USB Voodoo

[ChrisSmolinski]

After a few years of no use, I decided to get the SDR-14 up and running again (to record the 22m band for a few hours in the evening when foF2 is highest and the chances of picking up some beacons is best). But I ran into a problem, it would only run for a few seconds, then I'd lose the USB connection. I had to cycle power for the computer to see it again. This was with it plugged directly into the computer's USB port, no hubs. You know the first thing someone says when you have problems with a USB device is to plug it directly into the computer, don't use a hub. (keep that in mind as you read on)

I tried other USB cables, 12V power supplies, even another computer. No luck.

I contacted Pieter at RF Space to see if he saw this type of failure before. He said the SDR-14's USB port was "sensitive to the USB clock on the PC", and that they were soldering two 10 pF caps to the 6 MHz xtal on the FT245 USB I/O chip. I assume that was to bring it closer to spec. I checked the 6 MHz oscillator on the FT245 and sure enough it was at 6.032 MHz, well outside of the 0.1% USB tolerance spec.

On a lark, I decided to plug the SDR-14 into a USB hub plugged into the computer. Voila! It works fine, and has run for hours without stopping. Perhaps the hub is more tolerant of timing errors?

Then I decided to solder a cap on the 6 MHz xtal to pull it down to 6 MHz. I did that, but still have the same problems when directly plugged into the computer's USB port. But still works fine with a hub. At this point I am going to stop trying to "fix" it, and use it as is, which works.

[Rob.]

That's more common than you think. I worked on a project where the vendor couldn't get their device to work on over 200 of our computers. By chance the only ones that worked were the few that had hubs and were plugged into that hub. The vendor had to supply over 200 USB hubs to us. The hubs they supplied were the cheapest POS they could find but it worked. It took a long time for the sales rep to admit that was the problem so I talked with the design engineer and he mentioned some vague about timing. Company ended up being a fly by night outfit.

Seen it happen with a couple other specialized devices, too. Never took the time to try to figure out why, just added a hub to the mix. (Time is money thing, you know.)

[Josh]

I wonder how it performs in fax reception. Normally using a hub can degrade sdr performance.

[ChrisSmolinski]

I wonder how it performs in fax reception. Normally using a hub can degrade sdr performance.

If no data packets are lost, there's no degradation at all. It's digital, not analog 

[Josh]

When I tried fax via utwente it was always mucked up due to timing. Psk was fine, am fine too, fax, ugh so figured yours might be a similar case.

[ChrisSmolinski]

That's most likely because the sound data is getting mangled while sent over the intertubes. Possibly due to compression? Locally over a USB or ethernet connection, the sound quality is as good as it can get. Certainly better than radios with knobs 

[Josh]

USB really stands for Unpredictable Serial Bus.

[Rob.]

... and it takes three tries to plug in a USB cable.