I am very familiar with your problem and may be able to help you.
Before going any further, CONFIRM that both the SDA unit and the STEALTHWARE are set to the same BAUD RATE.
The first SDA-5000 units (non-QAM) were able to download with a standard DB9 to DB9 cable with the addition of a Null Modem. This was common practice when the Wavetek provided download cable from the factory was missing. It was also cheaper than spending $60+ for a factory cable from the manufacturer as it did work well with the non-QAM units.
The newer QAM units were designed to operate with the factory cable only with respects to the data transfer download (and other features). The Null Modem and DB9-DB9 cable will not work. Wavetek/Acterna/JDSU used to have a website that was taken down years ago with the actual correct PIN wiring diagram configuration (along with Stealthware software downloads) after they were bought by Viavi.
If you are handy with a soldering iron, two DB9 PIN connectors and a section of CAT5 cable, you should be able to manufacturer either a complete cable or an adaptor that will work. I will look in my old "bag of tricks" and get the actual configuration from my homebrew adaptor. I will also look to see if I have the wiring information from an old manual.
Please send me your email address to my PM and I'll get you the information you may need.
Have a great 4th of July!