Friday, January 11, 2008

SDR & Edirol FA-66, spurs etc.

Per reports of spurs when using the SDR1000 with the Edirol FA-66, I thought I'd check them out with my system.

I'm using a Toshiba laptop, and my latest version of k6jca code from the SVN directory (this version has a "Freeze" button which is very useful when making comparisons, as we'll shortly see).

I set up the Console as follows:

o Spur Reduction: OFF
o VFO A: 3.797 000 MHz
o Preamp: MED
o DDS Clock Offset: 0 (Go to "Setup>Hardware Config" tab and click on the "expert" box)
o Waterfall High Level set to -80, and Low Level set to -150.
o Display "Average" turned ON.
o Edirol Sample Rate set to 96 KHz.
o SDR's antenna connector attached to dummy load.

The spurs are shown below (you can click on the image to enlarge it).






The waterfall contains three different configurations. I merged them into one Waterfall display by "freezing" the display while I changed cables (thus the utility of the Freeze button (it's the 'F' button on the display)). The three configurations are:

1) The bottom 1/3 is with the SDR turned OFF (I also get the same thing with the SDR ON and the cables between it and the Edirol disconnected). Note the spur at 3.796 MHz. That's from the Edirol itself.

2) The middle 1/3 is with the SDR ON, and the SDR audio connected directly to the Edirol. Note the numerous spurs. Interestingly, the only spur that moves as I change the VFO in 1 Hz increments is the bright green spur to the right (at about 3.7948 MHz). All of the others are static! I don't know how to explain this - but I suspect that these are generated within the Edirol. My best guess is that this is a ground loop between the Edirol, the PC, the SDR, and their interconnection with the firewire, audio, and parallel port cables. But frankly, I don't know what the cause is, and it could easily be something else. It'd take *alot* more work to figure that out.

3) The top 1/3 is with a Radio Shack Ground Loop Isolator (p/n 270-054) in series with the cable from the SDR's "To Line In" signal and the Edirol inputs (3 & 4). Note that most of the spurs (with the exception of the spur at 3.796 and one further to the right at about 3.7948 MHz) are attenuated.

One test you can do is to tune the vfo in 1 Hz steps (that's Hz, not KHz), and watch if the spurs move, and how much they move by.

Spurs that are picked up by the SDR from external sources will follow in 1 Hz steps, and you can easily follow their movement in the waterfall.

Spurs that jump around crazily as the vfo is stepped from Hz to Hz are most likely DDS spurs.

Spurs that remain stationary are most likely spurs related to the Edirol itself, although I don't have a satisfactory explanation as to why they disappear if the cables are disconnected.

By the way - I also ran the tests with the two switching supplies removed (I turned them off and ran the laptop from its batteries and the Edirol from a bench supply (linear, not switcher)). No change in the spurs.

The above image is presented as an example of what I'm seeing. Others can use it to compare their system(s) to mine, if they think they have a problem.

For the moment, that's it.

1 comment:

Rick - W7RCB said...

Jeff, Great work. Will attempt to do the same tests when I get back to SD. Have a great day...Rick