/pls/portallive/docs/1/7293022.PDFIf you need more than 3GHz bandwidth then the next step up is the WaveMaster 8000A (which starts at 1GHz and goes up to 6GHz) which is essentially a WavePro with higher bandwidth and some additional features. Note that the WaveMaster has 50 ohms inputs only while the WavePro 7300A has 1M/50 ohms switchable.
The WaveMaster 8kA spec sheet is here:
http://wiki.epfl.ch/carplat/documents/PDF/LC6800A_WaveMaster_Series_Brochure_in_PDF_format.pdfAs to the Agilent DSO80604 and DSO81204 that was suggested elsewhere: we had several of these Agilent DSO80k scopes when they were still current several years ago, and while there are various options available for them as well they are really no match for a WavePro 7kA or WaveMaster 8kA, neither in performance nor in its capabilities. For example, even if you have the max sample memory option for the DSO80604 it only works at up to 4GSa/s which is rather pathetic, as is the waveform update rate (4800 wfms/s at 64pts sample memory and up to 10GSa/s, up to 800 wfms/s with larger memory sizes and faster sample rates). The WavePro/WaveMaster is specified for up to 150k wfms/s. Or the DSO80k's smaller screen (8.4" 640x480 vs 10.4" 800x600) which is limited to displaying four grids only (WP/WM eight grids). Or the UI (designed for mouse UI on the Agilent, full touch UI on the LeCroy). Lots of things that make working with the LeCroy so much easier.
The DSO80k is also out of support since 2013 while the WavePro/WaveMaster are still supported (until end of 2016 if I remember correctly). This also shows in the software support (latest version for the Agilent is from April 2009, latest version for the WavePro/WaveMaster is from 2nd of March 2015, and it will get further updates).
There's an ebay seller called 'technosaurus' who regularly sells LeCroy scopes. He often has some WavePro 7300A ex-demo scopes with all software options and 48M memory for less than $5k which is a steal. I paid in the same region for mine. You could send him a message, maybe he has something available.