Hi Stewart and Kevin Calculation for white noise are a bit tricky - here goes:
The power output is specified as Excess Noise Ratio. This is how much more power the N-gen produces compared to a 50 ohm resistor at a temperature of 290 Kelvin. How much power that is depends on bandwidth. If you double the bandwidth, you get twice as much noise power. The equation is Pn=kTB. So, at 2300 Hz bandwidth, at 290 K, we get 1.38x10(-23)*2300*290=9.2x10(-18) watts out of a 50 ohm resistor. Since we're 35 dB above that, we've got a power out of 9.2x10(-18)x3162=2.9x10(-14) watts. Since this is a 50 ohm source, voltage output is only about 1.2 micro volts. Of course, this is in RMS. Now, the oscilloscope bandwidth is going to be much larger, let's say 50 MHz. So the noise bandwidth increase by 50000000/2300, which in dB is 43 dB. So voltage will look higher on the scope, by the square root of that, or about 137 microvolts. Again, this is RMS, but not like for a sine wave. You should often see peak values up around 0.4 millivolts in a very random fashion. Might be a bit hard to catch unless you have a delayed sweep circuit and a fast sweep rate. Michael, AB9GV >Hi Kevin, > >If your calculations are right ( and I have not checked them) then 56 uV would >be very difficult, if not impossible to see on the average oscilloscope. > >On most oscilloscopes the maximum Y axis sensitivity is 1 to 5mV/cm, so you >are >trying to measure something approx 100 times smaller. > >73 >Stewart G3RXQ > Hi guys, > > I have built the Elecraft Noise Generator. The specs say that the noise > output is 35 dB. > > Am I correct in assuming that 0 dB corresponds to 1 microvolt, so that > an output of 35 dB corresponds roughly to 56 microvolts? > > If this is so, it would explain why I can not see the output on my > oscilloscope. > > Many thanks in advance, and 73 > > Kevin > VK3DAP / ZL2DAP _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
