power-controlled amps the voltage-sensing (fig. 1a) and current-sensing (fig. 1b) methods are often used for pa output-power con- trol. in the voltage-sensing method, a high-speed control loop is incorpo- rated to regulate the collector voltage of the amplifier while the pa stages are held at a constant bias. by regulating the power, the stages are held in sat- uration across all power levels. as the required output power is decreased from full power down to 0 dbm, the collector voltage is also decreased. the current-sensing method senses the cur- rent supplied to the pa through the power supply. a complementary-metal- oxide-semiconductor (cmos) controller controls the base voltage of the field- effect transistor (fet) with an error voltage generated by applying the ramp voltage, from the dac, and the cur- rent measured to an error amplifier. these are both indirect closed loop methods as there is no direct mea- surement of the pa? out- put power. linear power control of gsm amplifier power this technique of controlling pa module output power has advantages in dynam- ic range and accuracy compared to tra- ditional current-sensing and voltage- sensing power-control methods. ower-control methods for integrated global system for mobile communications (gsm) power-amplifier (pa) mod- ules are many. new methods include approaches based on sensing current and sensing voltage. but the best perfor- mance can be achieved with a linear-in-db technique that provides an accurate and predictable method of controlling pa output power. in comparing these different approaches, the usual measures include output power and power-added efficiency (pae). but other areas to consider include the pa? output-power stability as functions of temperature, frequency, load vswr, and battery power; the dynamic range of the power-ramping function; the ease of calibration; trade-offs between pae/bat- tery current and output power; and the impact of the power-control circuitry on efficiency. p philip sher applications engineer analog devices, inc., 804 woburn st., wilmington, ma 01887-3462; (781) 937-2815, fax: (781) 937-1026, e-mail: phillip.sher@analog.com, internet: www.analog.com. microwaves & rf 51 july 2003 design visit planetee.com 1. these simple diagrams illustrate the voltage-sensing (a) and current-sensing (b) methods of pa output-power control. current sense (b) v supply v ramp rf in rf out voltage sense (a) v supply current sense resistor v ramp power control rf in rf out mwjuly03_051.ps 7/8/03 1:20 pm page 51
| power-controlled amps | figure 2 shows a simplified block diagram of a more accurate way to control power by directly detecting the rf power from the amplifier. the mod- ule comprises a pa, a silicon power controller, and a number of passive components. the pa output power is controlled by adjusting the bias on the bases of the power transistors. the out- put power is regulated by a classic auto- matic-gain-control (agc) loop. this measures the actual output power of the pa by coupling off a small proportion of the output power using a direction- al coupler. the sensed power level is applied to a logarithmic amplifier (logamp). the logamp measures the power and compares it to a set point, v set . if there is a difference between the measured power and the correspond- ing v set , an error amplifier adjusts the voltage to the bias controller, v apc . neither the current-sensing method nor the voltage-sensing technique have any feedback from the output of the pa. thus, power control is nonlinear in both cases (see eqs. 1 and 2). the out- put-power control function in both methods has a nonconstant slope and smaller dynamic range, making ramp- ing to low power levels difficult. see eq. 1 in box below see eq. 2 in box below figure 3 shows that the output power for the rf power-detection method is linear-in-db over all gsm power levels. this relationship is stable over temper- ature, and within each frequency band. the part-to-part transfer functions of the dac that drives the controller, and the logarithmic detector both vary, howev- er, so it is necessary to calibrate the cir- cuit to obtain precise output power. the straightforward linear-in-db relationship between the output power and v set , allows for a single two-point calibration in each frequency band. the procedure simply involves applying a v set that results in an output-power level close to full specified power (e.g., +33 dbm for gsm). once achieved, the digital-to-ana- log converter (dac) code is noted for that power level, which is denoted p high . then, a v set is applied that results in an output power close to the minimum out- put-power level for that standard (e.g., +5 dbm for gsm). similarly, the dac code is noted for this power level, which is denoted p low . with these four data points, the power output versus v set can be calculated. this translates into a sim- ple two-point calibration and straight-line approximation. see eq. 3a in box below see eq. 3b in box below once the slope and the intercept are known, the required code for any trans- microwaves & rf 52 july 2003 design pmvv p dbm set loop threshold off =? () + log ( ) 1 p vv r dbm ramp sat l = ? () ? ? ? ? ? ? ? ? ? 10 2 810 2 2 3 log ( ) slope db code pp code code a high low high low (/ ) () = ? ? 3 intercept code p slope code b high high () =? () () 3 code p intercept slope out = ? () () 4 10 1 10 1 1 20 1 1 7 2 log log log ( ) ? () ? + ? = + ? l l ll 2. in the rf power-detection method of pa output-power control, the output power versus control voltage (v set ) transfer function is linear-in-db over a 40-db dynamic range. l +? r flt c flt 30 db v set input match bias control v bat gsm pa v apc pin gsm pout gsm output match mwjuly03_052.ps 7/8/03 1:20 pm page 52
| power-controlled amps | mit power level can be calculated using the formula: see eq. 4 on p. 52 the rf power-detection method suffers only ? db error for an output- power range spanning +5 to +34.5 dbm, while the current-sensing and voltage- sensing methods exhibit higher errors for narrower power levels. the rf power-detection approach uses an internal 30-db coupler. because of this low coupling factor, the inser- tion loss of this coupler is extremely low (approximately 0.05 db), and has a minimal impact on pa efficiency. this impact can be described by a loss fac- tor of 10 ?.05/10 . the pae of the ampli- fier module is then the pae of the ampli- fier integrated circuit (ic) multiplied by this loss factor. it should be noted that all pae specifications for the rf power-detection approach include the effect of directional coupler insertion loss. the fet used in the voltage-sensing approach has a voltage drop of about 180 mv at full power. this will also reduce the efficiency of pa chip inside the module. this loss in efficiency is approximately: this loss factor is similar to the rf power-detection approach at nominal supply voltages. however, it is worse loss factor v v bat bat = ? () 018 5 . () at low supply voltages. amplifier mod- ule pae specifications for the voltage- sensing approach also include the pae loss due to the fet. the log detector controller has a control function that is linear when scaled in db/v. to achieve the desired raised-cosine rf power profile from the pa, the ramping signal from the ramp dac should also follow a raised- cosine form. during initialization and comple- tion of the transmit sequence, the pa bias voltage should be held at its min- imum level by keeping the external con- trol voltage at some level below 150 mv (this is generally achieved by setting the ramp dac code to 0). the pa has a clamping mechanism designed to keep the pa off, with high isolation, when the v set voltage is below 150 mv. to optimize switching transients a step is applied to the ramp (fig. 4) . the step is used because there is no point in ramping from 0 to 200 mv because the pa is designed to stay off for this voltage range. when ramping to lower power lev- els, the same initial offset voltage should be applied before ramping begins. the raised-cosine portion of the ramp should be scaled to set the desired power level. the ramp-down profile can be a sim- ple mirror image of the ramp-up sig- nal (i.e., the same codes can be used). alternatively, the ramp dac signal can be a simple raised-cosine signal that falls all the way to 0 v. this is not true for the voltage-sensing and current- sensing control methods. these meth- ods require more than one ramp pro- file, especially at low power levels, in order to achieve good ramping and switching transients. a filter (c flt and r flt ) must be used to stabilize the loop and ensure opti- mum conformance to the time mask and switching transient specifications (as shown in fig. 2). the choice of c flt and r flt will depend to a large degree on the gain-control dynamics of the pa. the optimum values for the con- trol loop have been determined to be 220 pf for the capacitor and 3 k ? for the series resistor for gsm and 4.3 k ? and 150 pf for the resistor and capacitor, respectively, in dcs/pcs systems. this gives the loop sufficient speed to follow the required ramping profiles, while still meeting the switching transient requirements at all power levels. depend- ing on the board layout and choice of transmit components, these values may have to be adjusted slightly. general- ly, meeting these requirements is most difficult at full power. the specific-absorption rate (sar) is an indication of the amount of radi- ation that is absorbed into the body (usually the head) when using a cellu- lar telephone. due to strict sar regu- lations, it is important to accurately control the output power of the pa. a large-impedance mismatch, e.g. 10:1, can occur at the antenna. however, due to losses in the switchplexer placed microwaves & rf 54 july 2003 design 3. linear-in-db power control allows for easy two-point calibration, whereas other control methods require more ramping profiles at lower power levels. +40 +30 +20 +10 0 ? 10 ? 20 ? 30 0 0.4 0.8 1.2 1.6 2.0 v set /v ramp ? v output power ? dbm voltage sense current sense linear in db 4. the filtered rising edge of ramp dac output signals (v set drive signals) used for high-power ramping curves are based on 4 interpolation). mwjuly03_054.ps 7/8/03 1:21 pm page 54
isolators model freq isol insertion vswr outline price # range ghz min loss max max # per unit d3i0890 .8-.9 20 .40 1.25 8 $235.00 d3i0116 1.4-1.6 20 .40 1.25 8 $235.00 d3i0118 1.6-1.8 20 .40 1.25 3 $210.00 d3i0120 1.7-2.0 20 .40 1.25 3 $210.00 d3i0223 2.0-2.3 20 .40 1.25 3 $210.00 d3i2040 2.0-4.0 18 .50 1.30 1 $215.00 d3i2060 2.0-6.0 14 .80 1.50 1 $250.00 d3i2080 2.0-8.0 10 1.50 2.00 1 $395.00 d3i3060 3.0-6.0 19 .40 1.30 2 $195.00 d3i4080 4.0-8.0 20 .40 1.25 3 $185.00 d3i6012 6.0-12.4 17 .60 1.35 6 $195.00 dmi6018 6.0-18.0 14 1.00 1.50 11 $275.00 d3i7011 7.0-11.0 20 .40 1.25 4 $185.00 d3i7012 7.0-12.0 20 .40 1.25 4 $205.00 d3i7018 7.0-18.0 15 1.00 1.50 5 $225.00 d3i8012 8.0-12.4 20 .40 1.25 4 $180.00 d3i8016 8.0-16.0 17 .60 1.35 5 $205.00 d3i8020 8.0-20.0 15 1.00 1.45 5 $230.00 d3i1020 10.0-20.0 16 .70 1.40 5 $220.00 d3i1218 12.0-18.0 20 .50 1.25 5 $180.00 d3i1826 18.0-26.5 18 .80 1.40 5 $225.00 d3i1840 18.0-40.0 10 2.00 2.00 5* $1300.00 d3i2004 20.0-40.0 12 1.50 1.65 5* $950.00 d3i2640 26.5-40.0 14 1.00 1.50 5* $700.00 circulators model freq isol insertion vswr outline price # range ghz min loss max max # per unit d3c0890 .8-.9 20 .40 1.25 8 $235.00 d3c0116 1.4-1.6 20 .40 1.25 8 $235.00 d3c0118 1.6-1.8 20 .40 1.25 3 $210.00 d3c0120 1.7-2.0 20 .40 1.25 3 $210.00 d3c0223 2.0-2.3 20 .40 1.25 3 $210.00 d3c2040 2.0-4.0 18 .50 1.30 1 $215.00 d3c2060 2.0-6.0 14 .80 1.50 1 $250.00 d3c2080 2.0-8.0 10 1.50 2.00 1 $395.00 d3c3060 3.0-6.0 19 .40 1.30 2 $195.00 d3c4080 4.0-8.0 20 .40 1.25 3 $185.00 d3c6012 6.0-12.4 17 .60 1.35 6 $195.00 dmc6018 6.0-18.0 14 1.00 1.50 11 $275.00 d3c7011 7.0-11.0 20 .40 1.25 4 $185.00 d3c7018 7.0-18.0 15 1.00 1.50 5 $225.00 d3c8016 8.0-16.0 17 .60 1.35 5 $205.00 d3c8020 8.0-20.0 15 1.00 1.45 5 $230.00 d3c1218 12.0-18.0 20 .50 1.25 5 $180.00 d3c1826 18.0-26.5 18 .80 1.40 5 $225.00 d3c1840 18.0-40.0 10 2.00 2.00 5* $1750.00 d3c2004 20.0-40.0 12 1.50 1.65 5* $1350.00 d3c2640 26.5-40.0 14 1.00 1.50 5* $900.00 5114 e. clinton way. #101 fresno, ca 93727 tel: 559-255-7044 fax: 559-255-1667 email: sales@ditom.com internet: www.ditom.com buy online 45 products can be bought online with credit card. delivery within 24hrs aro. ditom stocks over 25 units of each device at all times. units over 26.5 ghz come with k-female the leader in broadband and high frequency isolators and circulators outline # a b c d e f g h j 1 1.58 1.62 0.70 0.25 0.25 1.265 0.10 1.380 0.690 2 1.25 1.25 0.70 0.25 0.25 0.900 0.10 1.050 0.525 3 1.00 1.00 0.50 0.25 0.25 0.675 0.10 0.800 0.400 4 0.86 0.98 0.50 0.25 0.25 0.625 0.10 0.660 0.330 5 0.50 0.70 0.50 0.25 0.18 0.455 0.08 0.340 0.170 6 0.62 0.78 0.50 0.25 0.25 0.425 0.10 0.420 0.210 8 1.25 1.25 0.72 0.26 0.26 0.900 0.10 1.050 0.525 11*** 0.50 0.58 0.38 0.19 0.19 0.10 0.300 inches 0.17 ref only tolerance: xx+-0.03 xxx+-0.010 isolator circulator termination sma-f, * k(f) 2-56 x 0.15 dp 3 places (tapped holes, both sides) *** 0-80 x 0.80 dp 2 places cl between the antenna and the pa, the vswr at the pa will be reduced, but still can be as high as 6:1. figure 5 shows the variation in output power due to a vswr of 6:1 for all phase angles. the baseline output power level (i.e., a vswr of 1:1) is +30 dbm. as expect- ed, the baseline output power level drops due to the increased vswr. as the phase changes, the output power of the indirect closed-loop approach varies by well over 3 db. in contrast, the 14- db coupler directivity of the rf power- detection approach reduces this varia- tion to 1.3 db. this translates to a 14-db reduction in the reflected power presented to the pa. the indirect closed-loop approach operates open loop, and the output power will vary directly with the vswr, yielding an error of: the indirect closed-loop approach will have greater error in its transmit- ted output by a factor of: see eq. 7 on p. 52 the error created by the rf power- detection approach will always result in a lower transmitted power, and thus will error l l = + ? 10 1 1 6log ( ) microwaves & rf 56 july 2003 design 5. for a vswr of 6:1, the output power of the indirect closed-loop approach varies by more than 3 db with phase. the high-directivity directional coupler used in the rf power-detection method ensures a worst-case variation of 1.3 db. 6. these curves show output power versus v set for pcs (static) at +3.5 vdc. phase?deg. linear in db vswr = 6:1 indirect closed loop vswr = 6:1 pout vswr = 1:1 output power ? dbm 31 30 29 28 27 26 25 32 24 ? 190 ? 130 ? 70 ? 10 50 110 170 output power at ? 30 o c error at +85 o c error at +25 o c error at ? 30 o c output power at +85 o c output power at +25 o c output power ? dbm 40 30 20 10 0 ? 10 ? 20 ? 30 4 3 2 1 0 ? 1 ? 2 ? 3 0 0.5 1.0 1.5 2.0 normal operation v set ? v error ? db mwjuly03_056.ps 7/8/03 1:21 pm page 56
never exceed maximum safety limits. the accuracy of the coupler and the detector in the rf power-detection method allows for precise power con- trol. this error of control is within 1 db. the required peak power out at the antenna for gsm 900 is +33 dbm. the specification allows a 2-db mar- gin. the rf power-detection approach allows cellular telephones to be oper- ated at a lower power level of +32 dbm and still meet specification, in the pro- cess saving battery life. when the battery voltage decreases, the pa output power generally decreas- es. in the rf power-detection method, however, the pa output power will not decrease until the battery voltage drops below +2.9 v. this is because the closed loop senses the decreasing output power and drives the bias circuit (v apc ) hard- er, thereby keeping the output power constant. in the indirect closed-loop approach at high battery voltages, the output power is well regulated. however, when the battery voltage to the pa decreas- es, the pa has trouble delivering the requisite output power. in addition, to avoid excessive switching transients at high power levels, v ramp must be lim- ited according to the equation: as battery voltage decreases, the output power of the indirect closed- loop approach at high power also decreases. assuming that a v ramp of 1.6 v is required to achieve full power, this limits the minimum battery voltage vv ramp batt ? + 3 8 018 8 .() 58 design the accuracy of the coupler and the detector in the rf power-detection method allows for precise power control. mwjuly03_058.ps 7/8/03 1:21 pm page 58
have a question? need to connect? if you want to interface with other professionals in the high- speed design industry or discuss topics featured in microwaves & rf , check out planetee? forums section! if you need to know, ask those who do! go to: www.planetee.com/forums microwaves & rf 60 july 2003 design to +3.25 vdc. as the battery voltage drops below this level, v ramp must also be reduced. in the case of the rf power-detection approach, the battery voltage can drop to +3 vdc before v set must be adjusted to prevent exces- sive switching transients. in practice, this adjustment will not be necessary as most cellular telephones are turned off at around +3 vdc. as temperature varies, so does pa out- put power. the detector in the rf power-detection method detects this power change and adjusts the biasing to the pa to correct it. this method has good performance over temperatures from ?0 to +85?. figure 6 shows that the error is within ? db and shows good linearity over all power levels within the normal output-power oper- ating range. the indirect closed-loop approach has higher variation at lower temperatures, and requires more than one ramp profile. this increases pro- duction test time and thus final cost. the linear-in-db method described in this article is used in the models ADL5551 and adl5552 6 8-mm gsm quad-band x-pa pa modules from analog devices. the inventions used in these advanced products are protected with patents and other intel- lectual property rights, including unit- ed states patents nos. 4,990,803, 4,929,909, 4,604,532, 5,572,166, 6,144,244, 6,172,549, 6,525,601, 6,489,849, and corresponding patents in other countries. mrf as battery voltage decreases, the output power of the indirect closed-loop approach at high power also decreases. mwjuly03_060.ps 7/8/03 1:22 pm page 60
|
