Agilent 778D Service Manual

778D Dual Directional Coupler

Operating and Service
Manual

Manual part number: 00778-90013
Printed in USA
March 2001
Supersedes: June 2000

Revision 2.0

Notice

The information contained in this document is subject to change without
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© Copyright 2000–2001 Agilent Technologies, Inc.

ii Agilent 778D Operating And Service Manual

What You’ll Find In This Manual…

Overview, page1

•

•Specifications, page2

•Installation and Shipping, page3

•Operation, page5

•Performance Testing, page9

Agilent 778D Operating And Service Manualiii

Warranty

Custom systems are warranted by contractual agreement between Agilent
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Certification Agilent Technologies, Inc., certifies that this product met its published

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formerly NBS), to the extent allowed by the Institute’s calibration facility,
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Warranty This Agilent Technologies system product is warranted against defects in

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product, or improper site preparation or maintenance.

iv Agilent 778D Operating And Service Manual

NO OTHER WARRANTY IS EXPRESSED OR IMPLIED. AGILENT
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For assista nce, ca ll your local Agi lent Technologies Sales and Service Of f ice

(refer to “Service and Support” on page vi).

Agilent 778D Operating And Service Manual v

Service and Support

By internet, phone, or fax, get assistance with all your test and measurement
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vi Agilent 778D Operating And Service Manual

Safety and Regulatory Information

Review this product and related documentation to familiarize yourself with
safety markings and instructions before you operate the instrument. This
product has been designed and tested in accordance with international
standards.

WARNING The WARNING notice denotes a hazard. It calls attention to a procedure,

practice, or the like, that, if not correctly performed or adhered to, could result
in personal injury. Do not proceed beyond a WARNING notice until the
indicated conditions are fully understood and met.

CAUTION The CAUTION notice denotes a hazard. It calls attention to an operating

procedure, practic e, o r the like, which, if not correctly pe rformed or adhered
to, could result in da mage to the product or loss of important data. Do not
proceed beyond a CAUTION notice until the indicated conditions are fully
understood and met.

Instrument Markings

When you see this symbol on your instrument, you should refer to the instrument’s

!

1SM1-A This text indicates that the instrument is an Industrial Scientific and Medical Group 1

instruction manual for important information.

This symbol indicates hazardous voltages.

The laser radiation symbol is marked on products that have a laser output.

This symbol indicates that the instrument requires alternating current (ac) input.

The CE mark is a registered trademark of the European Community. If it is
accompanied by a year, it indicates the year the design was proven.

The CSA mark is a registered trademark of the Canadian Standards Association.

Class A product (CISPER 11, Clause 4).
This symbol indicates that the power line switch is ON.

This symbol indicates that the power line switch is OFF or in STANDBY position.

Agilent 778D Operating And Service Manual vii

Safety Earth Ground

This is a Safety Class I product (provided with a protective earthing
terminal). An uninterrup ti ble saf et y ear th gr ound must be pr o vided from the
main power source to the product input wiring terminals, power cord, or
supplied power cord set. Whenever it is likely that the protection has been
impaired, the product m ust be made inoperative and secured against any
unintended operation.

Befor e Applying Power Verify th at the product is configured to match the available main power

source as d escribed in the input power configuration in structions in this
manual. If this product is to be powered by autotransformer, make sure the
common terminal is connected to the neutral (grounded) side of the ac power
supply.

viii Agilent 778D Operating And Service Manual

Overview

This coaxial dual directional coupler has a frequency range extending over
four octaves from 100 MHz to 2 GHz. Although the coupling factor
increases 6 dB/octave below 100 MHz, directivity remains above 36 dB.
Thus, the coupler can be used below 100 MHz as well as above. The
usefulness below 100 MHz is restricted only b y the amount of signal source
power and/or the sensitivity of the RF detector connected to the coupler.

The unidirectional characteristic of the Agilent 778D dual directional
coupler makes it an i m por ta nt t ool in microwave measurement systems. The
778D has the property of inducing, in an auxiliary line, a contra-directional
flow of power which is proportional to the power fl owing in the main guide.
This coupler may be used in the measurement of reflection coefficients or
SWR over a very wide frequency range. The relatively constant coupling
across several frequency bands and the stability of coupling with time make
the 778D useful, also, as a wideband attenuator or for power monitoring
applications.

Description To achieve its broad frequency coverage, the 778D’s directivity is optimized

for mainline energy flowing in one direction. This means that there is a
preferred orientation for the 778D. To achieve the measurement accuracies
in the specifi cations, the t est de vice or system shoul d be connected wher e the
778D is labeled TEST PORT. To accommodate test devices with Type N or
APC-7 connectors, a choice of connectors is available on the TEST PORT
(see Options outlined in “Specifications” on page 2).

With the APC-7 precision sexless connector on the output, the coupler can
be conveniently converted to other types of connectors by the use of an
adapter. Adapters are available to the SMA (OSM), TNC, GR900, and
others, so that the coupler becomes a versatile device for making
measurements on many dif ferent types of connector s. The Amphenol APC-7
is a sexless connector with very low SWR. This precision connector makes
possible the construction of adapters with low SWR.

Agilent 778D Operating and Service Manual 1

Specifications

Specifications

Specifications for the 778D ar e shown in the following t able.

Characteristic Value

Frequency range 100 to 2000 MHz (usable below 100 MHz)
Directivity

Auxiliary arm 0.1 to 1.0 GHz

Auxiliary arm 1.0 to 2.0 GHz

Coupling Attenuation nominal coupling above 450 MHz is 20 dB

Coupling variation ±1.5 dB from nominal

Cyclical variation Frequency is similar to Figure 7 on page 10.
Tracking Auxiliary outputs typically track within 0.7 dB.

SWR Primary Line ≤1.1

Maximum main line power 50 W average

Accessories available adapters

See figure on page 1
A 36 dB

B 30 dB
A 32 dB

B 30 dB

nominal coupling above 450 MHz is 20 dB increasing with a
slope of 0.006 dB/MHz for decreasing frequency.

Phase tracking typically 4°.

Auxiliary Arms ≤1.1

500 W peak

2

11533A APC-7 to SMA male
11534A APC-7 to SMA female
11525A APC-7 to Type N male
11524A APC-7 to Type N female

1

Connectors Choice of main line connectors (see options)

Auxiliary Arms have Type N female
All Type N connectors stainless steel, compatible with

MIL-C-39012 and MIL-C-71.

Dimension 16-3/4 in long x 4-3/8 in high x 1-3/16 in wide

(425 mm x 111 mm x 30 mm)
Net weight 3-3/4 lb (1.5 kg)
Options available 778D (Type N female output, Type N male input connectors)

Option 011 (APC-7 output, Type N female input connectors)

Option 012 (Type N male output, Type N female input

connectors)

1. See Figure 6 on page 8 for a typical coupling curve.

2. Adapters from APC-7 to TNC, GR-900, GR-874 as well as the above are available from Amphenol

RF Division, Danbury, CN)

2 Agilent 778D Operating and Service Manual

Installation and Shipping

Installation and Shipping

Unpacking and Inspection

CAUTION Protect the face of the co nnecto rs (espe cial ly the APC-7) from damage. An y

The coupler was carefully inspected mechanically and electrically prior to
shipment. Inspect it for mechanical damage incurred in transit, and test
electrical performance using the test procedure given under Performance
Testing. If there is damage or deficiency, notify the carrier and the nearest
Agilent Technologies offic e. In the e v ent of mecha nical damag e, the packi ng
materials and carton should be held for th e carrier’s inspection.

scoring or burring of the mating surfaces causes discontinuity; the resulting
increase in SWR degrades performance (see Figure 1).

Figure 1 Care of APC-7 Connectors

Repackaging for Shipment

Using Original Packaging The same containers and materials used in factory packaging can be

obtained through Agilent Technologies.
If the coupler is bei ng r et urned to Agilent Technologies for servicing, attach

a tag indicating the type of service required, return address, and model
number. Also, mark the container FRAGILE to assure careful handling.

In any correspondence refer to the coupler by full model number.

Agilent 778D Operating and Service Manual 3

Installation and Shipping

Using Other Packaging The following general instructions should be used for repackaging with

commercially available materials:

1. Wrap the coupler in heavy paper or plastic. (If shipping to a Agilent

Technologies office or service center, attach a tag indicating the type of
service required, return address, and model number.)

2. Use a strong shipping container. A double wall carton made of 2.4 MPa

(350 psi) test material is adequate.

3. Use enough shock-absorbing material (3-4-inch layer) around all sides

of the coupler to provide firm cushion and prevent movement inside the
container.

4. Seal the shipping container securely.

5. Mark the shipping container FRAGILE to assure careful handling.

Load-Bearing When installing be sure auxiliary equipment supports its own weight. The

coupler, particularly the connectors, is not designed to carry weight. Load
limits for APC-7 connectors are given in Figure 2.

Figure 2 Load Limits for APC-7 Connector

4 Agilent 778D Operating and Service Manual

Operation

Operation

Signal Flow Unlike some other dual-directional couplers, this coupler is polarized. For

best performance the signal must flow through the coupler as shown in

Figure 3. Observe the TEST PORT designation on the label. DO NOT

reverse the coupler since the directivity may be somewhat l ower i n the
reverse direction. To test devices with other types of connectors, another
coupler with the correct mating test port connector is recommended. Using
an adapter will degrade the effective directivity of the measurement.

Figure 3 Signal Flow

Coupler Temperature Operate the coupler at normal room temperature and do not temperature

cycle the coupler in use or storage. Heat cycling may degrade the
performance of the coupler.

Reflectometer Measurements

Figure 4 on page 6 shows an arrangement in which swept-frequency

measurements of reflection coefficient are easily made with the coupler.
Incident and reflected signals are detected from the auxiliary arms of the
coupler. The signals can then be displayed on an oscilloscope or X-Y
recorder to give a visual display of reflection coefficient or SWR.

Agilent 778D Operating and Service Manual 5

SWEEP
OSCILLATOR

Operation

OSCILLOSCOPE

SWEPT
FREQUENCY
INDICATOR

Figure 4 Typical Reflectometry Setup

The high directivity and close tracking of the auxiliary arms of this coupler
make it particular ly useful for thi s type of broadba nd measurement. The hi gh
directi vity insures a ccurate measur ements b y a true separat ion of the forw ard
and reflected signals. Close tracking of si gnal variation from the auxiliar y
arms gives a flat frequency response that ma kes readings ea sy to interpret.

Errors Maximum errors in reflection measurement due to the coupler are as

Γ

follows, where

Maximum phase error = ±sin

Table 1 Errors

Frequency
Range
(GHz)

0.1 to 1 ±(0.015+0.04|Γ

1 to 2 ±(0.025+0.04

Γ

= reflection coefficient of unknown.

L

= reflection coefficient of device under test:

L

-1

(∆ΓL/ΓL) ° (∅≤90°)

Maximum Magnitude Error ∆

Swept Frequency Fixed Frequency

|+0.05|ΓL|

L

|ΓL|+0.05|ΓL|

2

) ±0.015+0.05|Γ

2

) ±0.025+0.05|Γ

L

2

|

)

L

2

|

)

L

Errors include directivity, tracking, and source match errors but do not
include any detection errors. They are also based on the following
conditions: auxiliary arms are terminated in matched loads (VSWR

≤1.2).

the mean of open- and short- circuit readings is set to 1.0, and the
short-circuit phase reading is measured over a band of frequencies and the
mean is set to 180°.

6 Agilent 778D Operating and Service Manual

Operation

Impedance Measurement

The 778D is also well suited for measurements of impedance when used
with the Agilent 8508A vector voltmeter (see Figure 5). Again, a
reflectom etry technique is used. With the vector voltmeter, however, both
magnitude and phase of the reflection coefficient can be measured.

909A

50-Ohm
“T”

50-Ohm

“T”

909A

Figure 5 Setup for Impedance Measurement

Data can be read fr om th e two mete rs of the v e ctor v olt meter and tr ansf erre d
directly to a Smith Chart to provide impedance information of such devices
as antennas or other passive components. In addition, with an appropriate
bias supply, active components, such as transistors, can also be quickly
measured and their char acteristics determined with this setup. The 778D is
well suited for these types of measurements since both the phase and
magnitude var iation be tween the cou pling ar ms hav e been cl osely controll ed
in the design of the coupler.

Power Leveling The 778D has cycl ic co upli ng variations of up to ±2 dB. For this reason it is

not recommended for power leveling that requires flatness exceeding these
tolerances. A typical coupling curve is shown in Figure 7 on page 10 .

Maintenance The directional coupler should need very little maintenance besides keeping

it clean and protecting the connector faces.

Agilent 778D Operating and Service Manual 7

Operation

Figure 6 Typical Coupling Variation with Frequency

8 Agilent 778D Operating and Service Manual

Perf ormance Testing

Use the following procedure for initial inspection, performance testing, or
whenever the coupler performance is suspected. Table 2 lists equipment
necessary for testing. Other equipment may be substituted provided its
specifications equal or exceed the specifications listed under Critical
Specifications. The coupler should be tested on a swept-frequency basis to
assure that there are no out-of-specification narrow-frequency bands
between the spot fre que ncies tested on a fixed-frequency basis. If the results
of swept-frequency testing are doubtful or if the equipment for
swept-frequency testing is not available, the fixed-fr equency test may be
used. Table 3 on page 11 provides a place to record the results.

Table 2 Recommended Test Equipment

Performance Testing

Instrument Critical Specifications

Sweep oscillator Frequency: 0.1 to 2.0 GHz

Modulation: 1kHz square wave

Leveling: —V proportional to RF

Low-pass filter Rejects: second harmonics lowest frequency

Rejection: ≥50 dB at 1.25 f
Network analyzer No other network analyzer is recommended
SWR meter Frequency: compatible with sweep oscillator

Accuracy: ±0.1 dB
X-Y recorder Impedance: 200 K ohms/V

Sensitivity: ≥50 mV/in
Crystal detector (2) Frequency: 0.1 to 2.0 GHz

Sensitivity: >4.0 mV /µW
Coaxial attenuator Frequency: 0.1 to 2.0 GHz

Attenuation: 20 dB or 10 dB
Low reflection termination SWR: ≤1.005 0.1 MHz to 2.0 GHz

Type N connectors
Oscilloscope Vertical Sensitivity: >10 mV/cm

Horizontal Sweep: synchronized ext
Termination

(2) Frequency: band of interest

Load SWR: <1.01, de to 2 GHz

co

Dual directional coupler Frequency: band of interest

Directivity: >40 dB

Impedance: 50 ohms coaxial
Short 7 mm coaxial Type N

Agilent 778D Operating and Service Manual 9

Performance Testing

Table 2 Recommended Test Equipment (Continued)

Instrument Critical Specifications

Adapter (2) Type N male to male
1 kHz Filter Adapter Rejects 1 kHz and above

Figure 7 Typical Coupling Variation with Frequency

10 Agilent 778D Operating and Service Manual

Table 3 Performance Test Record

Tested by:
Date:
Dual-Directional Coupler:
Instrument Serial Number:
Directivity:
Auxiliary Arm: 0.1 to 1.0 GHz 1.0 to 2.0 GHz

A _________dB (≤—36dB) _________dB (≤—32dB)

B _________dB (≤—30dB) _________dB (≤—30dB)

SWR Measurement
Primary Line ___________ (≤1.1) ___________ (≤1.1)
Auxiliary Arm ___________ (≤1.1) ___________ (≤1.1)

Performance Testing

Swept-Frequency

Directivity (in d B) is equal to:

Directivity

P

()when Pmain flows forward

10′

where

P = Power
aux = auxiliary arm
main = main arm

NOTE The power input to the main arm is the same for both measurements.

To measure the directivity proceed as follows:

1. Connect the equipment as sho wn i n Figure 8 on page 12 (the purpose of
the 1 kHz low-pass filter input to the oscilloscope is to reduce noise).

aux

----------------------------------------------------------------------------------- - log

P

()when Pmain flows reversed

aux

Agilent 778D Operating and Service Manual 11

Performance Testing

The setup as shown will measure the directivity of port A. To measure
the directivity of port B reverse the coupler.

Figure 8 Swept-Frequency Directivity Setup

2. Set the controls as shown in Table 4 on page 12.

Table 4 Control Settings

Characteristic Setting
Network Analyzer

Frequency range 1 to 2 GHz (set Sweep Stability Vernier to phase lock

display)

Amplitude (test Channel Gain) 20 dB

Sweep Oscillator (see network analyzer manual for proper output level).

Function Start-Stop
Start/CW 1.0 GHz
Stop/∆ F 2.0 GHz
Sweep Selector Auto
Sweep Time 1.0–0.1 (set vernier to phase lock display)

12 Agilent 778D Operating and Service Manual

Table 4 Control Settings (Continued)

Characteristic Setting
Vertical Sens 50 mV, calibration to make 1 dB gain change on

Horizon Sens 1 V/cm
Sweep Mode Ext
Input DC

3. To establish a reference remove the low reflection termination. This
open will giv e a condition of 10 0% reflection. If a goo d short is a vail able
use also and take the amplitude reference as the average between the
shorted and open conditions.

4. Be sure network analyzer is phase locked over entire range. The display
on the oscilloscope should be an unbroken line. If not, adjust network
analyzer SWEEP STABILITY and/or AMPLITUDE controls to phase
lock over the entire sweep.

Performance Testing

network analyzer equal 1 cm vertical change on CRT

5. Zero network analyzer phase gain meter on 10 dB AMPLITUDE range
with AMPL VERNIER control or AMPLITUDE TEST CHANNEL
GAIN (dB).

6. Draw a horizontal line with a grease pencil on the CRT somewhere
above the middle X-axis line.

7. Place the average of the reference trace on the grease pencil line by
adjusting the oscill oscope VERTICAL POSITION control.

8. Put the low re flection termination back o n the coupler as shown in

Figure 8.

9. Increase the network analyzer TEST CHANNEL GAIN (DB) by the
amount of the directivity specification for this frequency range.

10. The entire trace should stay below the grease pencil line. If a portion of
the trace goes above the line the coupler may or may not be out of
specification (the reflection from the load may be adding to the
directivity of the coupler). Test these ambiguous frequencies with the
fixed frequency testing following.

11. Repeat this test at the other frequency range given in the

“Specifications” on page 2. Then turn the coupler around to test the

directivity at port B and repeat the above tests.

Agilent 778D Operating and Service Manual 13

Performance Testing

Termination In the preceding of the following tests use the best termination obtainable.

The accuracy of t he tests will depend to a gre at ext ent upon th e quality o f the
termination . Note that the quality of the termination in cludes any adapter
necessary for connection. The adapter reflection coefficient will add
vectorially to th e termination reflection co efficient. These instructions
assume the use of a termination with a total SWR of 1.005 or less.

SWR Measurement SWR in this procedure will be measured in terms of reflection coefficient.

Because the directional couplers used in this procedure have finite
directivity, the measurement error due to dire ctivity is taken into
consideration in determining the test limits.

1. Connect the equipment as shown in Figure 9.

Figure 9 SWR Measurement Setup

2. Set controls as shown in Table 4 on page 12.

3. Set sweep oscillator to sweep an octave band in the range 0.1 to 2 GHz
with no modulation.

4. To establish a reference, remove the termination. This open will give a
condition of 100% reflec ti on. I f a g ood short is available, use it also and

14 Agilent 778D Operating and Service Manual

Performance Testing

take the amplitude reference as the average between the shorted and
open conditions.

5. Be sure network analyzer is phase locked over entire range. The display
on the polar display should be an unbroken circle. If not, adjust netw ork
analyzer SWEEP STABILITY and/or AMPLITUDE controls to phase
lock over the entire sweep.

6. Push and hold the polar display’s BEAM CTR push button and adjust
HORIZ POS and VERT POS controls to place the dot in the center of
the graticule. To bring the dot onto the display, rotate each positioning
control about five turns counterclockwise or until a slight increase in
resistance to movement is encountered.Then turn each control two and
one-half turns cl ockwise.

7. Center the trace on the outer graticule circl e, using th e netwo rk analyz er
position and amplitude controls. If a good short is availabl e, average the
position between the open and shorted indications. Note TEST
CHANNEL GAIN reading.

8. Increase TEST CHANNEL GAIN by 26 dB. The outer graticule is now
calibrated to a reflection coefficient of 0.05 and the inner graticule to

0.01.

9. Connect the mainline of the 778D under test in place of the termination
shown in Figure 9. Terminate the mainline of the 778D under test with
the best termination available. A poorer termination may be used to
terminate th e auxiliary arms.

10. The indication should be less than the following limits:

0.1 to 1.0 GHz:0.032

1.0 to 2.0 GHz: 0.022

11. If thes e limit s are ex cee ded at a ny fr equenc y, set the sweep oscil lator fo r
CW operation at this frequency.

12. Remove the 778D under test from the reflectometer and connect a
low-reflection termination to the reflectometer.

13. With the VERT an d HORIZ Position controls set the dot to the cent er of
the graticule. This effectively cancels out the directivity signal at this
frequency.

14. Remove the low-reflection termination and reconnect the 778D under
test. The indication should be less than 0.045 (0.1 to 2.0 GHz).

Agilent 778D Operating and Service Manual 15