P6230

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P6230 termination box

The Tektronix P6230 and P6231 are passive ×10 (450:50 Ω) probes with 1.5 GHz bandwidth and DC offset capability, intended to minimize DC probe-loading effect when used on logic circuits.

The P6230 has an offset control potentiometer in the termination box and connects to the scope, e.g. 7000 series scopes, through a BNC connector with ×10 readout pin. It can be powered from 7000 series mainframes having LEMO S-series connectors for probe power, or compatible units such as the 1101.

The P6231 is a Tekprobe Level 2 probe intended for 11000 series plugins such as the 11A32 or 11A71. Offset control and power are provided by the scope mainframe through a Tekprobe BNC connector.

The P6230/6231 shares accessories with many other Tektronix subminiature modular probes although the input hybrid is unique. Tip color is yellow, connector end color is blue, part number 206-0279-00. Its end to end resistance is 400 Ω.

Key Specifications

Bandwidth	DC to ≥ 1.5 GHz
Rise time	≤ 230 ps
Attenuation	10×
Input Impedance	450 Ω // 1.3 pF (1.6 pF guaranteed)
Signal delay	8.7 ± 0.1 ns
DC offset	–5 V ≤ (Vsignal + Voffset) ≤ +5 V
Features	Readout Pin Switchable 50 Ω termination

Internals

The P6230/6231 probes are sometimes described as FET probes or active probes, however, as the circuit diagram clearly shows, this is not correct. After the 400 Ω input hybrid, the AC component of the input signal is connected directly to the scope input through a 1 μF capacitor, then terminated in 50 Ω either in the scope input or the switchable terminator. The active circuit in the P623x, a standard NE5532 dual opamp, has the function of restoring and offsetting the input signal's low-frequency and DC components. This approach is similar to the feed-beside technique used in the 7104 scope and 7A29 amplifier.

Tek explains the raison d'être for the P6230/P6231 thus:

The P6230 acts as a standard 500 Ω passive voltage probe with the additional capability of having an adjustable tip "nulling voltage." This feature reduces the dc-loading effects on the probe when it is used to measure signals whose mid-voltage value is other than zero volts, or in circuits where the termination impedance is returned to other than ground level. The Input Bias/Offset Voltage may be adjusted so that at a particular test-signal voltage both ends of the probe input-resistor are at equal potentials and no current is flowing through the resistor.

ECL logic is most commonly operated from a -5.2 V supply with Vcc connected to ground. The output of an ECL gate is the emitter of an NPN emitter follower stage. The output is pulled down to a negative supply (about -2 V) with an external resistor (50 Ω to 100 Ω). Since speed is a major consideration in ECL designs, the interconnections between gates are often transmission lines, and the pull-down resistor doubles as a line termination.

If a standard 500 Ω, 10× probe without the input Bias/Offset feature were used to examine an ECL output, the probe's 500 Ω resistance to ground would form a voltage divider with the gate's output-termination resistor. This divider can cause distortion of the output signal levels, shift the DC operation point of the output transistor, and reduce the gate's noise margin.

The DC-load nulling capability of the P6230 helps to solve this problem. By adjusting the Input Bias/Offset Voltage to the ECL low level or to the termination voltage, the only effect of the probe resistance will be a small decrease in the ECL output-termination resistance. The effect of the probe on output voltage levels is negligible.

Links

• P6230 / P6231 @ barrytech.com (incorrectly described as active probe)

Prices

Year	1984	1987	1989	1991	1995
P6230 Catalog price	$385	$395	$420	$525	–
In 2022 Dollars	$1,100	$1,030	$1,000	$1,140	–
P6231 Catalog price	–	$410	$410	$525	$799
In 2022 Dollars	–	$1,070	$980	$1,140	$1,550

Pictures

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  P6230 compensation box

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  P6230 Switchable terminator

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  P6230 Compensation box interior

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  P6230 Compensation box interior

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  P6231

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  P6230 probe tip with alligator clip ground lead

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  Input hybrid

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  Probe tip components

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  Low-inductance ground lead

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  Probe-to-PCB adapters

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  Probe-to-PCB adapters

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  P6230 circuit diagram

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  P6230 input impedance graph

Measurements

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  P6230 Rise time measured on a 7S12 with S-52 and S-4 heads, P6230 ground sleeve connected directly at S-52 output (no ground lead)

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  P6230 Rise time without ground lead: 9.2 * 20 ps = 184 ps (equiv. to 1.9 GHz bandwidth)

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  Effect of using the "low inductance" ground lead - rise time 400 ps, bandwidth 875 MHz

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  Using the 250 mm alligator ground lead - severe pulse distortion, rise time ~3 ns (115 MHz)!

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  Bandwidth vs. grounding, from manual