B+K precision 1856D User manual

1
Instruction Manual
Model 1856D 3.7GHz Frequency Counter

2
WARRANTY
Warranty service covers a period of one year from the date of original purchase.
In case of technical failure within one year, repair service will be provided by our
service center or sales outlet free of charge.
We charge customers for repairsafter the one-year warranty period has expired.
We charge for repairs regardless of the warranty period if failure resulted from the
user’s negligence, natural disaster or accident.
For more professional repair service, be sure to contact our service center or sales
outlet.

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Introduction
Thank you for purchasing our product. Electronic measuring instruments
produced by us are high technology products made under strict quality control. We
guarantee their exceptional precision and utmost reliability. For proper use of the
product, please read this operation manual carefully.
Note
lTomaintain the full precision and reliability of this product,use it within the
temperature range of 10 °Cto 35 °C(50 oF to 95 oF), and within a humidity of
45% to 85%.
lAfter turning on the power, allow a warm up period of about 30 minutes before
use.
lThis equipment should be used with a triple line power cord for safety.
lDue to ongoing product development and improvement, things such as exterior
design and technical specifications can change without prior notice.
lIf you have further questions concerning use of this product, visit
www.bkprecision.com.
Safety Summary
Please take a moment to read these operating instructions thoroughly and
completely before operating this instrument. Pay particular attention to WARNINGS
concerning conditions and actions that pose a hazard to the user and CAUTIONS
describing conditions and actions that may damage the instrument.
lBefore every use, inspect the instrument and other accessories for signs of
damage or abnormality.
lKeep your body isolated from ground.
lNever touch exposed wiring, connections or any live circuit conductors.
lDo not install substitute parts or perform any unauthorized modifications on the
instrument.
lUse caution when working above 60V DC or 30V AC rms. Such voltages pose
a shock hazard.
lRemember that line voltage is present on circuit points such as on-off switches,
fuse, power transformers, etc., even when the equipment is turned off.
lRemember that high voltage may appear at unexpected points in defective
equipment.
Safety Symbols
DANGEROUS VOLTAGE
AC-ALTERNATING CURRENT
DC-DIRECT CURRENT
SEE EXPLANATION IN MANUAL
GROUND
FUSE

4
CONTENTS
1. PRODUCT DESCRIPTION
1-1. Introduction -----------------------------------------------------
(5)
1-2. Technical Specifications ------------------------------------ (6)
1-3. Equipment Ratings ------------------------------------------- (9)
1-4. Supplied Accessories ---------------------------------------- (10)
2. INSTALLATION
2-1. Initial Inspection ----------------------------------------------- (10)
2-2. Connecting AC Power --------------------------------------- (10)
2-3. Cooling and Ventilation -------------------------------------- (10)
2-4. Position ----------------------------------------------------------
(10)
2-5. Warming-Up --------------------------------------------------- (11)
3.OPERATION
3-1. Controls, Indicators and Connectors --------------------- (11)
3-2. Operating Instructions---------------------------------------- (14)
3-3. Frequency Measurements ---------------------------------- (14)
3-4. Period Measurements --------------------------------------- (16)
3-5. Total Measurements ----------------------------------------- (16)
3-6. RPM Measurements ----------------------------------------- (16)
3-7. Use of RS-232C Serial Interfacing ------------------------
(17)
4. MAINTENANCE
4-1. Fuse Replacement ------------------------------------------- (18)
4-2. Adjustment and Calibration -------------------------------- (18)
4-3. Cleaning and Decontamination ----------------------------
(18)
5. OTHER
5-1. BNC Cable Considerations --------------------------------- (19)
5-2. Attenuator Probes -------------------------------------------- (20)
5-3. Line Frequency Measurements---------------------------- (20)
1. PRODUCT DESCRIPTION

5
1-1. Introduction
This reciprocal FREQUENCY COUNTER is a microprocessor-controlled
instrument for frequency measurement. Due to uniquely developed LSI circuitry in
an expanding/reciprocal system, high accuracy with 9-digit resolution is achieved
with a one second gate time. It covers a frequency range of from 0.1 Hz to 3.5
GHz based on a 10 MHz TCO (Temperature Controlled Oscillator). Other features
include:
lRPM (Revolutions Per Minute) Measuring Function
lExternal Frequency Standard Input with 9 Digit LED Display
lAttenuator
lPeriod
lTotal
lLow Pass Filter
lLine Filter
Also provided is a self-test function for a quick check of several operating modes.
Each mode can be selected by a front panel push button switch with automatic
decimal points and indicators. The high accuracy, sensitivity and versatility of this
counter make it an extremely valuable instrument to the scientist, engineer,
experimenter and communications technician. Lightweight and compact size make it
practical for use by the hobbyist or field technician.

6
1-2. Technical Specifications
gINPUT A CHARACTERISTICS
hFREQUENCY RANGE: 0.1 Hz to 100 MHz (DC coupled)
30 Hz to 100 MHz (AC coupled)
hSENSITIVITY: 30 mV rms
hCOUPLING: AC or DC selectable.
hIMPEDANCE: 1 MΩresistance shunted by < 40 pF
hATTENUATOR: x1 or x10 switch selectable
hLOW PASS FILTER: -3 dB point at approx. 100 KHz, switch selectable
hACCURACY: ±Time base error ±resolution (Table 1)
hResolution and Number of Displayed Digits
Time Base Selector INT EXT INT EXT INT EXT INT EXT
Gate Time 0.01S 0.1S 1S 10S
Number of Display
Digits
56677889
Frequency (Input A)
RESOLUTION
0.1 Hz-0.99 Hz 10 uHz 1 uHz 1 uHz 0.1 uHz 0.1 uHz 10nHz 10nHz 1nHz
1 Hz-9.9 Hz 0.1mHz
10 uHz 10 uHz 1 uHz 1 uHz 0.1 uHz 0.1 uHz 10nHz
10 Hz-99 Hz 1 mHZ 0.1mHz
0.1mHz
10 uHz 10 uHz 1 uHz 1 uHz 0.1 uHz
100 Hz-999 Hz 10mHz 1 mHz 1 mHz 0.1mHz 0.1mHz 10 uHz 10 uHz 1 uHz
1 KHz-9.9 KHz 0.1 Hz 10 mHz
10 mHz
1 mHz 1 mHz 0.1 Hz 0.1 mHz 10 uHz
10 KHz-99 KHz 1 Hz 0.1 Hz 0.1 Hz 10 mHz 10 mHz 1 mHz 1 mHZ 0.1mHz
100 KHz-999 KHz 10 Hz 1 Hz 1 Hz 0.1 Hz 0.1 Hz 10 mHz 10 mHz 1 mHZ
1 MHz-9.9 MHz 100 Hz 10 Hz 10 Hz 1 Hz Hz 0.1 Hz 0.1 Hz 10 mHz
10 MHz-99 MHz 1 KHz 100 Hz 100 Hz 10 Hz 10 Hz 1 Hz 1 Hz 0.1 Hz
100MHz 10 KHz
1KHz 1KHz 100 Hz 100 Hz 10 Hz 10 Hz 1 Hz
(Table 1)

7
hMax. Input Voltage Level
FIG. 1 MAX. Input Level. (Input A)
hPERIOD RANGE: 10 nS to 10 S
DISPLAY: n/u/m (nano/micro/milli) Sec. with decimal
point
hTOTAL RANGE: 10 Hz to 30 MHz
CAPACITY: 0 to 999 999 999, OVER FLOW : “OF”
hRPM RANGE: 600 to 600 x 106RPM, OVER FLOW : “OF”
gINPUT C. CHARACTERISTICS
hFREQUENCY RANGE: 80 MHz to 3.5 GHz
hSENSITIVITY: 15mV from 80 MHz to 2000 MHz
20mV from 2000 MHz to 3.0 GHz
30mV from 3.0GHz to 3.2GHz
50mV from 3.2GHz to 3.5GHz
hCOUPLING: AC only
hIMPEDANCE: 50 Ω±5%
hMAX. INPUT LEVEL: 3 V rms sine wave
300
250
150
200
100
25
50
MAX INPUT VOLTAGE (ACV & DCV)
50100 500
FREQUENCY (Hz)
1K 1M 5M 10M 50M 100M

8
hResolution and Number of Displayed Digits
Time Base Selector
INT EXT INT EXT INT EXTINT EXT
Gate Time 0.01S 0.1S 1S 10S
Number Of
Displayed Digits 56677889
Frequency (Input C)
RESOLUTION
100 MHz-999 MHz 10 KHz 1 KHz 1 KHz 100 Hz 100 Hz 10 Hz 10 Hz 1 Hz
1 GHz-3.7 GHz 100 KHz
10 KHz 10 KHz 1 KHz 1 KHz 100 Hz
100 Hz
10 Hz
(Table 2)
gTIME BASE CHARACTERISTICS
hTYPE: TCO (Temperature Controlled Oscillator)
hFREQUENCY: 10.000000 MHz
hSTABILITY: ±1 PPM (±1 count)
hLINE VOLTAGE STABILITY:
Less than ±1 PPM with ±10% line voltage variation
hTEMPERATURE STABILITY:
±5 PPM from 0°C to 50°C
hMAX. AGING RATE: ±5 PPM/year
hINT. STD. OUT: 10 MHz (Internal Standard Frequency Output)
LEVEL: 1V p-por more. TTL (10TTL)
CAPACITANCE: 15pF
hEXT. STD. IN: 10 MHz (External Standard Frequency Input)
LEVEL: 1.5Vrms to 5Vrms
IMPEDANCE: Approx. 600Ω
gDISPLAY CHARACTERISTICS
hDISPLAY: Nine Digit 0.56" LED with M (Mega) or K (Kilo) Hz; nano, micro,
or milli (n/u/m) Sec.; GT (GATE TIME); H (HOLD); and OF
(OVERFLOW) indicators. FUNCTION and GATE TIME are
user selected. OF display is shown when the count exceeds 999
999 999.
hHOLD: In the FREQUENCY, PERIOD, TOTAL, RPM modes,
measurement in progress is stopped; and the last complete
measurement is displayed. When HOLD is released, a new
measurement begins.

9
hGATE TIME: Depending on input frequency
< 10 mS----------- Somewhere between 0.9 and 9mS
< 0.1 S------------- Somewhere between 9 and 90mS
< 1 S----------------Somewhere between 90 and 900mS
< 10 S---------------Somewhere between 0.9 and 9S
NOTE:
LAST MEASUREMENT DISPLAY WILL REMAIN FOR 10 SECONDS AFTER
SIGNAL OFF.
gDIMENSIONS AND WEIGHT
hDimensions (WxHxD): 9.4 x 3.5 x 10.6” (240 x 90 x 270mm)
hWeigh (Approx)t: 5.5lbs. (2.5kg.)
1-3. Equipment Ratings
lPower: 115VAC±10% , 50-60 Hz, 9W
lPlug and Socket: 3 wire ac power plug and 3 wire outlet
lFuse: 500 mA/ 250V F type
lOperating Environment:
TEMPERATURE: 0°C to + 40 °C (Accuracy Specified at 23°C ±5 °C)
HUMIDITY: up to 85% RH (Relative Humidity) to 40°C without
temperature extremes causing condensation within the
instrument.
lStorage Environment:
TEMPERATURE: -20°C to +70°C
HUMIDITY: below 85% RH
lInsulation Category II: Portable equipment of local level.
lPollution Degree: 2
lProtection to IEC 529: Ordinary
Note: Specifications are subject to change without notice.

10
1-4. Supplied Accessories
lUser’s Manual --------------------------------------------------------------1
lBNC cable -------------------------------------------------------------------1
lPower cord-------------------------------------------------------------------1
lSpare Fuse-------------------------------------------------------------------1
2. INSTALLATION
2-1. Initial Inspection
This instrument was carefully inspected both mechanically and electrically before
shipment. It should be physically free of damage. To confirm this, the instrument
should be inspected for physical damage which may have occurred in transit. Also,
verify that all supplied accessories have been included.
2-2. Connecting AC Power
This instrument requires 115VAC, 50-60 Hz power through a 3-conductor AC
power cable to be plugged into a three-contact electrical outlet to secure grounding.
If adapted into a 2-conductor cable, use the ground terminal on the rear panel for
grounding the instrument.
CAUTION
THIS INSTRUMENT IS SET TO 115V, AC. BEFORE TURNING ON THE POWER
TO THE INSTRUMENT, MAKE SURE THE VOLTAGE OF THE POWER SOURCE
IS 115V, AC.
2-3. Cooling and Ventilation
No special cooling and ventilation is required. However, the instrument should be
operated where the ambient temperature is maintained.
2-4. Position
This instrument is built as a bench-type instrument with rubber feet and tilt stand
in place. The stand-up angle can be adjusted by changing the angle of the
carrying handle.

11
2-5. Warming-Up
Allow at least 30 minutes for the unit to warm up so that it can stabilize.
3. OPERATION
3-1. Controls, Indicators and Connectors
ON
POWER
OFF
k/u
SEC
RX
RMT
FREQ.
A
G.T
PRIOD
C
A
TOTAL
A
A
RPM
0
0
.
1
S
0.1S
1S
10S
m
H
TX
0.F
M/n
Hz
INPUT C
80MHz
3V MAX.
3.7GHz
100MHz
250V MAX.
INPUT A
1M
FUNCTION
19
18
15
16
10
12
9
2
3
1
4
5
6
14
8
GATE TIME
MULTIPLIER
HOLD
TOTAL RESET
OFF
LPF
INPUT A
COU
PLE
AC
ATT
1/1
ON
DC
1/10
20
50
FIG.2 FRONT PANEL
GATE INDICATOR: The gate light, when lit, indicates the main gate is open
and measurement is in progress.
RS-232C INDICATOR: TX (transmitting), RX (receiving) blinking
OVER FLOW INDICATOR: OF is displayed when there is an overflow.
DISPLAY: 9 digit (0.56“) green LED display used for all readings.
NOTE: LAST MEASUREMENT DISPLAY WILL REMAIN FOR 10 SECONDS
AFTER SIGNAL OFF.

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UNIT INDICATOR: When lit, indicates that the frequency displayed is in
MHz, KHz, Hz and period is in nano, micro, or milli
(n/u/m) Sec.
HOLD INDICATOR: The HOLD function is engaged the when lit.
INPUT C: Use this input for all frequency measurements above 80
MHz. Female N type connector terminated in 50Ω.
LOW PASS FILTER: With this switch pushed in, the A input is routed through
a low-pass filter with a -3 dB point at approximately 100
KHz. When it is released, the A input is applied directly
to the counter.
ATT. SWITCH: When this switch is set to x10 (pushed in) the A input is
attenuated 10:1 before application to the counter. With
the switch set to x1 (pushed out), the A input signal is
applied unattenuated. The attenuator has no effect on
the C input.
COUPLE. SWITCH: This switch is used to select the input coupling mode,
AC or DC.
INPUT A: Use this input for frequency measurements below 100
MHz and all period measurements. Female BNC
connector. Input impedance is1 MΩshunted by < 40 pF.
GATE TIME SWITCH: This switch selects the degree of resolution on the
display in all modes except TOTAL.
HOLD SWITCH: In the HOLD function, the display is held but the counter
continues to increment. When HOLD is released, the
display is updated and resumes counting.
FUNCTION SWITCH: This selects the desired operating mode.
a. FREQ. A: When this mode is selected, the counter reads the
frequency of the input at A.Resolution is selected
using the GATE TIME.
b. FREQ. C: When this mode is selected, the counter reads the
frequency of the input at C.All readings are in MHz.
c. PERIOD A: When this mode is selected, the counter reads the
period of the input at A.Resolution is selected using

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the GATE TIME.
d. TOTAL A: When this mode is selected, the unit counts the cycles of
the signal at input A and continuously displays that count.
e. RPM A: When this mode is selected, the unit displays the RPM
(Revolutions Per Minute) of the signal at input A.
POWER SWITCH: This push type switch, turns on the power when pressed.
TILT STAND: Pull out to adjust tilt.
REPLACE ONLY WITH FUSE OF SAME RATING AND TYPE.
DO NOT REMOVE COVER. REFER SERVICING TO
DISCONNECT POWER SUPPLY BEFORE REPLACING FUSE.
1
INT STD OUT QUALIFIED PERSONNEL.
EXT STD IN
TO PREVENT ELECTRIC SHOCK HAZARD AND FIRE,
RS-232C
WARNING
6523
4
FUSE
230V~, 50Hz/60Hz
FIG.3 REAR PANEL
AC INLET: AC power input connector
INT/EXT TIME BASE SELECTOR:
Selects the source of the time base. EXT.STD. IN
sets up BNC as a nominal 600Ωinput impedance
path for an external 10 MHz time base signal. INT. STD.
OUT sets up BNC to monitor the internal time base
signal.
GROUND TERMINAL
FUSE HOLDER: Replace fuse by unscrewing
INT/EXT TIME BASE BNC:
Provides a connector through which the internal time

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base signal can be monitored or through which an
external time base signal can be applied (see item
above). The external signal should have a voltage
range of 1.5 V~5 V rms.
RS-232CCONNECTOR: Connector for serial interfacing with a computer.
3-2. Operating Instructions
a. Connect the unit up to the AC power.
b. Turn on the unit by pushing in the POWER on-off switch.
c. Set the FUNCTION indicator position to FREQ A and the gate TIME
indicator to the 1 Sec, position.
CAUTION
1. APPLICATION OF INPUT VOLTAGES HIGHER THAN THE LIMITS LISTED IN
THE SPECIFICATIONS SECTION MAY DAMAGE THE COUNTER. BEFORE
APPLYING ANY SIGNAL TO THE INPUTS, MAKE CERTAIN THAT IT DOES NOT
EXCEED THESE SPECIFIED MAXIMUMS.
2. FREQUENCY COUNTER GROUND POINTS ARE CONNECTED DIRECTLY TO
EARTH GROUND. CONNECT GROUND OF THE CIRCUIT UNDER TEST TO
THE FREQUENCY COUNTER GROUND ONLY
3-3. Frequency Measurements
A. INPUT A (0.1 Hz to 100 MHz):
a. Apply the signal to be measured at input A.
b. Set the FUNCTION indicator to FREQ. A
c. Select the resolution using the GATE TIME selector switch.
d. The frequency is indicated on the display. The gate indicator is lit while
the measurement is in progress, and the display is updated at the end of
each measurement interval.

15
e. Engaging the HOLD switch “freezes” the display at the existing reading.
When HOLD is released, the display is updated and resumes counting.
f. Engage the ATTENUATOR if necessary. When set to x10 (pushed in),
the signal at input A is attenuated by a factor of approximately 10 before it
is applied to the counter. This helps prevent miscounting caused by noisy
or improperly terminated high amplitude signals.
g. Engage the LOW PASS FILTER (LPF) if necessary. When this switch is
pushed in, the signal at input A is routed through a low pass filter (-3 dB
point at approximately 100 KHz) before it is applied to the counter. This
helps eliminate counting errors in low frequency measurements by
minimizing effects of high frequency noise that may accompany the signal.
h. When making measurements near the lower cut-off frequency of input A
(10 Hz or less), use DC coupling (Push in the DC COUPLING switch).
B. INPUT C (80 MHz to 3.5 GHz)
CAUTION
THE MAXIMUM INPUT TO THIS INPUT IS 3 Vrms OVER THE INPUT
FREQUENCY RANGE. THE “X 10 ATTENUATOR” DOES NOT APPLY.
a. Apply the signal to be measured at input C.
b. Set the FUNCTION indicator to FREQ. C
c. Select the resolution using the GATE TIME selector switch.
d. The frequency is indicated on the display. The gate indicator is lit while the
measurement is in progress, and the display is updated at the end of each
measurement interval.
e. Engaging the HOLD switch “freezes” the display at the existing reading,
When HOLD is released, the display is updated and resumes counting.
f. The ATTENUATOR, LPF, and COUPLING switches have no effect with
input C.

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3-4. Period Measurements
a. Apply the signal to be measured at input A.
b. Select the resolution using the GATE TIME selector switch.
c. The period is indicated on the display. The gate indicator is lit while the
measurement is in progress.
d. The ATTENUATOR, LPF, and COUPLING switches operate the same as
they do in the frequency measurements modes.
3-5. Total Measurements
The totalize mode is used to count the total number of events occurring during a
specific time period. The maximum frequency is 30 MHz.
a. With the FUNCTION switch, select the TOTAL A mode. Gate and unit
settings are ignored.
b. Apply the signal to be measured at input A. The accumulated counts will
be displayed continuously to a maximum count of 999999999. If this is
exceeded, the overflow message, OF, will be displayed.
c. The ATTENUATOR, LPF, and COUPLING switches operate the same as
they do in the frequency measurements modes.
NOTE
THE HOLD SWITCH MAY BE USED TO LATCH THE DISPLAY. THE COUNTER,
HOWEVER, WILL CONTINUE TO INCREMENT. WHEN THE HOLD SWITCH IS
RELEASED, THE UPDATED COUNT WILL BE DISPLAYED.
3-6. RPM (Revolutions Per Minute) Measurement
a. Apply the signal to be measured at input A. The count will be
displayed as revolutions per minute (RPM). Maximum count is 999999999.
If this is exceeded, the overflow message, OF, will be displayed.
b. The ATTENUATOR, LPF, and COUPLING switches operate the same as
they do in the frequency measurements modes.

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3-7. Use of RS-232C Serial Interfacing
1) Hardware/Software Requirements
IBM PC/XT/AT or compatible computer
Microsoft Windows
Serial port for connection with counter
2) Output Data Formats
(1) Baud rate : 9600BPS
1 start bit (0)
8 data bit
1 stop bit (1)
NONE PARITY
(2) To Frequency counter
Command Parameter Terminate Code
'H' : HOLD '0' : OFF
'1' : ON
'2' : TOGGLE
CR (0DH)
'G' : GATE '0' : 0.01 SEC
'1' : 0.1 SEC
'2' : 1 SEC
'3' : 10 SEC
CR (0DH)
'D' : DATA REQUEST DON'T CARE CR(0DH)
'F' : FUNCTION SET N* CR(0DH)
'R' : REMOTE '0' : OFF
'1' : ON CR(0DH)
N*= 01234567
3.5GHz,F/C FA NC FC PERIOD TOTAL RPM NC NC
(3) From Frequency counter
DATA UNIT
10BYTES include dp 4bytes CR

18
3) Installing program and using the software: Refer to the “Readme”file
on the software diskette supplied with unit.
4. MAINTENANCE
CAUTION
FOR SAFETY, IT IS ESSENTIAL TO PROPERLY MAINTAIN AND SERVICE THIS
INSTRUMENT
WARNING
VOLTAGES WITHIN THIS INSTRUMENT ARE SUFFICIENTLY HIGH TO BE
LETHAL. COVERS MUST NOT BE REMOVED EXCEPT BY PERSONS
QUALIFIED AND AUTHORIZED TO DO SO, AND THESE PERSONS SHOULD
ALWAYS USE EXTREME CARE ONCE THE COVERS HAVE BEEN REMOVED.
4-1. Fuse Replacement
a. Disconnect and remove all connections from any live power source.
b. Unscrew the fuse holder with a screwdriver.
c. Remove the defective fuse.
d. Install a new fuse of the SAME SIZE AND RATING.
e. Screw the fuse holder back in.
CAUTION
REPLACEMENT FUSES MUST HAVE THE SAME SIZE AND RATING AS THOSE
BEING REPLACED.
4-2. Adjustment and Calibration
Regular adjustment and calibration of this instrument is recommended. The
procedures involved should be executed by qualified and authorized personnel only.
4-3. Cleaning and decontamination
The instrument can be cleaned with asoft clean cloth to remove any oil, grease
or grime. Never use liquid solvents or detergents. If the instrument gets wet for any
reason, dry it using low-pressure clean air at less than 25 PSI. Use care and caution

19
around the window cover areas where water could leak into the instrument while
drying.
5. OTHER
5-1. BNC Cable Considerations
The accuracy of radio frequency measurements can be affected by the
connections between the signal source and counter. The main issues involved are
standing waves and cable shunt capacitance.
Waves propagating along a transmission line will be reflected at its ends if the
line is not terminated in its characteristic impedance. These reflections will set up
standing waves in the line, which may cause damage to the signal source or
produce inaccurate measurements. This effect increases as the cable length
reaches one-fourth of the wavelength for the frequency being measured. Standing
waves can be minimized by keeping cable lengths short, or eliminated by providing
a proper termination. RG58 cable, for example, has a characteristic impedance of
50Ω. The proper termination for this cable would, therefore, be a 50Ωresistor
between the center conductor and the shield. A standard BNC, 50Ωcable
termination is most often used for this.
Input A has an input impedance of 1 MOshunted by < 40 pF. Radio frequency
measurements made at this input will experience standing waves and attenuation
due to the shunt capacitance of the cable which adds to the shunt capacitance at
the input. For RF measurements at input A, it is recommended that a 50Ω
termination be used, or that the cable be no longer than three feet (91 cm).
Input C has a built in impedance of 50Ω, thus the need for an external
termination is eliminated.
In situations where a 50Ωtermination could affect bias voltages or other DC
voltages in the circuit under test, a blocking capacitor should be used.

20
5-2. Attenuator Probes
When measuring signals at input A that will exceed its maximum input voltage
level (See FIG. 1), a 10:1 attenuating probe such as an oscilloscope probe should
be used.
NOTE
DO NOT USE A 10:1 PROBE WITH THE INPUT C. THESE PROBES ARE
DESIGNED WITH A 10:1 ATTENUATION FOR AN INSTRUMENT WITH AN
INPUT RESISTANCE OF 1 M Ω. THE 50 ΩTERMINATION OF INPUT C WOULD
RESULT IN UNACCEPTABLY HIGH ATTENUATION.
5-3. Line Frequency Measurements
When making line frequency measurements, use of the LOW PASS FILTER, the
ATTENUATOR, and/or the 10:1 probe is recommended.
WARNING
USE CAUTION WHEN MEASURING THE LINE FREQUENCY OF AN AC
OUTLET. USE THE PROBE TIP ONLY, AND MEASURE BOTH SIDES OF THE
LINE. THE NEUTRAL SIDE WILL GIVE A ZERO READING AND THE HOT SIDE
WILL PROVIDE THE DESIRED MEASUREMENT. DO NOT USE THE "GROUND"
LEAD OF THE PROBE. REMEMBER THAT THE CHASSIS OF THE COUNTER
AND THE “GROUND” LEAD OF THE PROBE ARE ALREADY AT EARTH
GROUND (VIA THE 3-WIRE POWER CORD OF THE INSTRUMENT). TOUCHING
THE “GROUND" LEAD TO THE 'HOT" SIDE OF THE LINE WOULD PLACE A
DIRECT SHORT ON THE POWER LINE THROUGH THE PROBE CABLE,
RESULTING IN A BIG SPARK, DAMAGE TO THE PROBE CABLE, AND
POSSIBLE INJURY TO THE USER.