19-11-2024, 11:12 AM
(This post was last modified: 19-11-2024, 11:19 AM by Mike Watterson.)
The power is the issue. You need to limit the bandwidth. I'd need a 10W amp to get a decent level from mine, the wideband nature means it's quite low level, but I used to use it to see filter shapes with an HP141T spectrum analyser. I never use my now because my Rigol spectrum does 19 kHz to 1.5GHz and has a tracking generator (can be a signal generator if span = 0). Though there are other uses for a noise source.
A zener is quite noisy. Also good is a high value carbon composite resistor, with enough current to warm it.
Actually even a perfect resistor is an ideal noise source!
This is why low noise circuits need to be cool and have low value resistors, thus a 10 M ohm grid bias 100x noise of 100 k Ohm grid bias, and the grid resistors want to be at a cool place on the chassis / PCB, as the noise is amplified by the valve. The resistor noise is proportional to resistance x temperature x desired bandwidth.
So at room temperature, 10 MΩ provides about 3 mV of RMS noise for 20 kHz bandwidth and 100 kΩ thus 33 uV RMS noise. Note that 20 °C to 200°C isn't x10. It's 293 K to 473 K, only 1.6x worse, so the component placement and temperature is less critical than resistance value. Maybe only radio telescope LNAs need to be cooled, but a satellite dish certainly doesn't want to be reflective to IR at the equinoxes when there is more RF noise picked up from the sun as it passes behind the satellite.
An amp that does 100 mW max (= -10 dBW) without distortion can only do that with a single tone (frequency). If you have two equal signals, the level per tone is less. The white noise total power N integrated over a bandwidth B is N = BNo
https://en.wikipedia.org/wiki/Noise_spectral_density, thus it's challenging to get a significant signal level the wider band the noise source is.
A zener is quite noisy. Also good is a high value carbon composite resistor, with enough current to warm it.
Actually even a perfect resistor is an ideal noise source!
Quote:Thermal noise in an ideal resistor is approximately white, meaning that its power spectral density is nearly constant throughout the frequency spectrum (Figure 2). When limited to a finite bandwidth and viewed in the time domain (as sketched in Figure 1), thermal noise has a nearly Gaussian amplitude distributionhttps://en.wikipedia.org/wiki/Johnson–Nyquist_noise
This is why low noise circuits need to be cool and have low value resistors, thus a 10 M ohm grid bias 100x noise of 100 k Ohm grid bias, and the grid resistors want to be at a cool place on the chassis / PCB, as the noise is amplified by the valve. The resistor noise is proportional to resistance x temperature x desired bandwidth.
So at room temperature, 10 MΩ provides about 3 mV of RMS noise for 20 kHz bandwidth and 100 kΩ thus 33 uV RMS noise. Note that 20 °C to 200°C isn't x10. It's 293 K to 473 K, only 1.6x worse, so the component placement and temperature is less critical than resistance value. Maybe only radio telescope LNAs need to be cooled, but a satellite dish certainly doesn't want to be reflective to IR at the equinoxes when there is more RF noise picked up from the sun as it passes behind the satellite.
An amp that does 100 mW max (= -10 dBW) without distortion can only do that with a single tone (frequency). If you have two equal signals, the level per tone is less. The white noise total power N integrated over a bandwidth B is N = BNo
https://en.wikipedia.org/wiki/Noise_spectral_density, thus it's challenging to get a significant signal level the wider band the noise source is.
