New! PC-1 Precision Calibrator

The PC-1 Precision Calibrator is a unique product that was designed to solve a common problem facing every small company or individual using electronic test and measurement equipment. How can you be certain that your equipment is still accurate and within calibration? Larger companies that are ISO 9000 have quality control procedures that mandate calibrating all test and measurement equipment on a regular basis. The equipment is usually sent to an outside calibration lab at considerable expense. The cost of implementing this type of formal program is usually prohibitive for small companies and individuals, yet relying on equipment that has become inaccurate could result in serious consequences.  

The PC-1 Precision Calibrator is a low cost, portable, battery powered instrument designed for quickly checking the basic calibration of digital voltmeters up to 4-1/2 digits, frequency counters, oscilloscopes, and scopemeters. The PC-1 generates a precision +5.000 volt DC output for checking digital voltmeters. The PC-1 also generates highly stable, temperature compensated 125 KHz and 1.000 MHz square wave outputs with fast (<3 nanosecond) rise time, exact 50% duty cycle, and precision zero and +5.000 volt levels. The 125 KHz output is ideal for checking oscilloscopes and scopemeters as it corresponds to 8 horizontal divisions at a time base of 1 microsecond/division. Both the 125 KHz and 1.000 MHz outputs can be used to check frequency counters.

SPECIFICATIONS

The following PC-1 specifications are guaranteed over a 3 year calibration cycle:

Output voltage: +5.000±.0005 Volt

Frequency:         125 KHz ±0.5 Hz

                          1.000 MHz ±3 Hz

Duty cycle:         50±0.1% @ 125 KHz

Environment:     20-30°C & 20-80% RH 

The calibration certificate supplied with the PC-1 lists the actual output values.  

OPERATION

The PC-1 requires a standard 9V battery that is installed in a battery compartment on the back on the unit. A battery status LED illuminates when the unit is on and battery power is OK. Expected battery operating life is approximately 8 hours.

CHECKING DIGITAL VOLTMETERS

Modern digital voltmeters (DVM) and multimeters use a single internal voltage reference for all measurements. You can use the PC-1 +5.000V output to quickly check the basic accuracy of this internal reference. Set the DVM for DC measurements (auto-ranging or 0-20 volt scale). Connect the PC-1 as shown below and observe the DVM reading after 30 seconds. Basic DC accuracy can be calculated as follows:

Error% = 100 x (DVM – 5)/5

You can compare the calculated error (in percent) with the specified accuracy for the DVM. If the error is significantly higher, the DVM is suspect and requires recalibration or repair.

PC-1 Connection to DVM

 

CHECKING FREQUENCY COUNTERS

Modern digital frequency counters and multimeters that include a frequency function use a single internal crystal oscillator for all frequency measurements. You can use the PC-1 125 KHz and 1.000 MHz outputs to quickly check the basic accuracy of this internal oscillator. Set the frequency counter for auto-ranging (or appropriate scale) and TTL/CMOS (zero to 5 volt) input levels. Connect the PC-1 as shown below and observe the reading after 30 seconds. Basic frequency counter accuracy can be calculated as follows:

Error% = 100 x (Counter – PC1_Freq)/PC1_Freq

Accuracy on some instruments is rated in parts per million (PPM). To get PPM, multiply the percent error by 10,000. You can compare the calculated error with the specified accuracy for the instrument. If the error is significantly higher, the instrument is suspect and requires recalibration or repair.

PC-1 Connection to Frequency Counter

 

CHECKING OSCILLOSCOPES

Older analog oscilloscopes use a variety of internal circuits to generate the horizontal time base and set the vertical gain. Since there is no single frequency or voltage reference, checks conducted with the PC-1 will only offer assurance that the instrument ranges actually being tested are accurate. Modern digital storage oscilloscopes (DSO) are based on fast analog-to-digital converters that use a single internal voltage reference. The time base for a typical DSO is computer generated based on a single crystal oscillator. Thus DSO checks conducted with the PC-1 will offer a much higher level of assurance that the instrument is still within calibration.

PC-1 oscilloscope checks include: vertical gain accuracy (voltage levels), horizontal time base accuracy, and rise time (related to bandwidth or frequency response). Initial checks should be conducted with the PC-1 directly connected to the oscilloscope inputs by means of the male-to-male BNC adapter as shown below. Do not connect the unit by means of a BNC cable or use an oscilloscope probe as these can significantly affect the frequency response. After you check each oscilloscope input by means of a direct connection, you can repeat the tests with the probes.

PC-1 Connection to DSO

CHECKING VERTICAL SCALE

Set the vertical scale to 1 volt/division (you can also check the 2 volt/division and 5 volt/division scales). Set automatic triggering. On each vertical scale you are testing, first set the input to ground and zero the vertical position at some known point on the screen. Then select DC coupling. Turn off any bandwidth limit.

Use the PC-1 +5.000V, 125 KHz, and 1.000 MHz outputs. For the 125 KHz and 1.000 MHz outputs, use an appropriate horizontal scale (time base) so that several cycles of the square wave are displayed on the screen. You can manually read the upper (5 volt) level or use an available cursor function.

Basic vertical accuracy can be calculated as follows:

Error% = 100 x (Scope – 5)/5

You can compare the calculated error (in percent) with the specified accuracy for the oscilloscope. If the error is significantly higher, the unit is suspect and requires recalibration or repair. You should repeat this step for each input.

CHECKING HORIZONTAL TIME BASE

Set the vertical scale to 1 volt/division, DC coupling and automatic triggering. Turn off any bandwidth limit. Set the horizontal scale (time base) to 1 microsecond/division.

Use the PC-1 125 KHz output. Adjust the trigger position so that the waveform is displayed as shown in Figure 5, with one square wave cycle covering 8 horizontal divisions with a precise 50% duty cycle. You can manually read the elapsed time in microseconds for one cycle or use an available cursor function.

125 KHz Display with DSO

Basic horizontal time base accuracy can be calculated as follows (where measurements are in microsecond units):

Error% = 100 x (Scope – 8)/8

Repeat the test with the horizontal scale set to 200 nanoseconds/division and using the PC-1 1.000 MHz output. Adjust the trigger position so that the waveform is displayed with one square wave cycle covering 5 horizontal divisions with a precise 50% duty cycle. You can manually read the elapsed time in nanoseconds for one cycle or use an available cursor function.

Basic horizontal time base accuracy can be calculated as follows (where measurements are in nanosecond units):

Error% = 100 x (Scope – 1000)/1000

If your oscilloscope has measurement functions, you can also check the accuracy of the duty cycle measurement. At 125 KHz output (for minimum jitter), the duty cycle should read 50±0.1%.

CHECKING BANDWIDTH

Set the vertical scale to 1 volt/division, DC coupling and automatic triggering. Turn off any bandwidth limit. Set the horizontal scale to the fastest available time base (typically 2 to 10 nanoseconds/division).

Use the PC-1 1.000 MHz output. Adjust the trigger position so that waveform is displayed as shown below. Rise time is measured from 10% to 90% of the transition (i.e. from 0.5 to 4.5 volt for a 5 volt square wave). You can manually read the rise time in nanoseconds or  use an available cursor or measurement function.

Rise Time Measurement with DSO

Oscilloscope bandwidth is defined as the point where a sine wave of a given frequency is displayed at half amplitude. This is also referred to as the -3dB point. Bandwidth (BW) is related to rise time (RT) as follows:

BW = .35 / RT    or   RT = .35 / BW

For example, an oscilloscope with 100 MHz bandwidth should have a rise time of 3.5 nanoseconds. When the signal source (the PC-1) also has a finite rise time, the observed rise time is the root mean square (RMS) of the individual rise times:  

Observed_RT = √ (Scope_RT² + PC1_RT²)    

If the PC-1 rise time is known, one can solve for the oscilloscope rise time and calculate the bandwidth as follows:

BW = .35 /(√ (Observed_RT² - PC1_RT²))   

The PC-1 rise time is approximately 3.0 nanoseconds (this is not a calibrated value).  Substituting and rewriting the equation to use observed rise time in nanoseconds and give a bandwidth result in MHz:

BW = 350 /(√ (Observed_RT² - 9))   

For the example in Figure 6, the calculated bandwidth (MHz) is:

53.5 = 350 /(√ (7.2² - 9))   

This method will give reasonable results for oscilloscopes with 100 MHz or lower rated bandwidth. Input amplifier peaking and DSO sampling algorithms may cause a false low rise time. If the observed rise time is below 5 nanoseconds, the calculated results  may be questionable.   

Download PC-1 User Instructions