Taking Better Noise Floor Measurements

[Shannon Parks]

Hi Folks,

The first in a series of miniBoonton guides. Everyone is welcome to post their own ramblings. :)

Using the balanced I/O of the Boonton can help measure the real noise floor of your amp.

Lately, I've been running my Boonton source from the High BNC Output to my amp input RCA. This consists of a 36" BNC-RCA cable that I made myself from a scrap BNC-BNC cable and a Radio Shack male RCA connector. You could just as easily get a 'female BNC-male RCA' adapter from Ebay or a hamfest to use in conjuction with a 36" (or shorter if you have the table space) BNC-BNC. You could also use BNC to alligator clips or BNC to minigrips, if you have access to the inside of the chassis.

I load the output of the amp with the adequate resistive load, and then use the balanced inputs of the Boonton. Using a pair of BNC to alligator clips, I connect to the High BNC Input and attach the red lead to the '+' end of my test load. I usually connect my black clip to the '-' end. It doesn't seem to hurt measurements and helps the test in the next paragraph. I then connect another BNC to alligator clips to the Low BNC Input and the red clip goes to the '-' end of the load. I leave the black clip floating.

You can make sure your setup is working before running any automated tests very simply. Put the Boonton in Local mode by pressing LCL if it doesn't respond do to being in Remote mode. For the input control, press Level. With your amp running and at idle (no source voltage!), you should be measuring less than 10mV of noise - hopefully much less. You can now disconnect and reconnect the Low BNC Input to go from single ended to balanced measurements. See a difference?

Watch out for external sources of noise.

I had my monitor shut off after half-an-hour of inactivity the other day while setting up a Poseidon Mark III for testing. The noise level had been reading around .320mV, or 320uV. When the monitor shut off, the noise dropped to around .220mV - a significant change! I didn't try to sleuth out why at that time, but I'll use the filters next time to see where that noise is getting into the system (radiation? AC line power? mixed into Boonton?). The monitor sits right beside the Boonton. But for now, I'll be shutting my monitor off right after I press my 'Start' button in the test software. Hey, now I was able to get 100dB SNR with the Poseidon MKIII at 60W. Yellow_Light_Colorz_PDT_05

Check other sources, too, like fluorescent lights. I left my noise test hooked up, disconnected the main bench lamp, but it was OK.

Shannon

[jackinnj]

I find that the best time to noise test is Saturday morning (here in NJ) -- minimal crud on the power lines. I turn off the fluorescents and soldering station when measuring noise.

Best noise tester -- well I really like the HP3581 -- you can set the bandwidth to 3 Hz -- residual noise on the device is under 10 nV -- this is without a JFET front end.

[Shannon Parks]

Thanks for the tips, Jack.