{"id":2534,"date":"2021-05-06T22:23:01","date_gmt":"2021-05-07T06:23:01","guid":{"rendered":"http:\/\/wonghoi.humgar.com\/blog\/?p=2534"},"modified":"2021-05-06T22:24:08","modified_gmt":"2021-05-07T06:24:08","slug":"hp-54502a-datasheet-typo-about-ac-coupling","status":"publish","type":"post","link":"https:\/\/wonghoi.humgar.com\/blog\/2021\/05\/06\/hp-54502a-datasheet-typo-about-ac-coupling\/","title":{"rendered":"HP 54502A Datasheet typo about AC coupling"},"content":{"rendered":"\n

The cutoff frequency of 10Hz on the datasheet is a typo. Better scopes at the time claims 90Hz. 10Hz is just too good to be true.<\/p>\n\n\n\n

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Found the specs from the service manual:<\/p>\n\n\n\n

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Don’t be fooled by the -3dB cutoff and ignore how wide the transition band can be (depends on the filter type and the order). Turns out this model has a very primitive filter that AC couple mode still messes square waves below 3kHz up despite the specs says the -3dB is at 90Hz. You better have a 30+ fold guard band for old scopes!<\/p>\n\n\n\n

Remember square wave pulse train in time domain is basically a sinc pulse centered at every impulse of the impulse train in frequency domain superimposed. Unless you have a tiny duty cycle (which is not the case for uniform square waves, they are 50%), the left hand side of the sinc function at 1kHz fundamental still have sub-1kHz components that can be truncated by the AC coupling (high pass filter). <\/p>\n

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The cutoff frequency of 10Hz on the datasheet is a typo. Better scopes at the time claims 90Hz. 10Hz is just too good to be true. Found the specs from the service manual: Don’t be fooled by the -3dB cutoff … Continue reading →<\/span><\/a><\/p>\n

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