I'm probably going to do it the same way as last time using an old Tek RTSA as the noise measuring receiver.? This analyser has a low noise floor and a very flat response and a very flat noise floor right down to audio frequencies. The technique last time was to use a BJT based wideband 50R amplifier to drive an InGaP MMIC to act as a preamp.?
The assumption is that the noise figure of the BJT stage (BFR91 based) should be consistent from about 40MHz down to maybe 2MHz or even lower. So hopefully, the preamp will not contribute any slope to the noise response seen on the analyser at lower frequencies.
?
The RTSA frequency response and noise floor is very flat when used below 40MHz because it acts as a baseband analyser below 40MHz. However, I am going to try and improve the BJT stage to improve the input and output return loss. I can also tweak the InGaP MMIC board to improve the frequency response below 10MHz to make it ultra flat. It's going to be quite difficulat to manage all the contributors to uncertainty, but this is realistically the only way I can do this here at home.
?
If the ENR of the noise source was totally flat then the noise level seen on the analyser should rise but still be flat once the noise source was set to the hot state. The last time I did this I saw a slight droop down in noise level between 5MHz and 2MHz. Hopefully, I can modify the BJT amp and the MMIC to minimise mismatch uncertainty and also improve the frequency response. This should provide a more reliable result.? I probably won't be able to do all this until the weekend though.
?
It's important to use a preamp with low 1/f noise and so I used the BJT stage for the first stage of the preamp.?
