Crescend Technologies 7VB916-0140 800VC015-0R2RF User Manual

Crescend Technologies LLC 800VC015-0R2RF Users Manual

Users Manual

VOCOM ProductsRF AmplifiersInstruction ManualFor Model800VC28-015-XXRRF Power AmplifiersTHE FOLLOWING INSTRUCTIONS ARE FOR USE BY QUALIFIEDRF SERVICE PERSONNEL ONLY. SERVICE PERFORMED ONVOCOM’S AMPLIFIERS MAY VOID THE MANUFACTURER’S WARRANTYVOCOM Products Company, L.L.C.2255 Lois DriveSuite 1Rolling Meadows, IL 60008Quality since 1979
TABLE OF CONTENTS800VC28-015-XXRRF POWER AMPLIFIERSECTION 1.0 OPERATING INSTRUCTIONSSECTION 2.0 SPECIFICATIONSSECTION 3.0 TESTSECTION 4.0 PROPER AMPLIFIER SETUPSECTION 5.0 APPLICATION NOTE
SECTION 1: OPERATING INSTRUCTIONSGENERAL:To effectively use the 800VC28-015-XXR RF power amplifier, the operation capabilitiesof the unit must be known. This section describes input/output connections andenvironmental operating conditions. Understand the operating of limitations of theamplifier before installation is attempted.OPERATING VOLTAGES:Warning: This unit is designed for operation from 28 volts DC negative groundsystems. The chassis must be kept at earth potential. The negative(black) DC power lead is connected directly to the chassis. Operationin positive ground systems may cause injury.All power measurements and specifications are met at 28 volts DC as measured at theDC input terminals. For best performance, a regulated power supply is recommended.Operating voltage range is from 24-28 volts. Power will be reduced slightly at DCvoltages below 28 volts. Operation a 28 volts or higher is to be avoided as this willsignificantly reduce transistor life.POWER CORD IDENTIFICATION:Two cables extend from the amplifier. The RED lead is to be connected to thePOSITIVE power supply terminal. The BLACK lead is to be connected to the NEGATIVEpower supply terminal and earth ground. Use caution not to reverse the DC polarity anddo not connect to an AC power line.OPERATING TEMPERATURE:The amplifier may be operated when the ambient air is between O and 50 degreescentigrade. If operated in a closed cabinet, verify that the air temperature flowing overthe heat sink is in this range. The amplifier/power supply combination may generateover 1500 watts of heat. The amplifier is cooled by convection flow across the heat sinkfins of the unit. Maintain a minimum of 4 inches clearance. A thermal shut-off circuit isprovided that will interrupt the DC power input to the first RF stage should the heat sinkrise above 60 degrees C. Operation in confined areas may cause the unit to shut-offthermally. Installations where thermal cut-off occurs frequently should be avoided. Theunit may be stored in temperatures between -20 and + 70 degrees centigrade.Keep the fan intakes free of dust.TROUBLE SHOOTING:
LOW POWER OUTPUT:The most common amplifier problem is low power output. If the installation test shown inSection 3 is run the power input/output specification on the nameplate should be met,especially when working into a dummy load, since this is essentially the test run at thefactory.If working with BIRD wattmeters, remember that the power output reading is specified at+/-5% of full scale. In other words, a wattmeter with a 100w element gives readings of+/-5 watts when the wattmeter is reading full scale. Mid-scale readings have muchlower accuracy. Always use an element closest to full scale for maximum accuracy.Treat your element with extreme care. Dirty or dropped elements always read low.Make sure that your power supply is adjusted to 28 volts. The input/output powersspecified on the nameplate were taken at 28 volts. Measure the voltage at the amplifierterminals to be sure all the DC connections are tight and that the amplifier is seeing 28volts during operation. Check the manufacturer's ratings to be sure the CONTINUOUSrating of the power supply is sufficient to power the amplifier. If the power supply isslumping, the amplifier output will go down with the voltage.Look at the drive power while the amplifier is operating. Many exciters will put outdifferent powers into even slightly different loads. Low drive power will produce lowoutput power. Do not assume that your exciter output power with the exciter runninginto a dummy load will be the same as when the exciter is running the amplifier. Astrong RF field can sometimes cause an exciters to increase or decrease power as wellas go spurious. In-band spurious signals maybe amplified. The change in signal levelwill show up on the input wattmeter. RF fields can also cause linear power supplies tofall out of regulation. This should be seen on a voltmeter (CAUTION: RF can effectdigital voltmeters, too.)Low (or high) output powers can be the result of a high output SWR. Carefully checkyour loads and connecting cables.NO OUTPUT POWER:After checking the above items, be aware that there is protection circuitry. Check withthe factory if you suspect that the internal protection has or is operating. As a rule, if thefront panel light and fan are operating and output is zero, the SWR circuit has operated(usually with an audible click upon key-up.) If the SWR protection were operating, youwould notice the output wattmeter "jump" briefly upon key up and then return to zero.If the front panel light does not light, either the input drive isn't high enough to trigger theamplifier on or the amplifier has disconnected itself from DC power. The amplifier willreset itself when it cools down. (Some power supplies will "spring up" in voltage when aheavy load is released and settle down to normal after 100-200 milliseconds. This candamage an amplifier.)If all else fails, give us a call. We'll be glad to help!
SECTION 2: SPECIFICATIONSWARNING: Do not exceed 15 watts output. Do not exceed nameplate drive power by more than 5%. Do not exceed 28.0 volts while operating.FREQUENCY: 840-960 MHz range. Factory set to specific frequency on ID label.FREQUENCY RANGE: May be operated within + or - 10 MHz of the factory set frequency without tuning.INPUT DRIVE POWER: As specified on amplifier ID nameplate. This is the power measured during final inspection that was required to reach 15 watts output.MAXIMUM INPUT DRIVE POWER: Nameplate input drive power +5%MAXIMUM TEST OUTPUT POWER: 18 watts (Pulsed output period not to exceed 100 mS)MAXIMUM RATED OUTPUT POWER: Not to exceed 15 watts.MAXIMUM RATED LOAD VSWR FOR 1.2:1 (<5% reflected power)CONTINUOUS DUTY OPERATIONMAXIMUM VSWR RATING Infinite, all phase angles. (1 hour test time at 12V, noWITHOUT DAMAGE degradation in output power upon return to normal load and temperature). VSWR or thermal protection may operate above 1.5:1 VSWR.OUTPUT VSWR PROTECTION Inherent.THERMAL PROTECTION 60 degrees C + /- 7 degrees C heat sink temperature. Shut off; reset upon return to normal temperature.INPUT/OUTPUT CONNECTORS Type N FEMALE.SPURIOUS OUTPUTS < -75 dBc.HARMONICS - 65 dBc or below.COOLING DC fan cooled controlled by amplifier.
Section 3.0 TEST PROCEDURE 28 VOLT AMPLIFIER1. BLOCK DIAGRAM:2 EQUIPMENT NEEDED:1. Exciter2. Watt meters (2) with slugs for full scale readings3. Power Supply (28 VOLT)4. Dummy Load5. Short coaxial cables (RG-8 or better) with appropriate connectors or adapters.6. DC ammeter7. DC voltmeter3. INSTRUCTIONS:1. Mount equipment in permanent location. Operate the amplifier at 24 volts into adummy load for initial tests.2. The amplifier is of fixed gain design. In order to obtain a given output power (e.g.,exactly 100 watts) the input or drive power must be adjusted. The factory test input /output powers are shown on the serial number plate for your particular amplifier.3. You must measure input power, output power, DC voltage, and DC current draw andverify that all are within specifications. If you do not measure one of theseparameters, you have not done a proper installation.4. After proper operation into the dummy load is assured, connect the amplifier to theinstallation load (antenna, combiner, etc.) The output power and DC current drawshould not change significantly. If it does, a load SWR problem exists and must becorrected.Remember the specification output power is obtained only at 28 volts. Initial tests at 24volts will result in lower output powers than at 28 volts. A reduction in drive power fromthe value set at 24 volts will be required when the voltage is increased. Otherwise, higherthan rated output power will result.TRANSMITTER(EXCITER) WATTMETER WATTMETERAMPLIFIERBEING TESTED DUMMYLOAD+ -28 VOLT POWERSUPPLY. 24 VOLTFOR INITIAL TEST.
Section 4: Proper Amplifier Set-UpThe life of an amplifier is dependent on many factors, but probably the most important isinitial set-up. Let's look at some typical cases demonstrating amplifier life. Theseexamples are based on the VOCOM‘s 800 MHz 140 watt amplifier.Case 1: Run it "wide open."Power Input: 200 mWattsPower Output: 185 wattsCurrent Draw: 62A at 13.8 voltsPower consumed 62 X 13.8 = 855.6Power output = 185.0Power dissipated as heat 670.6 wattsAssuming a junction to case thermal resistance of 1.16 degree C per watt, and 7transistors in this design: 1.16 X 670.6/7 + 60 = 171.1 degree C junction temperature.The 60 in the above equation is an assumed heat sink temperature. Motorola haspublished data on MTBF (see for example, AN-790) For the MRF 847 used in thisamplifier this equates to an MTBF of a little over 10 years. Converting MTBF to %failures per year this equates to a 10% Failure rate per year PER DEVICE. In anamplifier with seven devices, Motorola predicts a 70% failure rate per year if theamplifier is operated at this point.Case 2: Recommended operating pointPower Input: 200 mWattsPower Output: 140 wattsCurrent Draw: 40 A at 13.8 voltsPower consumed: 40 X 13.8 = 552Power output = 150Heat dissipated = 402 wattsFollowing a similar calculation, the MTBF per device is 2000 years. In an amplifier with 7devices the 5 % failure rate per year would be only 0.35%. The extra 35 watts is verycostly in terms of amplifier lifetime.Conclusion: Set up the amplifier carefully. Measure the current draw and adjust thedrive for the best efficiency. At VOCOM, we carefully choose our power levels at eachstage of amplification to obtain total failure rates below 0.5% per year even in amplifierswith MRF transistors. But it's up to you to set them up properly.
SECTION 5: APPLICATION NOTEThere are a number of low power fixed output exciters on the market and it has come toour attention that may users are reducing the output power of these exciters by de-tuning them.NEVER REDUCE THE POWER INPUT TO THE AMPLIFIER BY DETUNING ANYSTAGE OF EITHER THE EXCITER OR THE AMPLIFIER!De-tuning the exciter will almost certainly cause it to produce spurious in-band signals.These spurious signals will be amplified by the amplifier. If they are high enough in level,amplifier damage may result not to mention FCC involvement. If drive reduction isnecessary to keep the amplifier within operating limits, always use a power attenuator ifthe exciter does not have a power control!In an emergency, a power attenuator may be constructed out of a length of RG-58 orsmaller coax (see cable attenuation specs for your frequency). Also be aware that wecan adjust the drive requirement at the factory to match the fixed output exciter.

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