TekTronix SG505 Repair and Abridged Validation
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- čas přidán 11. 09. 2024
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In this video we take a look at an ailing SG505 that I have in the lab. This is an ultra low distortion sine wave generator. On this unit the frequency stability is not healthy. But I also need to check if the repairs disturbed the distortion levels. This appears to be a LOT harder than it sounds given the distortion levels are so low. I cant even fully validate the SG505 as I do not have anything in the lab yet that can do spectral analysis of a 10 Hz signal. Well as always down the rabbit hole we go!
** Repair exposes the Tech to mains electric and should only be done by trained personnel. Errors can be fatal. **
Hello, CZcams recommended this video to me and it is a unfamiliar topic but I just wanted to drop a comment saying that I like your video and I stuck around cuz you've got a soothing voice ^_^
Love it! Nice work. Makes me want to ditch my HP 8903B in favor of an SG* and an AA501A.
Will be great if you can do a video of the Tektronix AA501A and Sg505 option 2 measuring IMD distortion
I will add it to the list.
You shouldn't really use the A-weighting function on the AA501 when evaluating the distortion performance of the SG505. The A-weighting filter frequency response varies considerably over the audio band and will distort the relative amplitudes of the harmonics, particularly when your fundamental frequency is higher than 2kHz.
The major reduction in distortion you were seeing at some frequencies is partly due to the harmonics being suppressed by the A-weighting filter filter but also the reduction of higher frequency noise in the AA501. If higher frequency noise is a problem it is better to use the 80kHz LPF rather than the A-weighting filter as it lets through more of the harmonics you want to measure. If mains hum on the signal being measured is a problem you can switch in the 400Hz HPF.
It is always a good idea to have a scope on the Function O/P of the AA501 to check that the signal, after the fundamental is suppressed, is primarily harmonics of the test signal and is not polluted by extraneous non-harmonic noise. Checking for excessive level of the fundamental leaking through also gives a good indication of the correct operation of the AA501's notch filter and will show up any instabilities in the test frequency that the AA501 is unable to track.
The testing procedure can be speed up considerably by using more modern equipment. You can use a scope with an FFT function rather than a spectrum analyser to monitor the notch residual signal. This will scan much faster than the spectrum analyser and goes all the way down to DC allowing you to check for spurious signals originating from mains hum or instabilities in the power supply etc.
100% correct. I still use the AA501A and balanced + IMD optioned SG505 as primary distortion measuring in my lab. I feed the out to a Siglent DSO in FFT mode
The other 505-Option 2 Zen knows of might be mine... we swapped some emails a while back.
This is true. I was using A weight only around the 1Khz mark where it is flat and the validation is only done to 3 harmonics out from the fundamental. With out the A weight filter the AA501A is only speced to
Yep I know of 2.5, Yours, Mine, and the half unit I have here that does not have the attenuator board. I am not expecting to fine a board any time soon for that one.
@@ZenwizardStudios I am not suggesting that you directly scope the O/P of the special notch filter module but rather the Function O/P on the front panel of the AA501.
You should get a much more substantial signal out of that Function O/P connector. This signal is amplified and scaled after the notch such that full scale reading should be equivalent to around 2V RMS. Even if you are looking at the lowest possible distortion of an SG505 the harmonic signal on the Function O/P should still be in the range of several mV RMS, quite readable by a moderately sensitive scope. By viewing the signal after the fundamental is notched out you effectively gain another 60dB or more of dynamic range for your scope allowing viewing of the minuscule harmonic signals.
Using a scope to directly look at the O/P of the SG505 is futile as even the best available scopes would not have the dynamic range and noise floor to allow discernment of the harmonic content. Neither do most spectrum analysers have the dynamic range necessary to directly view the distortion products of the SG505.
Yep and I totally agree with everything. However there is a problem with the method. It is stated Here
“This signal is amplified and scaled after the notch such that full scale reading should be equivalent to around 2V RMS.”
The issue with this method is the AA501’s have a WORSE distortion spec than the SG505. As good as the AA501 is it is impossible to us it to validate the distortion of an SG505. Now it can sanity check one and see if it is way off. To answer the question if the SG505 is with in spec a different method needs to be used. Due to the amplification in the AA501 at the function OP you have the main signal plus the distortion components of the AA501 as well as the primary signal. This would work IF the AA501 had a better distortion spec than a 505 but the 505 beats the 501. However this is great for looking at amplifiers and devices under test.
Spectrum analyzers can get the noise floor but they do not have the dynamic range hence the usage of the external notch. Tek specified a 7L5 or 7L3 as the alignment Spectrum analyzer 80 dB of dynamic range and a -60 dB reference level.
Did I miss seeing you use the AM502?
The AM502 is only used to check the 10Hz range. I need to acquire a DSA to go that slow. My current slowest spectrum analyzer only goes down to 20Hz some HP DSA's will go down to 0.1Hz I will do a full validation once I acquire the necessary analyzer
@Zenwizard Studios Thanks for details, repeated I'm sure. There are lots of details in this video.
I just got an SG505 opt01, so seeing this was very interesting.
I think I'm lost