PR235 Output Stability Comparison: Pure DC Output vs. Pulsed Signals

In the precise world of process calibration, the stability of the signal generated by a calibrator is the bedrock of accuracy. The Emerson Rosemount PR235 Pressure Calibrator is renowned for its ability to both measure and simulate a wide array of signals, serving as a trusted reference in the field. Among its key functions are generating pure DC (Direct Current) signals (like mA and V) and pulsed signals (such as frequency or pulse output for flowmeters). However, the inherent stability characteristics of these two signal types differ significantly, influenced by both the calibrator's internal design and external environmental factors.

Understanding this distinction is not merely academic; it is crucial for a technician to properly set up a calibration, accurately interpret readings, and ultimately validate the performance of the device under test. Misinterpreting normal fluctuations in a pulsed signal as an error, for instance, can lead to unnecessary troubleshooting or incorrect calibration adjustments. This article provides a detailed comparison of the output stability between pure DC and pulsed signals on the PR235, explaining the root causes of their behavior and offering best practices for achieving reliable results.

#### **The Nature of Pure DC Output: Inherent Stability**

Pure DC signals, such as a 4-20 mA loop simulation or a 1-5 VDC output, are analog representations where the amplitude of the signal is directly proportional to the value being transmitted.

*   **Defining Stability:** For a DC signal, stability refers to the signal's ability to maintain a constant amplitude (current or voltage) over time. Any deviation from the set value is considered an error. The PR235, with its high-quality digital-to-analog converters (DACs) and stable reference sources, is designed to provide exceptionally stable DC output.

*   **Sources of Instability:** Even with a high-end tool like the PR235, minor instability in a DC output can be introduced by:

    *   **Thermal EMF:** Tiny voltages generated at junctions of dissimilar metals within the test leads and connections, which are temperature-dependent.

    *   **Electrical Noise:** Electromagnetic interference (EMI) from nearby motors, radios, or power lines can induce noise onto the low-voltage DC signal.

    *   **Power Supply Ripple:** Minor fluctuations in the calibrator's internal power supply.

*   **Observation:** When observing a pure DC output from the PR235 on a high-resolution digital multimeter (DMM) or a loop calibrator, the reading should be rock solid, perhaps fluctuating only in the least significant digit. This high level of stability is what makes it an ideal reference for calibrating analog transmitters and controllers.

#### **The Nature of Pulsed Signals: Inherent Jitter**

Pulsed signals, like a frequency output used to simulate a turbine flowmeter or a Coriolis meter, are digital in nature. The information is not encoded in the amplitude but in the timing of the pulses—specifically, the frequency (pulses per second) or the pulse width.

*   **Defining Stability (Jitter):** For a pulsed signal, stability does not refer to the amplitude but to the *timing* of the pulses. The variation in the timing of the pulse edges is known as **jitter**. A perfectly stable pulse train would have absolutely consistent time intervals between each pulse. In reality, all electronic pulse generators exhibit a small amount of jitter.

*   **Sources of Jitter:** Jitter in the PR235's pulse output originates from the fundamental operation of its internal clock oscillator and the circuitry that generates the pulses. Minor variations in the clock signal are inevitable due to electronic noise within the components themselves. This is a normal characteristic of digital systems and is typically specified in the instrument's datasheet.

*   **Observation:** When you connect the PR235's pulse output to a frequency counter or a device under test, you will likely observe the measured frequency value fluctuating slightly. For example, if set to output 1000 Hz, a frequency counter might display values like 999.98 Hz, 1000.02 Hz, and 1000.01 Hz. This is a direct result of jitter and is expected behavior. The key is that the value should fluctuate around the setpoint within a defined, narrow band.

#### **A Direct Comparison: Key Differences**

| Feature | Pure DC Output | Pulsed Output |

| :--- | :--- | :--- |

| **What is Measured** | Amplitude (Current, Voltage) | Timing (Frequency, Period, Pulse Width) |

| **Primary Stability Metric** | Constant Amplitude | Minimal Jitter (timing variation) |

| **Typical Observation** | Rock-solid value on a DMM | Value fluctuating within a narrow band on a counter |

| **Main Internal Influence** | DAC accuracy, reference voltage stability | Clock oscillator stability, internal electronic noise |

| **Main External Influence** | EMI/RFI noise, thermal EMF in connections | EMI/RFI noise (can cause additional jitter) |

| **Defining Characteristic** | **High inherent stability**; fluctuations usually indicate a problem. | **Inherent jitter**; small fluctuations are normal and expected. |

#### **Best Practices for Ensuring Optimal Stability**

Understanding these differences dictates how a technician should approach a calibration task.

**For Pure DC Output:**

1.  **Use High-Quality Leads:** Employ shielded, high-integrity test leads to minimize the introduction of thermal EMF and to protect against electromagnetic interference.

2.  **Ensure Secure Connections:** Loose or corroded connections can act as thermocouples, generating spurious voltages that destabilize the reading.

3.  **Avoid Noisy Environments:** Keep the calibrator and test leads away from obvious sources of EMI, such as variable frequency drives (VFDs) and large power cables.

**For Pulsed Output:**

1.  **Understand the Specification:** Consult the PR235 manual for its pulse output specification. It will define the expected accuracy and stability (jitter) of the signal. This sets your expectation for what is normal.

2.  **Averaging is Key:** Most high-quality frequency counters and modern process devices have an averaging or filtering function. Enable this feature. By averaging the frequency reading over a period of several seconds (e.g., 5-10 seconds), the effect of jitter is smoothed out, providing a stable and accurate mean value that reflects the true output of the calibrator.

3.  **Use the Right Measurement Function:** If the device under test allows it, measuring the **period** of the signal (the time for one complete cycle) can sometimes provide a more stable reading for low-frequency signals, as it is less susceptible to the effect of a single missed pulse count.

4.  **Mind the Wiring:** For long cable runs, use twisted-pair or shielded cable to prevent noise from adding extra jitter to the signal.

#### **Conclusion**

The stability of the PR235 calibrator's output is a function of both its exceptional engineering and the fundamental nature of the signal being generated. While its pure DC output exemplifies amplitude stability, providing a near-perfectly steady signal ideal for analog device calibration, its pulsed output exhibits normal, inherent timing jitter. For a technician, recognizing that small fluctuations in a frequency reading are standard behavior—and not a fault in the calibrator—is a mark of expertise. By applying the correct measurement techniques, such as using averaging for pulsed signals and ensuring clean connections for DC signals, the technician can leverage the full capability of the PR235. This ensures that every calibration, whether for a slow-responding pressure transmitter or a fast-pulsing flowmeter, is performed with a understood and trusted reference, ultimately guaranteeing the accuracy and reliability of the entire process measurement loop.