Hi group,
In reading the thread, it is clear in my mind that there are two type of noise source that are being discussed here.
1) One kind like this
This is designed to put out a high amplitude noise signal that can be used in place of a tracking generator to make measurements on filters etc.
2) There is a second kind, which are designed as low cost replacements for the HP346B/C like the one I made and this one:
This only generate a very small amount noise, typical 15dB more noise than 50 Ohm resistor.
If the original poster had built the second kind of noise source, he could very easily be fooled into thinking that it wasn't working.
Here are some measurement on my noise source:
In the first measurement I have the resolution bandwidth, RBW, set to 3MHz, remember noise amplitude is a function of square root of the bandwidth. I have set the video bandwidth, VBW, to 1kHz. I have set the span to 0 Hz. The center frequency is 200 MHz.
The vertical is 2 dB division and the sweep is 10 seconds ( 1s/div). The attenuation has been set to 0dB.
The Spectrum Analyzer is an HP8568A.
I am turning the noise source on and off during the sweep. There is barely 0.4dB change in amplitude if the source is on.
I then used this amplifier made by Avantek. The part number is hard to read, I think it is UTC5 -157H. It is marked 5 - 500MHz.
I measured the gain of the amplifier in this set up:
The attenuator was fed with the 200MHz -20dBm signal for the Spectrum Analyzer. With the attenuator set to 60dB, the Spectrum Analyzer showed:
Therefore this amplifier has a gain of 60dB at 200 MHz.
I then connected the noise source to the input of the amplifier. In this screen shot the upper trace is with the noise source on the lower trace is with the noise source off.
In this configuration you can calculate the Noise Figure for the amplifier.
I just wanted to show that with the second type of noise source, how small the signal is.
Regards,
Jay_Diddy_B