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

Duty Cycle & PWM DIN Rail Transmitter Frequently Asked Questions

Why does duty cycle accuracy degrade from 0.01% at low frequencies to 1% at 10 kHz, the transmitter's documented maximum?

Documented specification specifically ties accuracy to frequency, with the finest 0.01% figure spanning 0.005 Hz to 500 Hz, degrading to 0.1% at 5 kHz and 1% at 10 kHz — since the underlying measurement counts clock pulses within each period, a higher-frequency signal leaves proportionally less time per period for the counting circuitry to resolve, which is documented as the reason accuracy coarsens toward the transmitter's upper frequency limit.

Does the same underlying "divide pulse width by period" calculation apply identically in both duty cycle mode and PWM mode?

Yes — documented description specifically states that in PWM mode, the transmitter "also determines the duty cycle ratio," using the same t/P calculation documented for duty cycle mode; the only documented difference between the two modes is what happens to that ratio afterward — duty cycle mode displays it directly as a percentage, while PWM mode scales it mathematically into application-specific engineering units.

Why would a PWM-output sensor (such as a relative humidity sensor) use a constant carrier frequency like 120 Hz rather than a variable frequency?

Documented description specifically frames PWM as "duty cycle applied to a constant frequency" — keeping the underlying frequency fixed means only the duty cycle ratio carries the actual measured information, which simplifies the transmitter's job to purely measuring and scaling that ratio, rather than needing to separately account for a frequency that might otherwise vary independently of the actual measurement.

Does selecting ON duty cycle versus OFF duty cycle (via leading or falling edge selection) change the underlying accuracy specification?

No — the documented accuracy figures (0.01% at low frequency, degrading to 1% at 10 kHz) are listed once for duty cycle measurement generally, without separate figures for ON versus OFF duty cycle selection; since OFF duty cycle is simply 100% minus ON duty cycle for the same waveform, selecting which one to transmit is documented as an edge-selection configuration choice rather than a change to the underlying measurement accuracy.

Why does this transmitter's documented maximum frequency of 10 kHz matter when many real-world motor PWM signals commonly run at higher frequencies?

Documented specification lists 10 kHz as this transmitter's upper frequency limit for both duty cycle and PWM measurement — this means the transmitter is documented as well-suited to lower-frequency PWM signals such as sensor outputs (commonly in the tens to low hundreds of Hz) or the cited 120 Hz example, but a PWM signal genuinely operating above 10 kHz, as is common in some motor drive applications, would fall outside this specific transmitter's documented measurable range.

Does averaging duty cycle over a longer gate time trade off against how quickly the transmitter can respond to a genuine duty cycle change?

Yes — documented specification ties output update rate directly to gate time (gate time + 30 ms + 0-2 signal periods), meaning a longer gate time used to average more periods together for stability is consistent with producing a correspondingly slower-updating output; this reflects the same general averaging-versus-responsiveness tradeoff documented across other LT Series gate-time-based measurements.

Does the documented ±0.02% analog output accuracy apply on top of the separately documented duty cycle measurement accuracy, or does it replace it?

Documented specification lists analog output accuracy as "0.02% of output span plus conversion accuracy" — this phrasing specifically indicates the two figures are additive rather than one replacing the other; the total end-to-end accuracy of the 4-20 mA or 0-10V output reflects both the underlying duty cycle measurement's own accuracy (0.01% to 1% depending on frequency) and the additional error the analog output conversion stage itself contributes.

Why does PWM mode scaling to engineering units (such as relative humidity) require the Extended main board specifically, rather than being available on a Standard board?

Documented specification lists the Extended counter transmitter main board as part of the duty cycle and PWM transmitter's core configuration, without offering a separate Standard-board variant on this particular page — this is consistent with the mathematical scaling step that converts the raw duty cycle ratio into arbitrary engineering units being a documented Extended-board capability, similar to custom curve linearization and other engineering-unit scaling functions documented elsewhere as Extended-board features across the LT Series.

Does contact debounce (0, 3, or 50 ms selectable) affect duty cycle accuracy the same way it would affect a simple pulse count?

Documented specification lists contact debounce as a general pulse-input filtering option intended to prevent mechanical contact bounce from being misread as extra transitions — for duty cycle measurement specifically, an inappropriately long debounce setting relative to the actual signal's pulse widths could distort the measured ON or OFF time itself, since debounce filtering effectively ignores transitions occurring within its configured window; matching debounce setting to the actual signal source (mechanical contact versus solid-state logic) is consistent with avoiding this distortion.

Can the same transmitter be reconfigured between duty cycle mode and PWM mode without a hardware change, or does switching modes require different signal conditioner boards?

Documented description presents duty cycle mode and PWM mode as two configuration options of the same FR dual-channel signal conditioner board and Extended main board, distinguished by whether the ratio is displayed directly as a percentage or scaled into engineering units — this is consistent with mode selection being a software/setup configuration choice on shared hardware, rather than requiring separate physical transmitter models for each mode.

PWM Frequency Selection for Motor Control Questions From the Field

Why does raising PWM frequency above roughly 20 kHz specifically eliminate audible motor noise?

Documented explanation specifically describes PWM switching as causing motor windings and iron core to physically vibrate at the switching frequency itself — since this vibration is documented as directly audible whenever it falls within the human hearing range (roughly 20 Hz to 20 kHz), raising the PWM frequency above that range is documented as moving the same physical vibration above what humans can hear, even though the vibration itself doesn't stop occurring.

Does doubling PWM frequency to reduce audible noise come with a roughly proportional cost in switching losses?

Yes — documented analysis specifically states that doubling frequency roughly doubles switching losses, framing this as a direct, near-linear tradeoff rather than a minor side effect; one documented worked example specifically calculates driver switching loss dropping from about 7.2W at 30 kHz to roughly half that at 15 kHz, illustrating the real magnitude of this tradeoff in a practical H-bridge motor driver circuit.

Is there a documented reason industrial variable frequency drives (VFDs) often cap switching frequency lower than the audible-noise-eliminating threshold, despite the acoustic benefit of going higher?

Yes — documented guidance specifically notes that IGBT modules used in industrial VFDs and large servo drives derate aggressively above around 16 kHz, meaning the switching loss and thermal management penalty at higher frequencies becomes severe enough that many such drives are documented as capping frequency around 8-12 kHz despite the genuine acoustic benefit that a higher frequency would otherwise provide.

Does selecting too low a PWM frequency for a given motor risk a documented problem beyond just audible noise?

Yes — documented guidance specifically identifies current ripple in the motor windings as a separate concern from audible noise; excessive current ripple at low PWM frequency is documented as causing additional motor heating, meaning frequency selection genuinely balances multiple documented factors (switching loss, audible noise, and current ripple/heating) rather than optimizing for noise alone.

Is there a documented formula or rule of thumb for calculating a minimum usable PWM frequency based on a specific motor's electrical characteristics?

Yes — one documented engineering approach specifically recommends calculating the motor's electrical time constant (tau = armature inductance divided by armature resistance) and then targeting a PWM frequency at or above 5 divided by (2π × tau), providing a documented, motor-specific starting point for frequency selection rather than relying solely on generic frequency ranges.

Does resolution (the number of distinct duty cycle steps available) trade off against achievable PWM frequency in a documented, quantifiable way?

Yes — documented guidance specifically frames this as a direct trade-off within a given timer clock: a higher target PWM frequency reduces the maximum achievable counter "TOP" value, which correspondingly reduces the number of distinct duty cycle steps (resolution) available at that frequency; documented best practice recommends matching resolution to genuine required control precision rather than pursuing resolution beyond what the rest of the system can actually use.

Can a random or frequency-hopping PWM strategy reduce audible noise without simply raising the base switching frequency?

Yes — documented technique specifically describes spreading PWM switching harmonics across a wider frequency spectrum (a "random PWM" approach) rather than concentrating audible energy at one fixed switching frequency; documented analysis notes this can meaningfully reduce perceived audible noise even when the base switching frequency itself remains within the audible range, offering an alternative to simply increasing frequency when switching-loss constraints prevent doing so.

Does driving multiple actuators or motors from PWM signals at slightly different frequencies create a documented practical problem?

Yes — documented guidance specifically warns that inconsistent PWM frequency across multiple simultaneously operating channels can create beat frequencies and interference, potentially causing uneven motion or unpredictable behavior; documented best practice specifically recommends synchronized PWM generation across all outputs in a multi-actuator system to avoid this documented interaction effect.