Fluke 85RF User manual

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®
Instruction Sheet
85RF
INTRODUCTION
The 85RF High Frequency Probe is designed to convert a dc
voltmeter into a high frequency (100 kHz to 500 MHz) ac
voltmeter. Conversion from ac to dc is accomplished on a
one-to-one basis and includes a range of 0.25 to 30V rms.
The probe’s dc output is calibrated to be equivalent to the
rms value of a sine wave input.
SPECIFICATIONS
AC to DC RATIO: 1:1
RATIO ACCURACY: (At 25 MHz and loaded with 10 M e)
Above 0.5V: ± 0.5 dB
Below 0.5V: ± 1.0 dB
Add ± 0.2 dB in RF fields of 1 to 3 V/m
FREQUENCY RESPONSE: (Relative to 25 MHz)
100 kHz to 100 MHz ± 0.5 dB
*100 MHz to 200 MHz ± 2.0 dB
*200 MHz to 500 MHz ± 3.0 dB
*Referred to high and low inputs at probe tip.
EXTENDED FREQUENCY RESPONSE:
Useful for relative readings from 20 kHz to 700 MHz.
RESPONSE:
Responds to the peak value of an input and is calibrated to
read rms value of a sine wave.
VOLTAGE RANGE: 0.25 to 30V rms
MAXIMUM INPUT VOLTAGE: 30V rms, 200V dc
INPUT CAPACITANCE: Approximately 3 pF
TEMPERATURE RANGE:
Operating: +10°C to +35°C
Storage: -40°C to +75°C
HUMIDITY: <90% R.H.
OUTPUT CONNECTOR: Fits standard 0.75-inch dual
banana connectors
ACCESSORY: BNC to Probe Adapter, P/N 574756
VOLTMETER COMPATIBILITY
The 85RF is compatible with any dc voltmeter that has an
input resistance of 10 megohms, ±10%. Voltmeters with
higher input resistances can be used if an external shunt is
placed across their input terminals. The shunt value is
selected to reduce the input resistance to 10 megohms.
PN 630376
July 1981, Rev.6, 6/09
©1981-2009 Fluke Corporation, All rights reserved.
Printed in the U.S.A.
All product names are trademarks of their respective companies.
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OPERATION
Use the following procedure to operate the 85RF Probe.
1. Connect the probe output to a compatible voltmeter
using a suitable shunt if required. Probe output low must be
connected to voltmeter low.
2. Select an appropriate voltage range (1V dc reading per
1V rms probe input).
3. Connect the input low (clip lead or tip shield) to earth
ground. WARNING
INPUT LOW IS INTENDED TO BE CONNECTED TO EARTH
GROUND. TO AVIOD ELECTRICAL SHOCK, INPUT LOW (CLIP
LEAD OR TIP SHIELD) MUST BE CONNECTED TO POTENTIALS
OF LESS THAN 30V AC, 42V PEAK, OR 60V DC.
a. Measurements at frequencies below 100 MHz can be
made by grounding the 4-inch clip lead supplied with the
probe, then touching the probe tip to the circuit under test.
b. Measurements above 100 MHz must be made using
minimum cable lengths to terminated 50-ohm system. One
inch of unterminated coaxial cable (such as a BNC adapter
and 50-ohm termination) is sufficiently short for up to 300
MHz.
c. Measurements above 300 MHz must have the probe tip
and tip shield (input high and low) touching the circuit under
test to realize the rated accuracy of the probe. Lead lengths
even less than 1 inch may cause significant errors.
CAUTION
To avoid probe damage, the dc component of the input signal
must not exceed 200 volts, including transients. 30V ac rms and
200V dc are the maximum inputs to the probe. This allows
measurements of an ac signal riding on a dc voltage of up to
200V. The input capacitor charges to the peak value of ac plus
dc. If the dc level is then changed by more than 200V, the
resulting transient may damage the probe. The chance of
damage can be minimized by discharging the input capacitor at
the end of each measurement. This is done by momentarily
touching the probe tip to the ground clip.
THEORY OF OPERATION
Circuitry within the 85RF consists of a capacitor-coupled
rectifier circuit which responds to the peak value of the input
waveform. The output is a positive dc voltage which is
calibrated to be equivalent to the rms value of a sine wave
input. See Figure 1. R1, R2, R3. and CR2 form a divider to
properly scale the dc output when the probe output is
connected to a dc voltmeter having a 10-megohm input
resistance. Diode CR2 compensates for the low level
linearity error caused by diode CR1.
MAINTENANCE
Performance Test/Calibration
The performance test is designed to ensure that the 85RF is
operating within specifications. Use the procedure as an
acceptance test and/or calibration procedure (the probe has
one adjustment). The recommended calibration interval is
one year.
1. Set up the equipment as shown in Figure 2. Using the
leveled sine function of the scope output, set the AC
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calibrator signal generator for 1V rms at 25 MHz and the
DMM to measure 1V dc.
C1
C1
220 pf R1
220 k R2
500 k
AC INPUT DC OUTPUT
CR1
CR1
CR2
CR2
R1 R3
R3
1.3 M
R2
Figure 1. Schematic Diagram
1
25
4
3
Fluke 85RF Probe under test
6
Required Equipment List
The following equipment (or equivalent) is required for
testing the performance of the 85RF:
EQUIPMENT TYPE RECOMMENDED MODEL
AAC Calibrator Fluke 5520A-SC600
B50-Ohm Feed Thru
Termination Fluke Y9103
CBNC Jack to Banana
Plug Adapter Fluke Y9108
DBNC to Probe Adapter Fluke P/N 574756
EDigital Multimeter, 3-
1/2 Digit (DMM) Fluke 87-III
FBNC Jack to Jack
Adapter Pomona 3283
Figure 2.
Equipment Configuration for Performance Testing
2. The DMM should read between 0.950 and 1.050V dc. If
the probe is functioning but is not within these limits, access
and adjust R2 for a DMM reading between 0.990 and 1.010V
dc. Reassemble the probe.
3. Transfer the unit under test to the Normal output of the
calibrator and reduce the output level to 0.250V rms 1 MHz.
The DMM should read between 0.223 and 0.280V dc.
4. Set the AC Calibrator to STDBY, decrease the
frequency to 100 kHz, and increase the voltage to 30V ac
rms. (Do not exceed this value.)
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5. Return the AC Calibrator to OPER. The DMM should
read between 28.3 and 31.8V dc
6. Return the UUT to the scope output and set the level
sine to 1V rms 160 MHz. The DMM should read between 0.9
and 1.12V dc. Increase the frequency to 450 MHZ. The DMM
should read between 0.70 and 1.41V dc.
Troubleshooting
Isolate a problem to the probe by verifying the operation of
the probe’s input source and output DMM. Most probe
malfunctions are caused by the failure of diodes CR1 or
CR2. Suspect CR1 if there is no output when an input is
applied to the probe. Excessive linearity error may be due to
the failure of CR2. Use a continuity test for in-circuit diode
testing. If the diodes are good, check the input and output
connections to the printed circuit board and the continuity of
the output cable.
CAUTION
The diodes have glass bodies; care should be taken to avoid
stressing them, particularly when preparing the leads. The diodes
are also subject to failure from overheating. When soldering, hold
the lead near the diode body with a pair of needle nose pliers.
Avoid excessive heat.
Probe Disassembly
Use the following procedure to access the probe’s internal
components, including the calibration adjustment R2:
1. Unscrew the bend relief and slide it away from the
probe.
2. Lift off the probe cover. R2 is exposed.
3. Remove the probe tip by carefully grasping the tip with a
pair of pliers and drawing it out of the front of the probe.
4. Unscrew the ground lead from the probe body.
5. Lift the rear of the printed circuit board slightly and pull it
firmly away from the probe tip.
6. To reassemble the probe, logically reverse this
procedure. Make sure that the gold pin on the front of the
printed circuit board centers properly in the tip and that the
ground-lead nut is aligned with the hole in the probe body.
Cleaning
Use a soft cloth dampened in a mild solution of detergent
and water to clean the 85RF. Do not use solvents.
85RF PARTS LIST
DESCRIPTION PART NO.
C1 Cap, Ceramic Chip, 220 pF 758078
CR1 Diode, Germanium 629261
CR2 Diode, Silicon 639922
R1 Resistor, cf, 220k 348953
R2 Resistor, Var, 500k 574624
R3 Resistor, cf, 1.3M 442558
Body, Probe 606988
Cover, Probe 581751
Tip, Probe 607267
Wire, Ground Assy 607291
Decal, Probe 585842
Contact, Ground 586560
Contact, Twist Pin 294991
Tip, Insulated 583682
Tip, IC 583674
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