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Potential sources of
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When measuring impedance with a Polar CITS or any other TDR there are a number of sources of potential measurement error. This application note sets out to explain those error sources and how you can minimise them in order to achieve repeatable and accurate measurements
Which probe I should choose for most accurate measurements?
TDRs used for impedance measurement are precision RF measurement tools and need to be regularly calibrated. Reference air line standards are used for TDR calibration and are available in a variety of standard impedances. Precision air lines are a high cost item and an acceptable alternative for less critical applications is a set of precision semi rigid cables calibrated against air lines.
Air lines are made traceable by a precision metrology technique called air gauging which allows the internal bore of the air line to be measured and the impedance calculated using a standard formula.
Resting your hand or fingers on a test coupon will cause the impedance of surface structures to drop. Ensure operators are aware of this effect. - Its easy to demonstrate, simply make multiple tests on a surface trace and see how placing your fingers on the board surface alters the measurement.
Secrets of TDR Calibration
Here is some information previously only know by TDR specialists.. A TDRs calibration will only be correct if it is calibrated with the same DC conditions at the end of the test cable as those occurring when you measure.
Simply stated if you calibrate a TDR
using a precision broadband resistor as a standard you should also
terminate your test sample (coupon) with the same resistance.
As it is inconvenient to fit a terminating resistor to a test coupon always calibrate a TDR used for coupon testing with an air line.
The error between the two methods can be as much as 3 or 4 ohms.
The reason for this effect is that TDR sampling heads are precision fast switching diode bridges and are sensitive to temperature change. A change in DC conditions at the test sample will alter the DC output voltage of the pulser.
Worn Cables and probes.
RF Cables and probes have a finite life and should be regarded as consumable, with 24 / 7 operation they will in time become resistive and ultimately fail. Cable life can be extended by careful handling, no tight bends etc.
Avoid falling into the trap of one PCB fabricator whose well meaning operator intentionally used worn probes to help a batch of boards pass that were out of spec!
Polar RITS automated impedance test systems incorporates software and on board calibration systems that detect cable and probe wear and alert operators of maintenance requirements
All TDRs are susceptible to measurement drift
over time and with temperature range. Ask if your TDR makes compensation
automatically for this? If you use a Polar CITS or RITS internal
stable standards are incorporated and the units recalibrate at regular
intervals. For best performance check your config and options are set to
recalibrate every 10 minutes.
|Are mobile phones influencing your measurement|
Mobile phones are RF
transmitters, they regularly "sign on" to the nearest base
station - even when you are not making a call.
Test coupons are around the same length as a mobile antenna and themselves will receive the signal from your phone. This is especially the case with surface structures like microstrip.
Ensure phones are switched off in the test area.
the work surface?
Yes even the work surface can influence measurement results. The work surface has its own dielectric constant and a microstrip on the bottom of the test coupon or PCB will measure low if it is too close to the table surface. Two ways to resolve this - place the coupon in a test fixture or support jig. Or turn the coupon over so the bottom layer faces up.
Provide a reliable way of verifying PCB impedance, and will offer you more reliable repeatable measurements than attempting to measure the board itself. The advantages of using test coupons are described in detail in AP124
Important note: If your striplines are reading high - did you remember to short the planes Vcc and Gnd on the coupon? (NOT on the board!) Vcc and grounds need to be shorted on the coupon especially on narrow coupons to simulate actual RF conditions on the finished board.
Tip: If increasing the height of the top plane to 10 x its normal height on the CITS25 gives you a reading of impedance similar to your measured value you have probably not shorted the planes on the coupon.
Further information on measuring PCB controlled impedances is available by email from firstname.lastname@example.org.
Polar Instruments Ltd
Park, Guernsey, GY2 4AF, UK
Tel: +44 1481 253081 Fax: +44 1481 252476 © Polar Instruments 2000
|© Polar Instruments 2000. Polar Instruments pursues a policy of continuous improvement. The specifications in this document may therefore be changed without notice. All trademarks recognised.|