Wideband baseband spectrum measurement

I'm designing a direct conversion receiver that processes the baseband signal with about 300MHz bandwidth. However, I'm wondering how to measure this signal? Is it possible to have an active probe that connects to an SMA connector such that I don't need to drive a 50Ohm load? Thank you for the advices...

You can use a High-Impedance probe like this. They are 50 ohms output, not SMA, but you can use an adaptor.
http://www.testequity.com/products/1285/
http://www.atecorp.com/Equipment/Tek/P6201.htm
http://www.probemaster.com/4251FETprobe.htm