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Application Note - Port Saver, The Smart DC Block

Updated: Sep 27

Most instruments don't tolerate high voltages well. High bandwidth oscilloscopes, spectrum analyzers, and vector network analyzers can easily be damaged by less than 5V. These instruments can cost more than a hundred thousand dollars or even several hundred thousand dollars. In our own recent unfortunate experience, using existing DC Blocks, we managed to damage two oscilloscope front ends at the cost of a $13,000 repair bill, with a 2-month downtime to boot.

DC Block Product Image
The Smart DC Block

Measuring voltage ripple, noise, power supply rejection ratio, impedance, Bode plots, and dynamic step load response all require low noise, flat response voltage measurements. The common 12V computer intermediate bus can reach 16 or 18V. The fast-growing 48V bus can be as high as 72V, depending on the application.


Power rail probes work with oscilloscopes but not generically with 50-ohm instruments such as spectrum or vector network analyzers. They generally don't even work with other vendors' oscilloscopes. Power rail probes offer only a 1V dynamic range and typically a 60V maximum offset, both failing to meet the requirements of the above ranges.

As a result, engineers turn to the DC block for protection, along with low-impedance transmission line probes. This option offers low noise, flat response, and generically works with oscilloscopes, spectrum analyzers, vector network analyzers, and, in fact, any 50 Ohm instrument.


Challenges With Traditional DC Blocks


DC blocks improve the signal-to-noise ratio of measurements where smaller AC signals are superimposed on higher DC voltages. This benefits oscilloscope, VNA, and FRA measurements, such as PSRR, Bode plots, voltage ripple, and noise. The blocks stop the high DC voltage and allow the smaller AC voltages, which the instruments can tolerate. Unfortunately, high-speed transients can still pass through the DC Block, which is one way instruments can be damaged. Removing the DC voltage allows higher sensitivity instrument settings, improving signal-to-noise ratio.


Legacy DC blocks are made for RF and microwave applications. They have two severe limitations for measuring power rails on scopes, FRAs, and VNAs: they do not provide transient protection, and their frequency cutoff is too high for most power-related applications.


An additional problem is that existing DC Blocks suffer from both DC and AC bias effects, which come from the blocks' typical ceramic construction materials, making good calibration elusive.


Finally, these existing DC Blocks are only offered in a small variety of configurations and operating limits, mostly sub-optimal for power applications.


R&D lab and test have had to live with these significant shortcomings due to a lack of options. Employing a legacy DC block is better than nothing. However, it can create a false sense of security that the high-value asset is sufficiently protected against misuse, inexperienced users, and unknown transients. Users unwittingly accept the risk of asset failure each time they use the equipment.


Now, there is an available solution to protect these high-value test instruments from excess voltage exposure – adding comprehensive transient protection in addition to pre-charge and self-discharge that can all but eliminate any surges in around 30 seconds.


Port Saver™ – Enhanced DC Block for Asset Protection


The Picotest Port Saver™ DC block delivers critical transient protection to limit in-rush surge with a wide frequency range to catch low-end frequencies that cause asset failure. Advanced features include:

  • Transient protection to limit in-rush surge, tested against manufacturers' instantaneous peak and max RMS damage level

  • Tantalum construction materials to remove DC and AC bias effects

  • Self-charging and discharging

  • Near-Zero additive noise

  • Compatible with 1-port probes to add attenuation outside the block for even greater protection

  • Wide frequency range, down to 2Hz on a custom basis and up to 500MHz

  • Single-ended or bi-directional operation

  • Available in multiple frequency ranges/clamp voltages

  • Customizable to a user-specified input voltage, clamp voltage, frequency range


Conclusion and Tips to Protect Your Instrument


Picotest has introduced a series of improved 100Hz-500MHz DC Blocks designed for voltages up to 75V. They don't suffer from DC or AC Bias effects and include transient protection for sensitive instruments. They can pre-charge to eliminate the inrush surge, and the blocks self-discharge so they won't cause damage the next time you use them.


Even with safer DC blocks, there are a few tips to keep your instrument safe.


1. Connecting a 48V power supply that soft-starts will greatly reduce the potential for damage to the instruments. If you connect a probe, the 48V power supply connection is instantaneous. We recommend a 5X P2104A probe for 48V.

2. Including an attenuation of 5X in the probe will provide even lower frequency on the low end and much better inrush protection for the instrument, particularly for an instantaneous connection like this. The attenuation is a trade-off between noise, instrument protection, and measurement dynamic range. 5X or 10X is generally a good compromise.

3. If you connect the probe and DC block to the power source first, the DC Block will charge within a few seconds, and you can safely connect the DC block to the scope without an energy surge.

4. Disconnecting the DC Block would typically keep the capacitors charged. Self-discharge will discharge the capacitors within a few seconds, making them safer.


For more information and product details please visit, https://www.picotest.com/Port-Savers.html.

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