An odissey with some low cost noise generators
Recently I have bought five noise generators rated from 0.2 to 2000 MHz .
After a quick check I have realized that no one of them worked correctly or at all , all of them with common problems of designing and/or building , and other with also additional problems ( loss of components , bad soldering , cables broken during constructions ....
I supposed no one was really tested .
Checking the basic circuits is is made by a noise diode polarized with an LM317 with variable voltage with a trimmer , a 3 dB attenuator , a cascade of three monolitic amplifiers and another 3 dB attenuator .
The first hardware generated noise , while the second didn't because the output cable was cut in the middle after having rolling it while screwing the SMA till the cable broke into two pieces !
What you see is the module after repair ( you can note that the cable is shorter because I used the cut part remaining connected to the SMA connector ) .
In the first module you can notice two capacitors missing on the right of the LM317 regulator .
This yelded errating instability during power up , I am sure due to autooscillations of the LM317 .
Anyway in both modules , the shape of the noise was very strange