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Science with your Sound Card!

Sound Card Frequency Counter Fcal Feature

Measure DC Levels With Your Sound Card

Daqarta's Frequency Counter includes an Fcal button that opens a custom calibration dialog. You can use this with a simple external voltage-to-frequency circuit to measure DC sensor values like temperature or pressure. Negative values can be displayed.

Unlike the methods discussed under DC Measurements, this method only handles a single channel (the Trigger Source), and produces a numeric display instead of a normal waveform trace. But it has the advantage that it easily supports non-linear sensors (such as thermocouples) by means of calibration tables.

If you need a time-series of values instead of just a numeric display, you can use Data Logging to send the readings to a text file.

Daqarta includes tested circuits for use with inexpensive precision temperature sensors or other unipolar DC voltage sources, bipolar voltages, and thermocouples. Complete board layouts are provided for use with direct laser toner transfer or simple hand-drawn circuit board construction. There are also CAD files if you wish to have boards made by a third-party commercial supplier.

[Daqarta Fcal Dialog (7544 byte image)] The Fcal dialog uses a simple 2-point calibration scheme. You apply a High sensor value (such as boiling water for a temperature sensor) which causes the voltage-to-frequency circuit to produce an associated "High" frequency... typically a few thousand hertz. While Daqarta is monitoring that raw frequency, you then enter the value you want Daqarta to read instead of Hz... such as '100' for degrees Celsius. You conclude that entry with CTRL+Enter to automatically fill in the raw value with the current frequency. (Or you could enter that manually, if the counter is not actually running and you know the proper value.)

Then you repeat the process for some Low sensor value, such as ice water for a temperature sensor. From these High and Low data pairs, Daqarta can compute other values by interpolation (or extrapolation) as needed.

The example Fcal dialog shown here is for a temperature sensor like the LM335 that puts out 10 mV per degree Kelvin, so boiling water is 3.73 V (373 K) and ice water is 2.73 V (273 K). The Raw values are for a hypothetically-perfect conversion to frequency at 1000 Hz per volt.

In this example, with the sensor in boiling water you'd enter 100 for High Set, at which point the frequency would be 3730 Hz. Then with the sensor in ice water you'd enter 0 for Low Set, while the frequency was 2730 Hz. Daqarta interpolates or extrapolates linearly from these two points. For example, if the incoming frequency drops to 2630, the display would show -10.00 .

Not all sensors are perfectly linear, but some nonlinear sensors like thermocouples are at least highly predictable. Daqarta allows you to load a calibration table for your sensor, which causes the interpolation to follow the specified nonlinear curve. Tables for all standard thermocouple types are provided, and since these are simple text files you can easily create your own for other sensors.

The tables include a header that specifies units, such as 'DegC' for standard thermocouple tables in degrees Celsius. Daqarta "understands" Celsius and Fahrenheit, and can convert between them at the click of a Units button in the Fcal dialog.

There are also two tiny tables (LinearC and LinearF) that are perfectly linear, for use with linear temperature sensors such as the LM335. The only function of these tables is to tell Daqarta what units were entered for the High and Low Set values, so it can label the Fcal Units button, and show the proper 'C' or 'F' after the main Frequency Counter display.

The images below show the Frequency Counter raw frequency (with Fcal off), plus with Fcal conversion to Celsius and to Fahrenheit by means of the LinearC table and the units button.

[Daqarta Fcal Raw Freq (16K image)]

[Daqarta Fcal Celsius (15K image)]

[Daqarta Fcal Fahrenheit (16K image)]

Measure MHz Frequencies With Your Sound Card

The same Fcal High and Low controls can be used to allow a simple frequency prescaler circuit to work with Daqarta. This is a single inexpensive CMOS chip (CD4040) that divides the incoming frequency by 1024 (or any power of 2 up to 4096).

You just set the Fcal High Set value to the division factor (1024), and manually set the Raw value to 1. The Fcal Low Set and Raw values must both be 0. This literally tells Daqarta that an incoming value of 1 Hz should be displayed as 1024 Hz, and thus should multiply any raw value by 1024.

So, if the original signal is (for example) 1.024 MHz, which is much higher than the sound card can handle, the CD4040 divides it down to an easily-handled 1000 Hz. Then Daqarta reads that value, multiplies it by 1024, and shows it as 1024000 Hz.

The Fcal scaling is not limited to integer ratios, so you can tell Daqarta to show kHz by dividing the Fcal High factor (1024) by 1000 and entering the resultant 1.024. Then a 1024000 Hz input frequency would be shown as 1024.000.



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