Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS
Posted: Fri Apr 12, 2019 6:49 am
Sdrom 33 you surely do have a keen eye!! Yes, the pic in the previous post showed I was using a switch mode PSU. Now, if you look at the pics I recently posted here:
https://www.sdrplay.com/community/viewt ... 3&start=70
there is strong interference at low frequencies and I wrote that this was "Broadcast stations stealing into the measurement" I sincerely apologise for this mistake: this interference was caused by strong LF RADIATION BY THE PSU, as I discovered some time ago. If you look a the first plots for the LF low pass filter here:
https://www.sdrplay.com/community/viewt ... f=5&t=3943
you note strong interference here as well and I was compelled to mark the plot with a red line: even then I was using the switch mode PSU! The reason why it took me such a long time to discover the problem was that the PSU noise, being at LF, does not show up as soon as you sweep filters at a few Mhz and beyond.
I might have misled and/or convinced some readers to use a noisy PSU for the job and I sincerely do apologise. In effect a conventional, AC transformer-series regulator, 9 V supply MUST be used for feeding the noise generator. I have built one myself and its excellent results can be seen in the very clean, last plots here:
https://www.sdrplay.com/community/viewt ... 3&start=70
Pics of the Noise Generator with its new and CLEAN (forgive the pun!) 9 V. supply are shown below. I am going to correct the original posts showing the noisy plots.
https://www.sdrplay.com/community/viewt ... 3&start=70
there is strong interference at low frequencies and I wrote that this was "Broadcast stations stealing into the measurement" I sincerely apologise for this mistake: this interference was caused by strong LF RADIATION BY THE PSU, as I discovered some time ago. If you look a the first plots for the LF low pass filter here:
https://www.sdrplay.com/community/viewt ... f=5&t=3943
you note strong interference here as well and I was compelled to mark the plot with a red line: even then I was using the switch mode PSU! The reason why it took me such a long time to discover the problem was that the PSU noise, being at LF, does not show up as soon as you sweep filters at a few Mhz and beyond.
I might have misled and/or convinced some readers to use a noisy PSU for the job and I sincerely do apologise. In effect a conventional, AC transformer-series regulator, 9 V supply MUST be used for feeding the noise generator. I have built one myself and its excellent results can be seen in the very clean, last plots here:
https://www.sdrplay.com/community/viewt ... 3&start=70
Pics of the Noise Generator with its new and CLEAN (forgive the pun!) 9 V. supply are shown below. I am going to correct the original posts showing the noisy plots.