LT DIN Rail Digital Transmitter with Serial Data Communication and Analog Outputs for Pulse to 4-20 mA Converter for Frequency, Rate and Period Applications P/N LT61FR

Frequency, Rate, and Period Transmitter Frequently Asked Questions

Why does Channel A support a higher maximum frequency (1 MHz) than Channel B (250 kHz)?

The two channels are designed with different circuit paths, with Channel A optimized for high-frequency signals up to 1 MHz — suited to fast pulse trains from sensors like encoders — while Channel B is tuned for the somewhat lower frequencies typical of flow meters and rate sensors used in dual-channel arithmetic applications.

What's the practical difference between frequency, rate, and period output modes?

Frequency mode reports pulses per second directly. Rate mode scales that pulse rate into application units, like gallons per minute. Period mode reports the inverse of frequency — the time between pulses — which is useful for slow-speed applications where frequency alone would report an inconveniently small number.

Why does frequency measurement use the inverse of period instead of directly counting pulses per second?

Timing the period between pulses against a precise quartz reference and inverting it gives an accurate reading in far less time than counting pulses over a full one-second gate, which is especially important at low frequencies where waiting a full second for a stable pulse count would make the reading too slow for control applications.

When is custom curve linearization needed for a turbine flow meter?

Turbine flow meters often produce a pulse frequency that isn't perfectly proportional to flow rate at the low end of their range. Custom curve linearization, using up to 180 data points and a spline-fit approach, corrects for that nonlinearity and extends the usable accurate range of the meter rather than relying on a single linear scale factor.

What's the difference between the A-B and A/B arithmetic functions?

A-B computes the numeric difference between the two channels, useful for net flow (inflow minus outflow). A/B computes a ratio between the two channels instead, useful for applications like ingredient mixing where the proportion between two flows matters more than their absolute difference.

Can this transmitter power the sensor connected to it?

Yes — the isolated 5V, 10V, 12V, or 24V DC transducer excitation output can power magnetic pickups or two-wire proximity switches directly, so an external power supply for the sensor isn't necessarily required.

Does contact debounce affect measurement accuracy on a clean electronic signal?

Contact debounce settings (0, 3, or 50 ms) are meant to filter out the mechanical bounce of physical switches and relays; a clean electronic signal like a proximity switch or magnetic pickup typically doesn't need debounce filtering and can use the 0 ms setting without affecting accuracy.

How fast does the analog output update relative to the input signal?

The output update rate is the programmed gate time plus 30 ms plus 0-2 signal periods; for example, a 60 Hz signal would update roughly 20 times per second, which is fast enough for most alarm and control applications.

Can this transmitter measure AC line frequency directly from mains voltage?

Yes — it accepts AC voltage inputs up to 250 Vac directly and can output line frequency to 6-digit accuracy (50.0000 or 60.0000 Hz) within a few line cycles, without needing a separate step-down or isolation transformer ahead of the input.

Can multiple frequency/rate 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 an LTE series Ethernet transmitter can be used instead for a direct Ethernet connection.

Frequency, Rate, and Period Transmitter Questions From the Field

My flow rate reading is accurate at high flow but reads inconsistently at low flow — what's the likely cause?

Inconsistent low-flow readings from a turbine flow meter often point to the meter's inherent low-end nonlinearity rather than a transmitter fault; setting up custom curve linearization with data points specifically covering the low-flow region typically resolves this.

My RPM reading is intermittent or drops out occasionally — what should I check first?

Intermittent dropouts from a magnetic pickup are commonly caused by the sensor gap being too large or misaligned relative to the target, producing a signal that falls below the minimum detectable level intermittently; verifying sensor gap and alignment is the standard first troubleshooting step.

My combined A+B total doesn't match the sum of the two individual channel readings — why?

This usually points to a scaling mismatch between the two channels rather than an arithmetic fault, since A+B sums the two channels' scaled engineering-unit values; if each channel isn't scaled to consistent units before the arithmetic function is applied, the combined result won't match a manual sum of the raw displayed values.

Why does my line frequency reading fluctuate slightly even though the frequency should be stable?

Some small fluctuation is normal and reflects genuine minor variation in actual line frequency, which is common in real power grids; the transmitter's period-based measurement is highly accurate, so persistent fluctuation is more likely reflecting real grid behavior than a measurement artifact.

My pulse input from a mechanical switch produces multiple counts per actual switch closure — what's the fix?

This is a classic symptom of mechanical contact bounce; increasing the contact debounce setting (from 0 ms to 3 ms or 50 ms as needed) filters out the rapid multiple transitions a mechanical switch produces during a single physical closure.

Can electrical noise from nearby VFDs affect a high-frequency pulse input?

Yes — high-frequency pulse inputs are susceptible to noise coupling from variable-frequency drives and similar switching equipment; checking cable shielding, grounding, and physical separation from the noise source, along with using an appropriate noise filter setting, are the standard remedies.

My totalizer resets unexpectedly — what could cause that?

An unexpected reset is often traced to the external reset input being triggered inadvertently, whether by wiring, a control signal, or a configuration setting; checking the reset input wiring and the conditions configured to trigger it is the standard first diagnostic step.