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VXI Frequency Counter Can Measure Down to 0.06 Hz

The manufacturer says . . .

ChipCenter's Alex Mendelsohn says . . .

The V635 is a single-width, C-size, register-based, VXIbus module with eight frequency-measurement channels. "D" connector and BNC connector options are available. This counter module can be used to monitor a variety of pulse sources. Its unique circuitry allows the monitoring of a wide range of frequencies without changing any module settings. Differential-input circuits with filtering and hysteresis provide high noise immunity. The switching threshold is programmable, and the input voltage should be at least twice the threshold voltage for noise immunity. AC or DC coupling of the differential inputs is programmable on a per-channel basis. TTL inputs are also provided on the "D" connector options. Frequency measurement is armed by software command, and begins when the first "edge" of the input signal is received. The input pulse stream for each channel is sampled during a user-selected observation window. The window period is programmable, and the selection is common to all eight channels. At the end of each window period, 24 bits of data representing the time-base count from the master clock, as well as 18 bits representing the number of whole periods observed, are stored in the Current Value Table (CVT) for that channel. If the period of the input pulse stream is longer than the window period, the window remains "open" until one whole period of the input signal is observed. The CVT memory can be read by software at any time, with the data from the latest observation being read. The frequency can be calculated by the host-computer software using the following formula: Frequency = whole input periods × clock rate/time-base counts. The V635 can operate in a single-scan or in a continuous mode. The clock rate for the module is programmable to provide a tick rate of 1 MHz or 10 MHz with a clock accuracy of ± 0.0001 percent. The counting accuracy depends on the time-base accuracy as well as the monitoring resolution. The longer the observation window, the higher the accuracy. A 10 ms observation window will result in an accuracy of approximately ± 0.01 percent with a 1 MHz clock, and ± 0.001 percent with a 10 MHz clock. A 100 ms window will provide accuracies an order of magnitude better. Programming the clock to 1 MHz allows a measurement down to 0.06 Hz, while a 10 MHz clock increases the resolution by a factor of 10, but makes the lower counting limit 0.6 Hz. An overflow status bit is asserted for that channel whenever the input frequency is below the measurable limit. A separate input connector is provided for a "health check" signal. The input circuitry can be switched—under program control—from each of the channels to this input, providing a test of the operating characteristics of that channel. Maskable interrupt source bits are set by an overflow from the time-base clock counter. An interrupt can be generated by any one or a combination of these bits. The V635 supports both static and dynamic configuration. Access to the data is through memory locations indicated by the Offset Register within the VXIbus Configuration Register set using A32/A16, D32/D16 data transfers. KineticSystems Corp. 900 N. State St. Lockport, IL 60441-2200 Phone: (815) 838-0005 FAX: (815) 838-4424 E-mail: tech-serv@kscorp.com

When we review frequency-counter products in this column, we're usually interested in a product's maximum upper frequency capability. This product from KineticSystems Corp. is a bit different. It's a low-frequency counter that can count down to a fraction of a cycle. The precision-time-base Model V635 VXI-based (IEEE Std. 1155) low-frequency counter, with its noise-immune differential analog inputs, as well as TTL input options, supports monitoring a range of frequencies and signal types. It accepts signals from transducers and RS-422/RS-485 transmitters. As a VXI "building block" that uses the open-standard VMEbus protocol, the V635 module can be ordered in a variety of flavors. These are primarily distinguished by the type of input connectors that are specified when ordering. By the way, regardless of connectorization, these boards will set you back about a tad under $6000 each, with software. Speaking of the VXIbus, recall that it permits communication using either register- or message-based protocols. With the register-based approach as embodied in this product, communication across the backplane is accomplished via an 8-, 16-, or 32-bit parallel path to the I/O registers within the V635 module. In contrast, a message-based system would use an ASCII interpreter on each VXI module, and the binary representations of ASCII characters would be transmitted across the system backplane. The advantage of using a message-based system in the V635 is that English-like commands and responses can be used, and throughput is typically orders of magnitude higher than that of message-based devices. Programmable Inputs Shielded DC and AC coupling (the latter can eliminate the effect of DC offsets) is used in the V635's inputs. The system also includes a programmable gain control as well as a programmable high-frequency RC-type noise filter for each channel (the block diagram at the top of this review shows one channel). The gain control sets the input switching thresholds, while the single-pole filter low-pass provides a 3 dB roll off (at 50 kHz). The V635's DC- or AC-coupled inputs can also be terminated as either high-Z inputs or low-Z types (using 100 W resistor terminators that are switch-selectable). The inputs are also diode-clamped at ± 10 V for spike and over-voltage protection. Clock Selections In terms of clocking, the V635's internal clock is driven from a temperature-stable 10 MHz clock, which imparts 1 ppm accuracy. Beyond that, the V635's clock can be set to 1 MHz under software control (a disadvantage of using the 10 MHz clock setting is that it causes a faster rollover of the tick count, and limits the lowest frequency to be measured to 0.6 Hz; using the 1 MHz clock you can count down to 0.06 Hz). As mentioned in the vendor's description (the left-hand column), all of the unit's input channels are monitored over a single user-selectable period that KineticSystems Corp. calls the observation window. What's powerful is that this period is programmable from 1 ms to 1.024 seconds in increments of 1 ms. The basic resolution of the measurement is ± 1 clock period. Making the observation window longer increases accuracy because the measurement period is longer (compared with the period of one cycle of the internal clock). If the window period is shortened, you get faster updates, which can be useful if a signal at the input is changing frequency. Excluding any jitter in the input signal, the table indicates the accuracy you can expect for various clock rates and observation periods. Observation Period (ms) 1 MHz Clock Accuracy (%) 10 MHz Clock Accuracy (%) 1 0.2 0.02 2 0.1 0.01 5 0.04 0.004 10 0.02 0.002 20 0.01 0.001 50 0.004 0.0007 100 0.002 0.0004 200 0.001 0.0003 500 0.0007 0.0003 1000 0.0004 0.0002 The board's programmable gain control, mentioned previously, sets up the V635's input ranges, and is selectable on a per-channel basis—a feature that ensures flexibility. The table shows the ranges and input thresholds for the two V635 options. click for full-size image To provide a safety margin, the actual switching threshold is approximately 30 percent of the input range minimums shown in the table (± 60 mV for the ± 200 mV to 20 V range, for example). KineticSystems notes that a gain setting higher than needed will result in a lower switching threshold and poorer noise immunity. Conversely, a gain setting that's too low could cause input pulses to be missed. In terms of counters, the V635 is only one product in a family of V6xx plug-ins. Others include a 6-channel 10 MHz optically isolated counter, a 4-channel up/down counter, and a 6-channel time interval counter. KineticSystems also offers an 8-channel 100 kHz counter as well as an 8-channel timing pulse generator and programmable clock generator. You can also order a 6-channel counter with either a 50 MHz or 100 MHz bandwidth. In addition to it V6xx line, KineticSystems Corp. offers quite a few other industrial VXI modules and components, as well as the application software and requisite drivers. VXI wares include everything from controllers and mainframes to memory boards, DSP boards, analog conditioners, and digital I/O boards. Go to the company's Web site to see more details about these products.

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