>From G3PLX: Thanks Paul. Thank goodness I am not alone!
I just did a simple calculation. The ultimate rock-bottom noise power is kTB, where k is Boltzmann's constant (1.38e-23), T is room temperature (300 degrees K) and B is the bandwidth in which you measure it - let's say 24kHz for a 48kHz soundcard, so this works out to 1e-16 watts. Let's say the circuit is 600 ohms (just so we can get some idea of the picture), so the r.m.s. noise level across a 600 ohm source connected to a soundcard is going to be the square root of 600e-16, or 0.244uV. Again, just for the sake of it, let's say the soundcard ADC is designed for 2.2 volts peak-to-peak (that's 0dBm in 600 ohms). A 24-bit ADC would then give 2.2/(2^32) volts quantisation level. That comes out to 0.13uV. That's not far off the circuit noise level. That 'feels' about right to me. It says that an 'ideal' 24-bit soundcard should just be able to 'hear' it's own input noise. This is how we have always designed the front-ends of things! I am just throwing numbers in there, and the experts may say that the circuit isn't 600 ohm, or I haven't taken into account the noise factor of the pre-amp, or the typical ADC full-scale isn't 2.2 volts. But I am surely not wrong by a factor of 256? Where is all the noise coming from? Paul's observation that the noise is 100dB down on full-scale could just mean that there is that much noise coming from the SDR1000 RF hardware, but it would be easy to test this by unplugging it and short-circuiting the soundcard input. If the noise stays at the -100dB mark then it says the soundcard is that noisy, which is exactly what I find here with the Firebox. Maybe Paul has already done this. Note that the software measuring the noise needs to be set to the full soundcard bandwidth for this to make sense - it will look a lot cleaner if you select a narrow filter. I am not familiar with the SDR software so I don't know how this works. 73 Peter _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
