Elecraft

N-Gen Output

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KBG Luxford

N-Gen Output

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
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Stewart Baker

Re: N-Gen Output

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

On Mon, 17 Jul 2006 20:28:17 +1000, kbgluxford wrote:

> 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: [hidden email]
> 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

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Masleid, Michael A.-2

RE: N-Gen Output

In reply to this post by KBG Luxford

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

Robert Friess

Re: N-Gen Output

In reply to this post by KBG Luxford

The N-gen produces noise power that is approximately 35 dB greater than that
produced by a 50 ohm resistor. At room temperature a 50 ohm resistor
produces a noise power of -174dBm per Hertz of bandwidth. So in a typical
CW bandwidth of 500 Hz, the N-gen noise power output is
approximately -174+10log(500)+35= -112 dBm or 0.56 microvolt.

Bob, N6CM

----- Original Message -----
From: "kbgluxford" <[hidden email]>
To: "Elecraft Reflector" <[hidden email]>
Sent: Monday, July 17, 2006 3:28 AM
Subject: [Elecraft] N-Gen Output

> 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: [hidden email]
> 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
>

Alexandra Carter

Re: N-Gen Output

In reply to this post by Stewart Baker

I seem to remember not being able to see much more than a hint that a
signal was there on my scope, but on a spectrum analyzer, which is
essentially a receiver, I was able to see it just fine - adjust it
just right, and the output looked like a fuzzy little caterpillar! 73
de Alex NS6Y.

On Jul 17, 2006, at 4:04 AM, Stewart Baker wrote:

> 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