SSA3021X goes down to 30Hz - Page 1
Forum section: electronics / projects

author: nenea dani
author_rank: Regular Contributor
timestamp: August 23, 2024, 11:32:08 pm
content: Dear colleagues,Let me show you an adapter that allows the use of the SSA3021X spectrum analyzer in sweep mode on the 30Hz-100kHz range. I used this adaptation to reveal the characteristic amplitude vs frequency of an amplifier with electronic tubes. Nothing new, just a mixer with the AD835 multiplier in front of the SSA3021X to increase the 30Hz-100kHz range by 1MHz, i.e. 1.000030MHz-1.100MHz, just like the old analyzers had 3,4 or even 5 frequency changes. Obviously, some characteristics will degrade to a small extent, but since we are talking about strong signals, the measurements will not be essentially affected if we aim to find non-uniformities or deviations of the DUT greater than 1dB. I consider that an amplifier with electronic tubes that approaches 100KHz with an attenuation of no more than 3dB is classified as very faithful. Regarding low frequencies, 30Hz is a limit imposed by the resolution filter of the analyzer. The adapter has inputs and outputs in 50 Ohms less the audio output which I tried to keep around 600 Ohms. The input that receives the signal from the load of the amplifier under test is also in 50 Ohm because I used RG316 cable. Ordinary/cheap shielded audio cables have been shown to introduce attenuation as they approach 100kHz. It is possible to view the entire 30Hz-100kHz spectrum or in portions, for example: 1kHz-100kHz, 30Hz-125Hz, 30Hz-1kHz with various speeds and resolutions, but not less than 30Hz. In fact, everything in a range of 30Hz is uncertain, masked or dubious. The analyzer does not have the possibility to measure in dBc, but it can measure in dBmV, mV, V, so we can make a simple calculation to highlight with the help of horizontal markers when the DUT lowers a certain threshold in dB, e.g. 1dB or 3dB. The greatest dynamics seem to be obtained for an input audio signal of approx. 800mVpp and the best accuracy, i.e. value close to reality for 200-400mVpp. In the loop back and without using the compensation function of the analyzer, I get a global non-uniformity of a few tenths on the dbm scale. We must remember that the analyzer is credited by the manufacturer with an accuracy better than 0.7dB, that is, the difference between the measured value and the real one. The scheme is very simple and it can be seen that the 10MHz reference signal generated by the analyzer is formatted and divided by 10 and then mixed with the signal obtained from the DUT, i.e. from the amplifier load through a resistive divider or even directly for a precise measurement. The input coupling is in D.C.The output of the mixer is applied to the input of the analyzer. Again the 1MHz signal obtained by division is mixed with that from the TG sweep and then filtered to be applied to the input of the audio amplifier under test. The filter has a corner frequency somewhere between 130-145kHz and eliminates the unwanted mixing products that would disrupt the amplifier with the NE5532 and which, if it could pass through it, would load the analyzer input. I used the Elsie program for the 6th order configuration, Butterworth at 136kHz. The audio output generates approximately 135mVeff with half of NE5532 and 310mVeff with both. I am attaching the diagram, pcb and some tests with markers in dBm or mV in loop back conditions, although in this case the audio output is not in the best condition. I have also attached a photo with a 1kHz signal and I did not find much difference regarding the 2nd and 3rd harmonics when I compared it with Analog Discovery 3.