The market trend in wireless telecommunications clearly is headed toward digital products. The advantages are significant: transmission quality, accessibility, and battery operating times are all improving.

These digital wireless telecommunications products transmit in short bursts and conserve power between transmissions, which improves battery operating life. Also, the products are operating at lower voltages, allowing for smaller batteries. As the transmit power remains the same, the current level increases with the lower voltage.

The high-volume production test of wireless products often dictates that test equipment, specifically the programmable power source, be remotely located from the test fixture. Cabling may be several meters in length. Connectors, contacts, and relays also may be required. As a result, the power supply path can have a few ohms resistance and microhenries of inductance which are not negligible.

These factors make it increasingly difficult to maintain a stable, transient-free voltage at the DUT. Most battery-powered products have low-battery voltage-shutdown circuits. If the resulting voltage drop at the DUT is large enough, the DUT may shut down and disrupt the test.

Local Sense Mode

Figure 1 shows two possible test-system configurations—local and remote voltage sensing. In the local sense mode (Figure 1a), the power supply regulates the output voltage at its output terminals. The voltage drop between the power supply and DUT is given by Equation 1:

Vd = RI + LdI/dt (1)

where: Vd = the voltage drop in the path

R = total path resistance

I = current

L = total cable inductance

t = time

V_load, illustrated in Figure 2, will occur at the load when the power supply is controlling the voltage at its output terminals, not at the load. If the path resistance is large enough, the transient voltage drop is sufficient to trigger the low-voltage detect circuitry in the product.

To illustrate, a typical setup may have 2-

W total cable and relay resistance, 2-µH cable inductance, and a DUT pulsed-current load stepping from 0.25 A to 1.25 A in 15 µs. The transient voltage drop at the DUT is 2.13 V, more than sufficient to trip the low-battery voltage-detect circuitry.

At the power source end of the cable, a small transient voltage occurs for each transition of the load. This transient voltage duration and amplitude are commonly provided transient-response specifications of the power supply. The transient response is a measure of how well the power source holds its output constant in response to load changes.

Most good system power sources have a transient response time of 50 to 100 µs. This time reflects how long it takes for the power supply to catch up with the change in load. The instantaneous transient-response peak amplitude at the output terminals is largely determined by the quality of the power supply’s output capacitor and directly proportional to the magnitude of the load change.

It is critical to determine the path impedance between the power supply and DUT, and the maximum acceptable transient voltage drop at the DUT. Generally, there should be no problem if the path impedance is below 0.1 to 0.2

W. If it is more than a few tenths of an ohm, extra steps must be taken to ensure acceptable performance of the test system.

Remote Sense Mode

To compensate for a significant path-impedance voltage drop, the power supply is used in the remote sense mode. As shown in Figure 1b, the sense leads of the power supply now are connected to the DUT to accurately control the voltage at the DUT.

In the remote sense mode, the power supply is a feedback control system. It senses the voltage at the DUT and adjusts the power supply output until the measured output voltage equals the desired output, compensating for any external voltage drop as well. The load and lead impedance become part of the voltage control system.

There is no problem controlling the DC voltage at the DUT since the control system maintains the average or DC voltage at the DUT constant. However, the accuracy to which the instantaneous voltage can be controlled for pulse loads depends on several factors:

The resistance and inductance of the cable connecting the power supply and DUT.

The input impedance of the DUT.

The amplitude and rise and fall times of the current pulse.

The power supply’s dynamic response characteristics.

While the average voltage may be ideal, the instantaneous value may not be. Most power supplies do not have sufficient control-loop bandwidth in the remote sense mode to respond to path-impedance voltage drops when testing remotely fixtured digital cellular phones. This bandwidth has been intentionally limited because of the need to stabilize the power supply for a wide variety of loads and path impedances. Even if a DC power supply has sufficient bandwidth, it typically still does not have the voltage slew-rate capability to raise the output voltage quickly enough to compensate for the path-impedance voltage drop.

By optimizing the general-purpose design to be load independent and to work for a variety of applications, power supplies do not have the necessary characteristics to work well in a digital-phone testing application when there is significant cable resistance. For fast rise-time current pulses, the transient voltage amplitude may not be much better than using local sensing, and the settling time may be much longer now that the path and load impedance are part of the feedback loop.

Alternate Solutions

Best Practice: Minimize Path Impedance

The most preferable solution minimizes the impedance between the power supply and the DUT. It is possible to accomplish this by:

Using larger gauge wire and reducing the distance between the power source and remote DUT.

Eliminating relays or using low-contact resistance relays.

Using twisted-wire pair to minimize inductance.

But this solution may not always be practical from an application point of view. It may be necessary to have additional resistance in the path due to connectors, contacts, and smaller relays for switching the power-supply output.

Filter at the DUT

A second solution places a large electrolytic capacitor across the terminals of the DUT. With the test conditions and power-supply configuration set up per the first remote sensing example, adding a 3,000-µF electrolytic capacitor limits the transient amplitude to 100 mV.

The voltage transient at the DUT is well within the minimum requirement due to the low impedance of the capacitor. However, there are drawbacks associated with this solution:

Electrolytic capacitors of this magnitude are large and may not be convenient to mount at the test fixture.

In many test applications for cellular phones, it is desirable to measure the phone standby and off-state current. In some cases, the leakage current of an electrolytic capacitor may be large compared to the phone off state or standby current. Also, the leakage current of the electrolytic is a function of time, voltage, and temperature.

A large capacitor at the DUT terminals will corrupt a pulse-current measurement because the pulse current will be supplied by the capacitor and not the power supply.

Power Source With Wide Bandwidth and High-Voltage Slew Rate

A third possible solution is to increase the bandwidth of the power-supply control loop to a minimum of 50 kHz and the voltage slew rate of the power supply to greater than 250 mV/µs. This required voltage slew rate is given by Equation 2 for a worst-case path resistance of 4

W and the current pulse characteristics as previously defined

Vs = RdI/dt+Ld2I/dt2 (2)

Figure 3 illustrates the results of a power source with these dynamic response characteristics. It now responds to the speed of the load change and compensates for the path-impedance voltage drop. The transient voltage at the DUT is reduced to well within acceptable limits. It is the dynamic response characteristics of the power source that ultimately determine what the remotely sensed transient response characteristics are for this situation.

Increasing the bandwidth of the power source cannot maintain stability for as wide a variety of loads. The path impedance and the DUT impedance now significantly influence the power-supply control loop. The power source no longer is load independent. It now is optimized for a particular application, limited by a range of load characteristics that it is designed to accommodate.

Special Considerations for Higher Wiring Inductance and Current

In all of these examples, the wiring inductive voltage drop has been considered to be small compared to the wiring resistive voltage drop. When the wiring is not twisted or is in excess of 2 meters, the inductance can be significant. The problem is compounded with the trend toward lower battery voltage and higher current as the current rise time remains the same.

In the typical setup, it was assumed that the load had a linear rise time of 15 µs. For digital wireless products, the rising portion of the load current is more accurately modeled as an exponential as shown in Equation 3:

I(t)= I_o (1- Î ^-t/t ) (3)

When the capacitance of the DUT is zero, the voltage drop in the cable is found by substituting Equation 3 into Equation 1. The resulting voltage drop now is given by Equation 4:

V_d = RI_o(1- Î ^-t/t )+ L(I_o/t )Î ^-t/t (4)

where: R = the cable resistance

L = inductance

I_o = final value of the current

t

= ½ the current rise time

Vd = cable voltage drop

An initial voltage drop in the cable, equal to (LI_o)/( t ), occurs in zero time even, though the current has a finite rise time. This is due to the cable inductance. With lower-voltage wireless products drawing higher current, this effect is very significant. For example, for a 15-µs rise time, 4 µH of cable inductance, and current pulse amplitude of 3 A, the inductive effect alone gives a 1.5-V drop in the cable. To compensate for the zero-rise-time inductive voltage drop in the cable, the power supply must have infinite bandwidth and slew rate, which are not possible to achieve.

In practice, cellular phones do have input capacitance, typically 5 µF or greater. The resulting inductive wiring voltage drop occurs in as little as 10 µs. Another consideration with the trend to lower voltage is the transient-voltage drop magnitude. Where a 300-mV drop is adequate for 6- to 7-V products, a150-mV drop is desirable for the latest- generation 3- to 4-V designs.

These factors dictate further measures. The optimized power-supply bandwidth should be 100 kHz or better. Alternatively, you can add a 20-µF low-leakage film capacitor to slow the transient voltage drop to 20 to 30 µs, negating the effect of wiring inductance.

Summary

Several solutions have been proposed to address the problem of power-source voltage transients resulting from testing remotely fixtured digital wireless products that draw currents in pulses. They are summarized in Table 1.

You need to understand the more subtle interactions of the power source in the test system and how various characteristics of the power source impact these voltage transients. By knowing the source and nature of these transient voltages and alternate solutions available, you can effectively plan the design of the test system and circumvent problems at the outset.

About the Authors

Ed Brorein is a business development engineer for HP’s Power Products Division, where he has held a variety of engineering positions in R&D, manufacturing, and marketing for the past 19 years. Mr. Brorein holds a B.S.E.E. from Villanova University and an M.S.E.E. from the New Jersey Institute of Technology.

Jim Gallo’s career at HP spans nearly 29 years. During that time, he has held several engineering and manufacturing positions. Currently, he is an engineer/scientist at the company’s Power Products Division. Mr. Gallo received a B.S.E.E. from the New Jersey Institute of Technology and an M.S.A.E. from Massachusetts Institute of Technology.

Hewlett-Packard, (800) 452-4844.

 

 

Solution

 

Requirement

 

Benefit

 

Liability

 

Minimize Path Impedance

Use heavy-gauge twisted-wire pair

Minimize distance

Eliminate relays and connectors

Uses standard power supply

Not always practical to achieve very low resistance needed

Add Filter at Load

Mount 3,000-µF capacitor at fixture

Very effective at lowering voltage transients

Uses standard power supply

Cannot measure pulse or microamp currents

Large capacitor difficult to mount

Power Source Optimized for Application

Power source with high band-width and voltage slew rate

Compensates for large values of path impedance

Can add relays or components in path

Power source stable with a limited range of load impedance

 

Table 1.

Copyright 1998 Nelson Publishing Inc.

September 1998