HP 35665A Dynamic Signal Analyzer with issue on the signal generator. - Page 1
Forum section: electronics / testgear

author: Sören_Marodören
author_rank: Contributor
timestamp: November 19, 2020, 08:55:54 pm
content: Some time ago I noticed that my HP 35665A had a bump in the signal level at around 3650 Hz when I generated swept signals at 10 mV and lower. (The signal generator was directly connected the the Ch1 or Ch2 input.) At lower levels the problem was more severe. It looked like a resonance of some kind where it first goes up to a positive peak and the then to a negative dip before continuing at more or less the same level. See Level_shift_at_3649Hz_1mV_signal.JPGI did a quick investigation by first comparing Ch1 and Ch2 input, and could not see any difference. Then I applied a signal from an external source to the inputs, and could not see any problem with the input channels. But when I connected the signal generator to an oscilloscope I could see increased amplitude at around 3.65 kHz. So it was for sure related to the signal generator.Unfortunately I don’t have the schematics for the signal generator so the faultfinding had to be done by just looking at the board and all the components on it. When I opened the instrument and took out the signal generator board I could see that the last part of the signal chain was made of Buffers, Op-Amps and the DAC. All of these components were powered from ±15 V that came from a connector at the top of the board. I soldered some wires to -15V, 0V, +15V and the generator signal just before the last buffer so I could measure on them with the instrument running. (My plan was to use the instrument to measure on its own internal signal. But I used a DC-block for isolation…)To my surprise there was a big resonance on the +15V supply. See Resonance_at_3649Hz_on_15V_supply.JPG. At lower frequencies the ripple was around 100 uVrms. At 3649 Hz the ripple was as high as 3.6 mV and at higher frequencies it dropped down again.-15 V was much more stable. The output signal before the buffer had the same resonance as what I measured on the front connector before. This problem was for sure related to the power supply.Further investigation of the design showed that the ±15 V supply came in at the top of the board and went through a 250 uH inductor and a 6.8 uF capacitor to ground. Further down on the board there was another 250 uH inductor followed by a 6.8 uF capacitor to ground. I made an LTSpice simulation on this filter and could see that it actually had a resonance at around 3 kHz. See Filter_before_upgrade_graph.GIFBy changing the two 6.8 uF capacitors to two 220 uF capacitors it was possible to lower the resonance to around 400 Hz and the level at 3.6 kHz was lowered by 50 dB. See Filter_after_upgrade_graph.GIF.I wanted to use tantalum capacitors but realized that they are way too expensive for me, so I had to use electrolytic caps. The four 6.8 uF capacitors C415, C438, C440 and C444 were replaced by 220 uF/50 V electrolytic capacitors from Jamicon. I also added some glue between them so they are not vibrating too much. See C415_C444_replaced_by_220uF.JPG and C440_C438_replaced_by_220uF.JPG.With the new capacitors in place I repeated the swept frequency test and found a very nice and flat response. Maybe the output level drops down a smidge in the upper part. See Responce_after_replacing_caps.JPG. Maybe it is possible to calibrate that…I hope this post can be useful for someone.