GENERAL RADIO COMPANY 874-GAL User manual

OPERATING INSTRUCTIONS
TYPE 874-GAL
ADJUSTABLE ATTENUATOR
DESCRIPTION
The Type 874-GAL Adjustable Attenuator is of the wave-guide-
below-cutoff type operating in the TE1
~ mode (inductive coupling).
The waveguide is excited by a coaxial trankmission line, as shown in Fig -
ure 1, and the output voltage is induced in a loop mounted on a sliding
plunger. One end of the loop is terminated in a 50-ohm resistor and the
otheris connected to the output cable. This arrangementmakes the output
impedance of the attenuator approximately 50 ohms. The attenuation is
varied by rotating the barrel whichmoves the plunger and loop along the
tube. The barrel is calibrated in relative attenuation in dB. The attenu -
ators can be adjusted from the stop at the minimum attenuation position,
which occurs at about -9dB on the calibrated scale, to the stop at the max -
imum attenuation end of the range, which occurs at about 140 dB on the
scale. Each revolution of the barrel changes the attenuation 20 dB. In
most applications the maximum usable attenuation is limited by leakage
from various parts of the driving circuit and not by the range of the atten -
uator. In this type of variable attenuator, as the attenuation is varied the
phase at the output connection is essentially constant with respect to the
input signal phase.
The method of coupling to the coaxialline permits the attenuator
to beused as a standard calibratedattenuator or as a device to couple a
small amount of energy from a high-level circuit for monitoring or meas -
uring purposes without introducing a large discontinuity in the main line.
The Type 874-GAL Coaxial Attenuator employs the new Type 874—BL
Locking Coaxial Connectors. These connectors can be used interchange -
ably with the standard GR874~ Connectors. When mated with another
locking connector, a firm mechanical coupling of the two is achieved
when the coupling shell is engaged. The shielding is also improved sig -
nificantly, and, in general, the leakage is reduced by at least 50 dB.
The “quick-disconnect” feature of thestandard GR874 Coaxial Con -
nectors is retained in the locking type if the coupling shell is not en -
gaged. However, in this case the shielding is not as effective.

ERRORS
In all waveguide attenuators, the excitation of modes other than the
desired one distorts the theoretical attenuation characteristic, particularly
at low attenuation levels. In the Type 874-GAL Attenuator,the TM0
1
mode (capacitive coupling) is the most serious offender. This mode
is attenuated more rapidly with distance from the source end of the wave-
guidethan is the desired mode and its intensity is proportional to the volt -
age across the exciting line at the coupling point,while the intensity of
the desired mode is proportional to the current flowing in the exciting line
at the point of coupling. The effect of the undesired mode on the attenu -
ation characteristic therefore decreases as the attenuation increases. Since
the output from the attenuator is the sum of the voltage generated in the
loop by both modes, serious errors can result if the voltage in the exciting
line at the coupling point is high compared to the current; that is, if the
impedanceis high. An error correction procedure is required to achieve
the stated accuracy for the 874-GAL Attenuator.
CORRECTION PROCEDURES
There are two configurations that may be employed in using the
874-GAL. First,it may be terminated in a 50-ohm termination (874-W5OBL)
as shownin Figure 2,in which case the special correction given in Fig -
ure 2 is required, and, in addition, thefrequency correction of Figure 3 is
required. In the second configuration recommendedto achieve the best
accuracy above 2 GHz, the termination is an adjustable stub employed to
place a voltage minimum at the coupling points. The frequency correction
graph of Figure 3 is required only in this case. The detailed procedure
is given in the following paragraphs.
If the exciting line is terminated in its characteristic impedance, for
instance with a Type 874-W5OBL Termination, the error resulting from the
coupling of the TM0 1 modecan be corrected by using the correction chart
of Figure 2. The ~ign of the correction depends on which end of the
exciting line is connected to the source of rf power. If the source is con -
nected to the end marked with the letter S, use the correction curve labeled
NORMAL and subtract the correction found for initial setting of the atten -
uator from the initial scale reading. Then subtract the correction found
for the finalsetting of the attenuator from the final scale reading. The
trueattenuation is the difference in thecorrected values. If the source
is connected to the opposite end, add to the indicated values the corrections
read from the curve labeled REVERSED. When these correction charts are
used, the indicated relative attenuation is accurate within ±(O.015 times
the difference +0.2)dB at frequencies up to 2000MHz. For accurate results
2 GENERAL RADIO

at higher frequencies for small values of attenuation, the method outlined
in the following paragraph shouldbe used.
To eliminatethe correctionand to make the indicated relative atten -
uation agree very closely with the actual attenuation between —9 and 140
dB on the scale, the voltage across the line, at the actual coupling point,
should approach zero. To accomplishthis, connect an adjustable short-
circuiting stub (Type 874-D2OL or -D5OL) to the end of the main line op -
posite the source. Adjust the stub until a voltage minimum appears at the
coupling point. To find this stub adjustment, substitute a Type 874-VRL
Voltmeter Rectifier and a Type 874-VI Voltmeter Indicator, or a Type 874-
TL Tee and a detector, for the attenuator. Adjust the stub until the indi -
cated voltage is minimized; then replace the attenuator. (If the Type 874-
VRL is used,connect the end containing the series resistor, engraved R,
to the generator.)If the frequency is changedthe stub mustbe reset, but a
frequency calibration can be marked on the stub, for convenience when a
number of measurements are to be made. At frequencies below 300 MHz, a
Type 874-WNL Short Circuit can be substituted for the stub, without fur -
ther adjustments. With the Type 874-WNL Short Circuit, accurate results
can be obtained at scale readings greater than zero. At frequencies below
1500 MHz, the indicated attenuation is accurate at scale readings as low
as —9 dB, and, at frequencies between 1500 and 4000 MHz, readings above
0 dB on the calibrated scale are accurate if the stub is set as previously
outlined.
An alternate method of setting a voltage minimum at the coupling
point,f or frequencies above 1500MHz, utilizes the factthat the attenuation
constant for the desired mode is smaller than that of the spurious modes.
If the attenuator is set to a relatively large value of attenuation, thestub
can be properly set by adjusting it until maximum output is obtained from
theattenuator. However, a voltage minimum does not occur at the coupling
point unless the effective sourceimpedance, seen looking back toward the
generator from the coupling point, is a pure resistance. When the genera -
tor and the detector are not matched, the above condition can be obtained
by the use of suitable pads (Type 874-G1OL or -G2OL) at the generator
and the detector.
At the higher frequencies the attenuation is reduced as the wave-
guide approaches its cutofffrequency. To correct for this effect, multiply
the indicated attenuation by the factor found in the frequency correction
graph, Figure 3, corresponding to theoperating frequency.
The impedance of the adjustable arm of the attenuator is nominally
50 ohms. However, the impedance varies appreciably with frequency, as
shown by the plot in Figure 4 of the VSWR seen looking back into the out -
put connector for a typical unit. The tolerance on the dc resistance of the
50-ohm resistor is ± 10%.
TYPE 874-GAL ADJUSTABLE ATTENUATOR 3

APPLICATIONS
(1) The attenuator can be used with a Type 874-YR or -VRL Volt -
meter Rectifier and a Type 874-VI Voltmeter Indicator to provide the level-
monitor and variable attenuator of a signal generator, when combined with
a General Radio Oscillator. TheTypes 1362, 1363 and 1218 are recom -
mended. A block diagramof a typical setup is shown in Figure 5.
In this case the stub is adjusted asindicated in a previous paragraph
and the adjustable line1
(Type 874-LAL) is adjusted to obtain a maximum
indication from the Type 874-VI Voltmeter Indicator. The voltage indi -
cated with the load connected is the effective open-circuit output voltage,
and the source impedance is accurately 50 ohms. The output can be reduced
by adjusting the attenuator;the actual voltage at any attenuator setting is
the indicated number of db below the original calibrating value.
Sincethe source impedance is the same as the characteristic imped -
ance of the line, the same effective open-circuit voltage and source im -
pedanceare obtained at the end of any length of 50-ohm line connected to
the outputof the voltmeter rectifier. If the line is not lossless, the ef -
fective open-circuit voltage will be reduced by the loss in the line.
(2) Theattenuator can also be used to measurethe attenuation of a
network by the substitution method, as indicated by the block diagramin
Figure 6.
Readings of the detector output and attenuator setting are first made
without the circuit under test connected. The circuit is then inserted and
the Type 874-GAL Adjustable Attenuator is readjusted to give the same
detector indication. Theattenuation of the unknown circuit is thenthe
difference in the attenuator readings. The pads indicated are used to
make the source and load impedances very close to 50 ohms. Oneor both
can be omitted if the source and detector impedances are matched.
(3) For monitoring purposes the input line of the attenuator can be
connected in series with the line under test without introducing an appre -
ciablereflection or loss. The changes in level in the main line can be
measured accurately using the attenuator and an uncalibrated detector which
may be 1) a Type 874-VRL Voltmeter Rectifier with a dc microammeter or
an audio amplifier such as the General Radio Type 1232-A Tuned Ampli -
fier and Null Detector if the signal is modulated or 2) if the signal is not
modulated, the General Radio Type 1241 Heterodyne Detectors may be
‘At the lowerfrequencies, it may be necessary to add lengths of Type 874-L30
AirLine in series with the adjustable linetoobtain the maximum output.
4 GENERAL RADIO

employed. The VSWR introduced in a flat line by the insertion of the at-
tenuator is shown in Figure 7.
(4) It can be used as a phase inverter if the flexible leg is driven,
producing equal outputs 180° out of phase at the remaining legs.
RW
~
5.0
~
~30
~
~
H20
~
1.0
0.9
0.8
0.7
0.6
05
~-~U-~ ~
~~, ~GE~ ~ ~ TYPE 874-W5OBL
~
~ HTYPE
874-GAL
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~t- ~ ~j~i1Jl~J111~
-~ L~ ~~
—~ When source Is connected to end of at- -~ - -
— tenuator marked S, subtract correction -
labeled NORMAL from reading. For t -
opposite connection, add REVERSED a - - -
~ correction.
ft it~t. 1I~ I II II
-10 0 0 20 30 40
ATTENUATOR READING A, dB
50 60
874 0.12
Figure 2. Correction chart for use with Type 874-GAL Adjustable
Attenuator when exciting line is terminated in its characteristic
impedance.
Figure 1. Cutaway view of adjustable attenuator.
TYPE 874-GAL ADJUSTABLE ATTENUATOR 5

874 G-lO
(I)
>
g
0
0.06
0.05
— —
— —
—
- -
0.04
— — — — — -
0.03
— — — - - -
- / -
o.or
ii 2
I III
0.0! — E E
— — — — — - — O.OOT
0.005
0.00~
— — — — - —
~‘
Actual AttenuatIon =
(1 - K) a Indicated AttenuatIon
,
1000 2000 3000 4000
FREQUENCY, MHz
Figure 3.
Frequency Correction Factor.
Figure 4.
VSWR seen looking
back into out put con -
nector of a typical
Type 874-GAL Ad.
justable Attenuator.
FREQUENCY. GHz 874 G.8
6 GENERAL RADIO

Figure 6. Block diagram of setup for measuring the attenuation
of a network.
TYPE 874—R22LA
PATCH CORD
1209-5
Figure 5. Block diagram of a simple signal generator usingthe adjustable
attenuator and other GR874 Coaxial Elements.
TYPE 874—D2OL
OR —D5OL
874 G-14A TEST
TYPE 874-GAL ADJUSTABLE ATTENUATOR 7

(I)
>
Figure 7. VSWR introduced in a flat line by the insertion
of a Type 874-GAL Adjustable Attenuator.
SPECIFICATIONS
A waveguide-below-cutoff type, useful as a calibrated
attenuator or as a sampling device. Calibrated in decibels,
on a micrometer-type scale. Absolute attenuation is the
sum of insertion loss and scale reading. Phase shift is
essentially constant as attenuation isvaried. The main line
is a short coaxial section with locking GR874 connectors,
one end for source, the other for load. It introduces mini -
mum discontinuity when inserted in a 50-ohm line. The
loop output is brought out through three feet of 50-ohm
cable with locking GR874 connector.
Calibrated Range: 120 dB (relative attenuation) with input
line terminated in 50 0; 129 dB with input line terminated
in adjustable stub to minimize the electric field at the
coupling point (scale reads —9 to 120 dB).
Insertion Loss (from input connector to end of output cable
at 1 GHz, when signal source impedance is 50 0):
With input line terminated in 50 0, and scale set at 0 dB,
30.4 dB ±2 dB; set at —9 dB, 17 ±2 dB (settings below 0
are not accurate).
With input line terminated in adjustable stub (Which ex -
tends the range over which the calibration is accurate to
the —9 dB scale setting), 19 ±2 dB minimum.
(Insertion loss is approximately inversely proportional to
frequency up to 1 GHz.)
Insertion Loss Directly Through Tee: Negligible.
Accuracy of Attenuation:
Stub-terminated input, ±(0.0l times difference in attenu -
ation reading +0.2) dB, direct-reading.
50-U terminated input, ±(0.015 times difference in atten -
uation reading +0.2) dB, when corrected. Correction chart
supplied.
VSWR Introduced into Line: <1.03 at 1 GHz; <1.12 from 1
to 4 GHz.
VSWR of Output: <4 at 1 GHz; <5 from 1 to 4GHZ.
Max Power: Input power limit inversely proportional to
square root of frequency. Power should not exceed 300 W
at 1 GHz. Output power should not exceed 1/2 W.
Patent No. 2,548,457.
GENERAL RADIO COMPANY
*WEST CONCORD, MASSACHUSETTS 01781
FREQUENCY. GHZ
8 74.0.13
Form No. 0874.0210-I 6/68
Printed in U.S.A.