AC or DC Coupled Probe Head

Pico also brings the flexibility of AC or DC coupling, again through selection of interchangeable probe heads. Low-impedance probes load the probed signal, slightly reducing the amplitude, and if the signal has a DC bias they will source or sink current to or from the signal source, potentially changing the device operating bias. The AC-coupled probe avoids the DC bias problem. Low- frequency 3 dB cut-off is less than 160 kHz and droop of pulse top or base is <0.1%/ns on all AC-coupled models.

High-energy Pulse and Burst Applications

Passive probes, neglecting relatively small self-heating contributions, are inherently linear and suffer from permanent damage or ratio shift only under overload conditions. The Pico Connect 910 family of RF, microwave and pulse probe heads uses MELF resistor technology and can be used for high-amplitude narrow pulse or burst applications such as high-energy physics, radar or RF imaging. The 910 family can withstand amplitudes in excess of the probe voltage rating under the following conditions:

• Peak pulse or burst voltage within ±150 V peak
• Peak pulse or burst width less than 500 ns[Note 2]
• Pulse or burst power not greater than (probe voltage rating)²/probe input resistance

Probing was a compromise ... until now

There is a rapidly proliferating need to probe, measure and view very high speed and broadband signals. Gigahertz signal frequencies were once the domain of radio and microwave engineering and were typically localized and narrow band within any given system. Today, a multiplicity of broadband signals are routinely found within digital and computing systems, personal communication, media and gaming devices, and position, sensing and control systems. Engineers now demand that these measurements, once the domain of relatively few highly skilled microwave engineers using high-cost instrumentation, be simpler and much cheaper to perform.

The Problem with Invasive Measurements

High-frequency and broadband signals typically propagate in transmission lines, whose transmitted efficiency and signal integrity rely on consistent characteristic impedance along their length. In turn, characteristic impedance is determined by conductor and insulator materials and their geometries relative to each other and adjacent grounds. These too must remain consistent if signal losses and reflections are to be minimized.

The challenge is to make contact with and measure these high-frequency or broadband signals without significant disturbance of the signal or, by inference, the transmission lines that carry them. This would be a non-invasive or, more realistically, a low-invasive measurement.

Until now, microwave probing solutions have tended to be bulky, too invasive, too expensive,
with poor measurement repeatability and in some cases mechanically or electrically fragile. As a
result, transmission lines under test have often been physically interrupted and either rerouted to,
or power-divided to, a terminating measurement instrument. While this method produces very
accurate measurements, it can also be inconvenient or impractical in cases such as these:

• Signal interruption, delay or attenuation may disturb the system function while under test.
• The interruption and subsequent repair of fine pitch or buried PCB tracking.
• Multiple system signal paths may need to be interrupted and fitted with matching pick-off networks and propagation delays, even if only one needs to be measured.

The 900 Series passive probes are a cost-effective and long overdue solution in all of these cases.

The Principle Behind the Pico Connect 900 Series Passive Probes

Low-invasive contact probing of a microwave transmission line is challenging because any addition of conductor material (Let alone circuitry) will add capacitance and disturb the line geometry and line impedance at the point of contact. However, if the additional loading of the probe is firstly minimized and then made consistent at all frequencies of interest, the overall impact of probing will be a small reduction of signal amplitude at the probe tip. Critically, this is not a change of wave or pulse shape or its relative spectral content and phases. A subsequent adjustment of probe or instrument gain, or scaling of a measurement result, can then compensate for the small amplitude reduction at the point of measurement. The result is excellent pulse fidelity and consistency as shown in the graphs below.

The 900 Series low-impedance, single-ended passive probes are suited to applications up to 9 GHz and transition times down to around 40 ps. Probe tip impedance lies between 1 kΩ and 200 Ω and all have a very low parallel input capacitance of typically 0.3 pF (0.4 pF max.).

Designed for Reliability

Low-cost, highly repeatable, very robust and high-integrity coplanar microwave design and screening are all simultaneously achieved through within-substrate assembly of components and edge-of-substrate assembly of test pins and connectors (patent applied for).

Note: Pico Connect 900 Series probes are not designed to be used as launcher probes. These probes are intended as low-invasive measurement devices for browsing of already terminated transmission lines. This purpose is distinct from launcher probes, which are intended to terminate an unterminated transmission line in a measurement or signal injection.