Tektronix Type 134 Probe Amplifier DC Bias Pitfall, and Parasitic Oscillation - Page 1
Forum section: electronics / testgear

author: niconiconi
author_rank: Frequent Contributor
timestamp: February 03, 2022, 10:14:16 pm
content: Last year I was fortunate enough to find a Tektronix P6021 AC current probe with the Type 134 probe amplifier in its original package. Fast forward to 2022, I was trying to measure low-level power supply noise of a filtered DC-DC converter, and I was delighted to find the Type 134 amplifier has a voltage mode, and it can work as a general-purpose 30/50 MHz voltage pre-amp too. It looks like a great idea since most oscilloscopes aren't sensitive enough at 5 mV or 1 mV range.While I was able to make the measurements, I find the Type 134 has a parasitic oscillation problem. When the rotary switch is set to VOLTS ONLY (for voltage mode) and the probe selector switch is set to P6019 (0.4 mV voltage mode, Gain = 125), under some circumstances, the amplifier can enter a 200 MHz parasitic oscillation. Only in P6019 / 0.4 mV mode, not P6022 / 1 mV mode. I found the easiest way to induce that is connecting an unterminated BNC cable at the input side (termination at the output side doesn't make a difference). Not all cable types can induce the problem, but I found my 1-meter RG316 BNC coax cable reproduces the problem reliably.I initially suspected it was a fault because I also found the amplifier stops working when a DC biased is applied, I had to use an external DC block. In the manual and theory of operation, it was repeatably mentioned the amplifier has an AC-coupled 50-ohm input, so I believed shouldn't have any problem when DC biased. Since both the input and the feedback network used the same capacitor type, I thought the AC coupling capacitors were defective.But later I found it was a trap for young players: The input is thenegativeside of the electrolytic capacitor, the positive side is a base-emitter junction. Thus the input DC bias should never exceed 0.6 V, otherwise the capacitor is reverse biased...The problem isthe max DC input in voltage mode is undocumented anywhere in the operating instructions, the only way to figure that out is reading the schematics. I think it's fair to call it an oversight... I can imagine the reason nevertheless: although it can be occasionally used as a voltage amplifier, first and foremost it is an AC probe amplifier, so the input normally stays at 0 V; thus explains the input-negative polarity design.If you have the same amplifier, beware of this gotcha and always use a DC block for voltage measurements!Finally, back to parasitic oscillation. It's apparent now the parasitic oscillation is not a capacitor problem, so now I'm not even sure whether the oscillation is a real fault or not - perhaps the amplifier is not even unconditionally stable by design to begin with?Does anyone have experience on "abusing" (not really, it's in the manual) the Type 134 as a voltage amplifier? Have you seen similar parasitic oscillation problems?Photo 1 & 2: Using the Type 134 amplifier to measure the input EMI of a DC-DC buck converter. The switching spike is clearly visible. A vintage HP 10240B DC block is used, it's a unique DC block with a very-low cut-off frequency, ideal for measuring power supply noise using RF equipment.Photo 3: 200 MHz Parasitic Oscillation in voltage mode with an unconnected BNC input cable.Photo 4: Input AC coupling capacitor pitfall. Note the capacitor polarity!

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2022, 11:08:07 pm
content: Page 2-2 of the manual further discusses the requirements for voltage mode. It says that you cannot AC couple the input because a ground return is required. At least that is how I read it although I do not understand why that is the case yet.I think your oscillation might be caused by lack of source termination of 50 ohms if you use a coaxial cable to probe. Try inserting a 50 ohm resistor in series with the coaxial cable at the probe point.

author: niconiconi
author_rank: Frequent Contributor
timestamp: February 04, 2022, 07:42:08 am
content: Quote from: David Hess on February 03, 2022, 11:08:07 pmAs far as I know, there are three types of DC blocks - capacitor at signal (standard), at ground, or both. I think this paragraph means that AC coupling is allowed, as long as the ground return is DC. I've successfully made measurements with a DC block.Yes, the oscillation stops if source termination is added. But if the amplifier is unconditionally stable, the lack of source termination should only degrade performance but not cause oscillation. Probing a low-impedance source power supply from an RF amplifier is a standard technique I've seen in many application notes. If the amplifier is not unconditionally stable it does explain the problem though, and it does make sense since it's expected to primarily used for the current probe. But the manual should've been more clear about that...

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2022, 01:34:28 am
content: Quote from: niconiconi on February 04, 2022, 07:42:08 amThe input amplifier is configured with shunt feedback (inverting amplifier) and the probe connects directly to the summing junction, so any capacitance at that point destabilized the amplifier. A source terminated transmission line will look resistive so not a problem. The inverting configuration provides the ideal termination for the current probe by minimizing the burden voltage which extends the low frequency performance as much as possible.The examples they give all involve probes, but I agree the manual should have been more explicit about the input requirements.