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Old 28th Sep 2019, 8:08 pm   #93
Pinörkel
Pentode
 
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 161
Default Re: Telequipment D75 scope.

I just did some frequency dependent testing on both channels of the D75. The horizontal calibration so far matched nicely with the square and sine waves from my directly connected function generator. However, I observed something strange regarding the amplitudes, I have no explanation for. I applied a 5 Vpp sine wave and varied the frequency over the full range of my function generator from 0 Hz to 30 MHz. From 0 Hz to approximately 1 Mhz the amplitude on both channels was spot on at 5 divisions. From then on, on channel 1, the amplitude lowered to 4 divisions at 30 MHz. This is something I kind of expected, because of the bandwidth limit of the scope. The manual says, that a 5 division display at 50 kHz should lower to no less than 4.2 divisions at 50 MHz. From that, I conclude that my channel 1 performs not optimal with respect to bandwidth. Now, on channel 2, the amplitude did something that I did not expect. From 1 MHz to approximately 25 MHz, the amplitude increased from 5 to 5.8 divisions and then lowered to 5.6 divisions at 30 MHz.

Since the issue affected the amplitude, I suspected the attenuator being involved. So I probed the issue depending on the attenuator setting and found out that it did not directly depend on the input voltage but on the Volts/Div setting. It only occurred on channel 2 for Volts/Div settings of 0.5, 1, and 2, being the strongest at the 0.5 setting. An issue linked in this way seems logical, since the Volts/Div rotary switch activates linked circuitry for each triplet of .5, 1 and 2. So, to me it seems likely that the issue is located in the attenuator, which is bad, since screwing around with the attenuator could affect a lot of things I can not recalibrate at the moment. The ringing may hint at some uncompensated inductance. So maybe an off-valued resistor?

Coincidentally I also noticed that a square wave had a significant rising edge overshoot including some ringing specifically for these three settings (see attached two images of channel 1 and 2). This can not be compensated with attenuator calibration via C906. I am not sure if these two issues are connected and how to further track them down without making things worse. Maybe there is something that oscillates with a resonance frequency of around 25 MHz, which also gets activated by signal edges with a rise/fall time near that frequency? Has anybody a good idea how to narrow these issues down?
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Last edited by Pinörkel; 28th Sep 2019 at 8:15 pm.
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