LT DIN Rail Digital Transmitter with Serial Data Communication and Analog Outputs for Duty Cycle and Pulse Width Modulation Applications

Duty Cycle and PWM Transmitter Frequently Asked Questions

What's the difference between duty cycle mode and PWM mode on this transmitter?

Duty cycle mode reports the raw ON/OFF ratio directly as a percentage (0-100%). PWM mode calculates that same underlying ratio but then scales it mathematically into a specific engineering unit that the transducer's duty cycle actually represents, such as relative humidity, rather than leaving it as a generic percentage.

Why does selecting leading vs. falling edges change whether I get ON or OFF duty cycle?

ON duty cycle measures the fraction of the period the signal spends high, while OFF duty cycle measures the fraction spent low; selecting which edge starts the measured interval determines which of these two complementary ratios (which together always sum to 100%) the transmitter reports.

Why does accuracy get worse at higher carrier frequencies (1% at 10 kHz vs. 0.01% up to 500 Hz)?

At higher frequencies, each cycle takes proportionally less time within the same gate time window, so fewer full cycles' worth of fine timing detail are available to average, and the same absolute timing resolution represents a larger relative error against a shorter cycle period.

What's a practical example of a sensor that outputs PWM instead of a standard analog signal?

Certain relative humidity sensors are documented as outputting their measurement as a duty cycle applied to a fixed carrier frequency (such as 120 Hz), where the duty cycle percentage itself directly corresponds to the humidity reading rather than the sensor providing a voltage or current output.

Does the gate time selection affect PWM scaling accuracy the same way it affects duty cycle measurement?

Yes — both duty cycle and PWM measurement rely on averaging an integral number of periods over the same selectable gate time, so the same gate-time tradeoff between averaging stability and update speed applies whether the ratio is being reported as a raw percentage or scaled into engineering units.

Can this transmitter be used to verify a PWM motor control signal rather than read a sensor?

Yes — the transmitter reads whatever duty-cycle-encoded signal is applied to its input regardless of whether that signal originates from a sensor or from a control system driving a motor or actuator, making it usable to independently confirm the actual duty cycle being commanded.

What resolution options are available for duty cycle readings?

Resolution is selectable at 1%, 0.1%, or 0.01%, letting the displayed and transmitted precision match what the specific application actually needs rather than defaulting to a single fixed resolution.

Can this transmitter accept the same wide range of input signal types as other LT Series pulse-input models?

Yes — the FR dual-channel signal conditioner accepts AC signals, pulses from NPN or PNP transistors, contact closures, and magnetic pickups, so the duty-cycle or PWM signal can originate from a wide range of sensor and switching types.

Can multiple duty cycle/PWM transmitters be networked together?

Yes — up to 30 LT Transmitters and/or Digital Panel Meters can be daisy-chained on RS485 for LAN integration, or a high-speed Ethernet or WiFi communication board can be used instead for network connectivity.

Does this transmitter require the Extended main board, and why?

Yes — duty cycle and PWM measurement use the Extended counter transmitter main board along with the FR dual-channel signal conditioner, since the ratio calculation and engineering-unit scaling for PWM mode require the additional processing capability the Extended board provides over the Standard board.

Duty Cycle and PWM Transmitter Questions From the Field

My PWM sensor reading seems off by a consistent scaling factor — what should I check?

A consistent scaling error usually points to the transmitter's PWM-to-engineering-units scale factor not matching the actual sensor's documented duty-cycle-to-value relationship; verifying the scaling configuration against the sensor manufacturer's specification is the standard first step.

My duty cycle reading fluctuates more than I expect for what should be a stable signal — what should I check?

Confirming the gate time is long enough to average sufficient cycles for stability is the first step, since a short gate time relative to the signal's actual period will show more apparent variation; increasing gate time is the standard remedy if the underlying signal is genuinely stable.

My reading shows ON duty cycle when I expect OFF duty cycle (or vice versa) — what should I check?

Verifying the edge selection (leading vs. falling) matches the intended measurement is the first step, since this setting directly determines which of the two complementary duty cycle values the transmitter reports.

My PWM humidity reading tracks correctly at moderate humidity but seems off at very high or very low humidity — why?

This can point to the sensor's own duty-cycle-to-humidity relationship being nonlinear near its range extremes, which a simple linear scale factor won't fully capture; checking the sensor's documented characteristic curve at the extremes of its range against the configured scaling is the standard diagnostic step.

Can noise on the input signal affect duty cycle accuracy more than it affects a simple frequency measurement?

Yes — since duty cycle depends on precisely timing both edges of the same pulse (not just counting cycles), noise affecting either edge's timing distorts the calculated ratio directly; checking cable shielding, grounding, and noise filter settings is the standard remedy for unstable duty cycle readings.

My reading is accurate at low carrier frequencies but degrades noticeably as frequency increases — is that expected?

Yes — this follows the documented frequency-dependent accuracy specification directly, with accuracy loosening from 0.01% up to 500 Hz to 1% at 10 kHz; if better accuracy is needed at higher frequencies, using a longer gate time to average more cycles can help within the constraints of the application's required update speed.

Can I use this transmitter to detect when a PWM control signal is stuck at 0% or 100% duty cycle?

Yes — the dual relay outputs can be configured to alarm on the duty cycle reading reaching either extreme, which is a common way to detect a control signal stuck fully off or fully on rather than actively modulating as expected.

My duty cycle reading works fine most of the time but occasionally spikes to an implausible value — what should I check?

Intermittent implausible spikes often point to a momentary noise glitch or a marginal connection being interpreted as a false edge on one channel; checking connector integrity and cable shielding, along with reviewing whether the spikes correlate with nearby equipment switching on or off, is the standard first step before suspecting a persistent transmitter fault.