Marconi Instruments TF 2502 User manual

TF-2502-Marconi-1.jpg

R.P. lc
11/78/E
Instruction Manual
No. EB 2.502
for
R.F. Power Meter
TF 2502
•
m1
rechnic~O
LLI::Jitcations
©
1969
MARCONI INSTRUMENTS LIMITED
ST. ALBANS HERTFORDSHIRE ENGLAND
EB 2502
lb- 11/74

Contents
Chapter 1 GENERAL INFORMATION
1. 1 Introduction
1. 2 Data summary
1. 3 Accessories
Chapter 2 OPERATION
2. 1 General ...
2.2 Controls .. ·.
2.3 Connections
2.4. Measurement precautions .
2.5 Making power measurements
2.6
Remote operation
2. 7 Determination of modulation
depth
Decibel conversion table
3
4
5
6
6
7
8
8
9
9
11
Chapter 3 TECHNICAL DESCRIPTION
3. 1 Circuit analysis ...
3. 2 Detector unit
3. 3 Terminating load ..
2
13
14
14
Chapter 4 MAINTENANCE
4. 1 General . . . 15
4. 2 Removal and replacement of
components . . . 16
4. 3 Removal of terminating load
and detector unit 16
4. 4 Removal of monitor front panel16
Chapter 5 CALIBRATION
5. 1 Apparatus required 17
5. 2
Preliminary checks 17
5. 3
Standing wave ratio 17
5. 4
Calibration accuracy at l. f. . 18
5. 5
Calibration accuracy at r. f. 19
Chapter 6 REPLACEABLE PARTS
Introduction and ordering
Chapter 7 CIRCUIT DIAGRAMS
Circuit notes ...
Fig. 7. 1 Circuit diagram
21
24
25
2502 (1)

General information
1.1 INTRODUCTION
TF 2502 is a 50 Q absorption power meter for measurements from d. c. to
1 GHz in ranges of 3 Wand 10 W full-scale. It is one of a series of similar instru-
ments having different power ranges.
The design is based on thin-film and microwave techniques and employs a
thermocouple detector in series with a terminating load. Incident power is sampled
by tbe detector and the resultant e. m. f. is displayed on a meter calibrated in terms
of totalincident power. This method gives a reading of true mean power irrespective
of waveform and makes the instrument particularly suitable for measurim:; the output
of a.m. or pulse transmitters. A socket is provided to allow the demodulated signal
to be monitored on headphones or an oscilloRcope.
The disadvantage of the long time constant inherent in a thennocouple meter is
overcome by the provision of an auxiliary fast response diode detector with a separate
small meter. This allows you to quickly tune a source under test for maximum output
before measuring its power level on the main meter.
Fig. 1.1
R.F. Power Meter TF 2502
2502 (1)
3

The instrument consists of three separable items a detector unit, a monitor
unit and a terminating load and requires neither batteries nor mains supply.
Fig. 1.2 Terminating load TM 8544
1.2 DATA SUMMARY
Frequency range:
Power ranges:
Accuracy:
Input impedance:
V.S.W.R.:
Overload capacity:
Detector characteristics:
Output facility:
Temperature range:
Dimensions and weight:
4
Fig. 1.3 Detector Unit TM 8723/2
D. C. to 1 GHz.
3 Wand 10 W full-scale.
±5% of full-scale.
50 Q via type N socket.
Better than 1. 1:1.
20% continuously.
Up to 50% for periods up to 30 s.
(1) Calibrated thermocouple giving an approximately
linear power scale.
(2) Uncalibrated fast response diode detector
operating down to 5 MHz.
Jack provided for monitoring modulation.
+10 octo +35 oc.
Height
5 1/8 in
(130 mm)
Width
8 5/8 in
(219 mm)
Depth
4 3/4 in
(121 mm)
Weight
3lb
(1. 4 kg)
2502 (1a)

1.3 ACCESSORIES
Supplied
Type N free plug, M. I. code 23443-708; for r. f. input.
Telephone plug, M. I. code 23421-610; for modulation output.
Knurled retaining ring, M. I. code 33265 -309R, for securing r. f. input
connector during transit.
2502 (1) 5

Operation
2.1 GENERAL
The power meter has dual readout facilities - a large meter directly calibrated
in mean power and a small meter with an arbitrary calibration for use as a fast
response tuning indicator.
2.2 CONTROLS
MARCONI INSTRUMENTS IJD
Fig. 2.1 Layout of controls
Q) Type N input socket accepts inputs up to 60 V peak.
0 Thermocouple meter shows mean power in 50 Q load. Near linear scales of
0 to 3 W and 0 to 10 W are selected by Range switch.
0 Power range switch selects 3 W or 10 W full-scale on WATTS meter.
0 DIODE DETECTOR meter provides fast response tuning indication for the
source under test. Operates down to 5 MHz. Scale arbitrarily engraved from 0 to
100.
@ Sensitivity control adjusts the gain of the diode detector. The crescent
symbol indicates that sensitivity increases clockwise.
6 2502 (1)

0 INSERT PROBE control adjusts the input to the diode detector.
0 MOD OUT jack accepts 2-pole telephone plug for connection to oscilloscope or
high impedance headphones to monitor amplitude modulation.
2.3 CONNECTIONS
0·223~ :!: ·009"
Fig. 2.2 Cross section of type N connector (male)
showing critical dimension
A knurled retaining ring is fitted to the type N connector to prevent movement of
the detector unit during transit. Before using the power meter, unscrew this ring and
pull off the plastic cap from the connector. Keep both these items for re-use if the
instrument has to be transported.
!cAUTION I When
making connection to the input socket it is important to use the
free type N connector supplied as an accessory. If another type N
connector is used then it is essential to ensure good reliable mating.
To achieve this the inner contact depth, i. e. the distance from the
shoulder on the pin to the interface as shown in Fig. 2. 2, must be
0.223 in ±0.009 in (5.92 mm ±0.23 mm) conforming to US Spec.
MIL-C-71B.
Type numbers of coaxial cables suitable for use with the plug are as follows:
United Kingdom: Amphenol No. 82-312
Joint Services 5935-99-943-4155.
United States: Military No. UG-1185/U.
NOTE: To prevent accidental interchange of parts with those of other instruments in
the series, the following items of the R. F. Power Meter TF 2502 are colour
coded YELLOW. In addition identification labels, bearing the same serial
number, are fitted on each of the units which make up the complete instrument.
(a) The plastic cover of the detector unit.
2502 (1a) 7

(b) The catch on the terminating load.
(c) The plug and socket of the cable joining thedetector unit and the monitor.
2.4 MEASUREMENT PRECAUTIONS
When malting a measurement, the following points should be borne in mind:
1) Ideally, the impedance presented by the power meter to a transmitter under
test should be purely resistive. In practice, the power meter impedance will,
inevitably, include a small reactive component which will be reflected back into the
transmitter tuned output circuit, partially detuning it. In order to obtain a true
measure of the power output capabilities of a transmitter, its output stage should
always be tuned to produce maximum indication on the power meter.
2) In circumstances where the power meter is required for use merely as a
terminating device, it is better to make connection directly to the terminating load
as this presents a particularly high quality resistive termination having a v. s. w. r.
of about 1. 04. To remove the load for this purpose see Sect. 2. 6.
3) Because of the time lag in thermocouple heating, and the low resistance of the
meter circuit, wait a few seconds before reading the power.
4) The peak voltage that can be applied to the power meter is limited by the in-
line detector insulation; this is rated to withstand 60 V peak, which exceeds the
peak voltage of a 100% sine wave amplitude modulated signal of 10 W mean power.
5) In the event of long-term storage in conditions of extreme humidity, check 50 Hz
calibration before use (refer to Sect. 5. 4). If necessary, dry out by placing the power
meter in an oven (+50 °C to +70 °C) for at least one hour.
2.5 MAKING POWER MEASUREMENTS
To make a power measurement proceed as follows:
1) Set the Range switch to suit the expected power level of the source. If this is
unknown, use the 10 W position as a precaution.
2) Turn the INSERT PROBE and Sensitivity controls fully counter-clockwise.
3) Connect the source under test to the Input socket and note the reading on the
DETECTOR meter. Bring this reading to about half scale by turning up the Sensi-
tivity control. If this is insufficient, turn the INSERT PROBE control clockwise.
NOTE: If the probe is inserted too far, the v. s. w. r. of the instrument may
increase. Do not insert it until the Sensitivity control is at maximum.
8 2502 (1a)

4) Tune the source under test for maximum reading on the DETECTOR meter.
5) If the probe was inserted in (3) above, turn the INSERT PROBE control back
to its fully counter-clockwise position. Then read the power of the source on the
WATTS meter.
2.6 REMOTE OPERATION
If the power meter has to be connected directly to the power source it may be
inconvenient to read or operate the instrument. If so, it may be better to use it
with the detector unit and load detached from the monitor unit.
To do this, pull out the INSERT PROBE knob to disengage the dog clutch,
depress the catch on the ternunating load and withdraw the load and detector assembly
as far as the interconnecting lead will allow.
Operation is the same as in Sect. 2. 5 except that probe insertion is controlled
directly by the slotted knob on the detector unit.
2.7 DETERMINATION OF MODULATION DEPTH
The depth of amplitude modulation of an r. f. signal having a sinusoidal envelope
can be determined as follows:
1) Measure the output power of the source under test with the signal unmodulated.
Let this reading be Pc watts.
2) Modulate the signal and again measure the output power. Let this reading be
Pm watts.
3) Calculate the modulation depth. This can be evaluated since P c, P m, and the
%modulation, m, are related by
2502 (1a)
pm p c
m2
(1 + 2)· .................... (1)
9

Transposing and simplifying, equation (1) gives
m
Example: From measurements, it is found that
1. 1 Wand Pm = 1. 5 W
Hence from equation (2) J2
(1.5 - 1.1) 1 (JJ
m = x. 001o
1.1
85.5%
10
2501 (1)

DECIBEL CONVERSION TABLE
Ratio Down
Ratio Up
VOLTAGE
POWER DECIBElS
VOLTAGE
POWER
1-0
1·0 0 1 0 1·0
·9886 ·9772
·1
1·012
1·023
·9772 ·9550 ·2 1·023
1·047
·9661
·9333 ·l 1·035
1·072
·9550 ·9120 ·4
1-047
1 096
·9441
·8913 ·5
1·059 1·122
·9333 ·8710 ·6
1-072
1·148
·9226
·8511 ·7
1-084 1·175
·9120 ·8318 ·8 1·096 1·202
·9016 ·8128 . ·9
1-109
1·230
·8913 ·7943 1·0 1·122 1·259
·8710 ·7586
1-2
1-148
1·318
·8511
·7244
H
1·175
1·380
·8318 ·6918 1·6
1-202
1·445
·8128 ·6607 1·8
1-230
1·514
·7943 ·6310 2·0 1·259 1·585
·7762 ·6026
2-2 1·288 1·660
·7586 ·57
54
2-4 1·318 1·738
·7413 ·5495 2·6 1·349
1-820
·7244 ·5248 2·8
1-380
1-905
·7079 ·5012
l·O
1-413
1·995
·6683 ·4467
3·5 1·496
2·239
·6310
·3981
4·0 1·585
2·512
·59 57
·3548
4-5 1·679
2·818
·5623
·3162
S·O
1-778 3·162
·5309 ·2818 5·5
1·884 3·548
·5012 ·2512 6
1·995
3·981
·4467
·1995
7
2·239 5·012
·3981
·1585
8 2·512
6·310
-3548
·1259 9
2·818
7·943
·3162
·1000
10 3·162
10·000
·2818 ·07943
11 3·548 12·59
·2512 ·06310
12 3·981
15·85
·2239 ·05012
13 4-467
19·95
·1995
·03981
14 5·012
25·12
·1778
·03162
15 5·623
31-62
2501 (1)
II

DECIBEL CONVERSION TABLE (continued)
Ratio Down
Ratio Up
VOLTAGE
POWER
DECIBELS
VOLTAGE
POWER
·1585 ·02512
16
6·310 39·81
·1413 ·01995 17 7·079 50·12
·1259 ·01585
18
7·943 63·10
·1122 ·01259
19
8·913 79·43
·1000 ·01000
20
10·000 100·00
·07943 6·310 X 10-3 22 12·59 158·5
·06310
3·981 X 10-' 24
15·85 251·2
·05012 2·512 X 10-'
26
19·95 398·1
·03981 1·585 X 10 3
28
25·12 631·0
·03162 1·000 X 10 3
30
31-62 1,000
·02512 6·310 X 10 4
32
39·81 1·585 X 103
·01995
3·981 x 1 o-•
34
50·12 2·512 X 103
·01585 2·512 X 10-4
36
63·10 3·981 X 103
·01259 1·585 x 1
o-•
38
79·43 6·310 X 103
·01 000 1·000 X 10 4 40 100·00 1·000 X 104
7·943 X 10-3 6·310 X
10-s
42
125·9 1·585 X
10'
6·310 X 10-'
3·981 X 1 o-s
44
158·5 2·512 X 104
5·012 X 10-' 2·512 X 1
o-s
46
199·5 3·981 X 104
3·981 X 10 3 1·585 X
10-s
48
251·2 6·310 X 104
3·162 X 10-3 1·000 X
10-s
50
316·2 1·000 X 105
2·512 X 10 3 6·310 x 1o-• 52
398·1 1·585 X 105
1·995 X 10-'
3·981 x 1 o-•
54 501·2 2·512 X 105
1·585 X 10-' 2·512 X 10 6
56
631·0 3·981 X 105
1·259 X 10-' 1·585 X 10-'
58
794·3 6·310 X 105
1·000 X 10 3 1·000 X 10-'
60
1,000 1·000 X 106
5·623 x 1 o-• 3·162 X 10-7
65
1·778 X 103 3·162 X
10'
.3·162 x 1 o-• 1·000 X 10-7
70
3·162 X 103 1·000 X 107
1-778 X 10 4 3·162 X
10-a
75
5·623 X 103 3·162 X 107
1·000 x 1o-• 1·000 X 10 8
80
1·000 X 104 1·000 X 108
5·623 X 10-s 3·162 X 10-'
85
1·778 X 104 3·162 X 108
3·162 X 1 o-s
1·000 X 10-'
90
3·162 X 104 1·000 X
10'
1·000 X 10-s
1·000 X 10-10
100
1·000 X 105 1·000 X 1010
3·162 X 10-6 1·000 X 10-
11
110
3·162 X 105 1·000 X 10
11
1·000 X 10 6
1·Q00 X 10-12
120
1·000 X 10'
1·000 X 1012
3·162 X 10-7 1-ooo x 1o-"
130
3·162 X 106 1·000 X 1013
1·000 X 10-7 1·000 X 10-14
140
1·000 X 107 1·000 X 1014
12
2502 (1)

Technical description
3.1 CIRCUIT ANALYSIS
R. F. Power Meter TF 2502 consists of an in-line thermocouple detector, a
diode detector and a terminating load as shown in Fig. 3.1.
IN-LINE DETECTOR with THERMO-ELEMENT
L~------~~'~"-A_D------~--.
; ~
TC1
THERMOCOUPLE
M2
lWATTSl
Fig. 3.1 Functional diagram of r.f. power meter
A smallpercentage of tlie incident power is absorbed by the heater of the thermo-
couple and the resultant rise in temperature is monitored by a series of bimetallic
junctions, TCl. The thermo~electric e. m. f. thus produced is then measured by a
d.
c. millivoltmeter, M2, calibrated in terms of total power in watts.
Power applied to the thermo-element produces a current, I, in the heater
resistance, R5, with a resultant heat dissipation of I2R5. The thermocouple output
voltage is consequently proportional to 12 since its e. m. f./ temperature response is
nearly linear. Hence the detector gives a d. c. output proportional to true mean
power, and independent of the waveform of the power.
The input voltage is sampled by a capacitance probe, rectified by diode MR1
and monitored by meter M1 which has an arbitrary 0-100 calibration. Potentiometer
RV3 adjusts the sensitivity of the meter. This circuit gives a fast-response indication
and also provides a demodulated output at the MOD OUT jack, JKA. for external
monitoring.
2502 (1)
13

3.2 DETECTOR UNIT
The thermocouple detector is an in-line type consisting of a modified slab
transmission line with a tubular thin-film resistive heater component forming part
of the centre conductor. Four series connected thermocouples are mounted close
to the heater surface. Beryllia* cement is used in this assembly.
3.3 JW TERMINATING LOAD TM 8587
The terminating load see Fig. 1. 2 - is a thin-film component with a substrate
made of Beryllia*. This has the high thermal conductivity of aluminium combined
with excellent dielectric properties of an alumina ceramic. It is housed in a 3 W
heat sink.
*See warning in Sect. 4.2
14
2501 (1)

Maintenance
4.1 GENERAL
The circuit diagram shows all the electrical components contained in the instru-
ment. The description of these components - their type, value, rating etc. -is given
in Chap. 6, and the layout of components in the monitor unit is shown in Fig, 4. 1.
Fig. 4.1 Rear of front panel
Screw fasteners
Screw threads used on this instrument are of the following sizes: 4BA, 6BA
and 8BA.
Cruciform head screws are of the Pozidriv pattern; to avoid damaging them a
Pozidriv screwdriver should be used.
2501 (lj
IS

Calibration
The following information, based on abstracts from the Company Test Schedule,
is intended to enable you to carry out a series of tests by which the main points of
performance of the instrument can be checked; it also gives details concerning adjust-
ment to preset components and the choice of value for individually selected components.
5.1 APPARATUS REQUIRED
(a)
Resistance bridge, e .. g. mT type TF 2700.
'(b) Signal generator, 400 MHz and 1 GHz, 50 Q output; e.g. mf types TF 801
and TF 1060.
(c) V. S. W. R. measuring device, 50 Q characteristic impedance with residual
v. s. w. r. of better than 1. 01 at 1 GHz.
(d)
Variable a. c. source, capable of delivering 10 W; e. g. a Variac.
(e) Dynamometer wattmeter, 0 to 10 W, 0.5%.
5.2 PRELIMINARY CHECKS
(Apparatus required: item a)
NOTE: When carrying out these checl{s, it is advisable to set the Range switch
to 10 W to avoid the possibility of overloading the meter.
1) . Connect the termi,nating load and measure the ·cold d. c. resistance of the
terminating load (R4) between the centre and outer conductors of the socket (SKTB).
The resistance should be 49 Q ±2%.
2) Measure the resistance of the complete power meter (i.e. , in-line detector
and the terminating load) between the centre conductor of the input socket SKTA and
chassis. The resistance should be 53 Q ±3%.
5.3 STANDING WAVE RATIO
(Apparatus required: items b and c)
For the power meter to act as a good match to a transmission line its v. s. w. r.
must be as close to unity as possible. In the TF 2502, v. s. w. r. is a function of the
2502 (1a)
17

general configuration and spacing of the transmission line, and no provision is made
for adjustment. The validity of subsequent calibration steps is dependent on verifi-
cation of good v. s. w. r. over the full frequency range. During manufacture a sweep
frequency method is used to achieve this, but another method can be used if tests are
made at a sufficient number of frequencies.
Connect the v. s. w. r. measuring device to the socket on the terminating load·
and check the v. s. w. r. at a selection of frequencies up to 1 GHz. The v. s. w. r.
should be better than 1. 05 over the whole frequency range.
With the INSERT PROBE control fully counter-clockwise, connect the v. s. w. r.
measuring device to the power meter input socket and check the v. s. w. r. of the
combined terminating load and in-line detector at a selection of ~requencies up to
1 GHz. The v. s. w. r. should be better than 1.1 over the whole frequency range.
5.4 CALIBRATION ACCURACY AT.L.F.
(Apparatus required: items d and e)
The power-reading accuracy is best checked at a low frequency - usually the
frequency of the local power supply - and then compared with that ·of a standardized
mean-reading power meter near the upper limit of the frequency range.
1) Connect the dynamometer wattmeter between the a. c. supply and the power
meter under test. The current and voltage coil connections should be arranged for
minimum error due to internal losses, and allowance should be made for the error
if it is appreciable. Owing to the frequency response limitations of dynamometer
wattmeters the a. c. supply must have low distortion (less than 1 %) .
2) Set the Range switch to 10 Wand set the power input until the dynamometer
indicates the input figure given on the label at the rear of the instrument.
3) Check that the TF 2502 reading is 8 W ±0. 5%, if not adjust RV1 until it is
correct.
4) Reduce the applied power to zero and allow sufficient time for the thermocouple
to cool.
5) Set the Range switch to 3 W and set the power input until the dynamometer
indicates the input figure given on the label at the rear of the instrument.
6) Check that the TF 2502 reading is 2. 5 W ±0. 5%, if not adjust RV2 until it is
correct.
16
2502 (1)

5.5 CALIBRATION ACCURACY AT R.F.
Calibration is difficult at r. f. since, unless great care is taken with impedance
matching, substantial errors can be introduced. If it is essential to make checks at
r. f. , then connections should be made with a wide band directional coupler having a
directivity of at least 35 dB and the power level monitored with a low level power
meter. Such an arrangement must, of course, be calibrated by reference to a
standard power meter.
NOTE If either a terminating load or a detector unit is to be returned to Marconi
Instruments for repair or recalibration, it is important to send the complete
detector /load assembly. This will enable the item to be recalibrated as an
assembly, which will ensure a better performance, e.g. v. s. w. r. and fre-
quency response, than can be obtained by recalibrating the detector and load
separately.
2502 (la) 19