Say Joerg,
Could you give us a more complete set of specs? I think a lot of people are kicking around the idea of building something like this as a hobbyist project...
Stuff like...
Frequency range: 100kHz-1GHz Dynamic range: 80dB IF bandwidths (realizing that software can make it arbitrarily narrow so long as the analog front-end isn't overloaded) Resolution bandwidth Maximum power input (i.e., are you planning to use this with an antenna or at the output of a transmitter!) Any need for built-in switched attenuators? Or will you just bring a handful of SMC or BNC in-line fixed attenuators and mount as needed? Were you expecting it could run off of the 2.5W you can (legally) pull from a USB port? Or a wall-wart? Sweep time? (I think you said 1s before...) Price target? (My own opinion is that ~$500 is the threshold of pain... I know you might be miffed to find $43 worth of parts in a $500 piece of equipment, but realistically for the kind of volumes I think a small company initially could build, $500 seems quite reasonable ...)
I took a look at Tektronix's 8GHz "real-time" spectrum analyzer today... it's a really nice piece of work, approaching something more like a combined oscilloscope/spectrum analyzer hybrid that anything else out there. The group at Tek that designed it picked up a bunch of the Agilent spectrum analyzer guys who were laid off a couple years ago, and this seems to have really helped them (since historically Tek SAs didn't seem particularly competitive to me). The guy in charge of engineering has been doing microwave RF for a number of years and seems to know his stuff... even has a little bit (but not too much) of an arrogant air about him (that translates into, "What? You want me to build a warp core? What do you think I am -- a kindergardener?"); this probably does him more good than harm :-)
But of course that SA runs into the high 5 digit $$$ and won't even qualify as carry-on luggage, much less something you can stick in your laptop's bag... :-)
---Joel