Attenuation equalizer for correcting a temperature and frequency-dependent cable attenuation

Attenuation equalizer for correcting within a frequency range the temperature and frequency-depending attenuation variation of cables, comprising a cascade arrangement of a fixed and a variable attenuation correction circuit, connected to a control circuit, the variable attenuation correction circuit comprising a bridge-T network having a series arrangement, of two identical series resistors bridged by a variable bridging impedance and a parallel impedance connected between these two series resistors, the fixed attenuation correction circuit having a lower attenuation for higher frequencies than for lower frequencies, the temperature dependency of the cable being eliminated in the variable attenuation correction circuit and the frequency dependence being eliminated mainly in the fixed attenuation correction circuit for which purpose the variable attenuation correction circuit comprises only two adjustable components.

The invention relates to an attenuation equalizer for correcting within a 
frequency range the temperature and frequency-dependent attenuation 
variations of cables, comprising a cascade arrangement of a fixed and an 
adjustable attenuation correction circuit, the adjustable attenuation 
correction circuit being connected to a control circuit and comprising a 
bridged -T network having a series arrangement of two identical series 
resistors bridged by an adjustable series impedance and having a parallel 
impedance connected between the two series resistors, the fixed 
attenuation correction circuit having a lower attenuation for higher 
frequencies than for lower frequencies. 
Such an attenuation equalizer is realized in the Magnavox trunk amplifier 
4-T300, as described in "instruction manual" MX 404-series, pages 91, 129. 
With signal transmission over cables a frequency-depending attenuation 
occurs, higher signal frequencies being attenuated by the cable to a 
greater extent than lower signal frequencies. Furthermore, an apparent 
increase or decrease in cable length occurs when the temperature of the 
cable increases or decreases, in a given proportionality to the 
temperature variation which is expressed in the temperature coefficient of 
the cable. At a higher cable temperature the cable attenuation varies to a 
greater degree with the same frequency variation than at a lower cable 
temperature, in other words the frequency-dependency of the cable 
attenuation is higher at higher cable temperatures than at lower cable 
temperatures. 
The known attenuation equalizer is used to restore a signal transmitted 
over a cable within a frequency range of 40 to 300 MHz at different cable 
temperatures within minimum and maximum cable temperatures occurring in 
operation, to its original equal signal amplitude level. A 
frequency-independent amplification of the signal is effected in an 
amplifier connected in cascade with the attenuation equalizer. 
By means of the fixed attenuation correction circuit of the attenuation 
equalizer the cable attenuation variation is compensated for each 
temperature with a frequency-dependent attenuation variation and adapted 
to the control range of the adjustable attenuation correction circuit. 
With a frequency-dependent attenuation circuit the frequency-dependency of 
the cable attenuation, being different for each temperature, is minimized 
for each temperature by means of the adjustable attenuation correction 
circuit and in a frequency-independent attenuation circuit the 
frequency-independent attenuation levels, which are different at different 
temperatures, are compensated to an attenuation level which is the same 
for each temperature and frequency. 
Both attenuation circuits are implemented as bridged-T networks, each of 
them comprising in the bridge of the series branch as well as in the 
parallel branch an adjustable PIN-diode as a control resistor. To obtain a 
correct value relation between these 4 PIN-diodes at each cable 
temperature a complicated control circuit is required. 
It is an object of the invention to provide an attenuation equalizer in 
which the number of elements to be adjusted is smaller than with the 
state-of-the-art attenuation equalizer, so that a cost effective 
production is possible and, furthermore, a simple control circuit is 
sufficient. 
An attenuation equalizer of the type according to the invention, defined in 
the preamble is therefore characterized in that the value of the bridging 
impedance at higher temperatures is smaller and the value of the parallel 
impedance is greater than at lower temperatures to obtain for one and the 
same frequency variation an attenuation variation of the adjustable 
attenuation correction circuit which is greater at lower temperatures than 
at higher temperatures, the bridging impedance being inductive and the 
parallel impedance being capacitive in said frequency range and the 
attenuation difference of the fixed attenuation correction circuit being 
at least substantially equal to the sum of the attenuation difference of 
the cable, ensuing at the maximum cable temperature occurring in operation 
and the attenuation difference of the adjustable attenuation correction 
circuit, then occurring. 
Application of these measures according to the invention enables the 
simulation in the adjustable attenuation correction circuit of the 
attenuation of a cable having a negative temperature coefficient. 
The temperature-dependent apparent length variation of a cable connected to 
the attenuation equalizer is compensated for in the adjustable attenuation 
correction circuit, which simulates a similar cable having the same, but 
opposite temperature-dependent length variation, to a given length which 
is equal for each cable temperature occurring in operation. To this end it 
is sufficient that only the parallel and the bridging impedances are 
implemented in an adjustable manner. The control circuit required for 
adjusting these impedances can therefore be simple. 
Whereas the elimination of the temperature-dependency of the cable 
attenuation is effected in the adjustable attenuation correction circuit, 
the elimination of the frequency-dependency of the cable attenuation is 
effected in the fixed attenuation correction circuit. 
Both attenuation correction circuits are linear circuits so that the 
circuit sequence of these attenuation correction circuits is not important 
for the functioning of the attenuation equalizer according to the 
invention. 
A preferred embodiment of an attenuation equalizer according to the 
invention, in which the product of the impedance values of the bridging 
and the parallel impedances is at least substantially equal to the square 
of the resistance value of the two series resistors, is characterized in 
that the bridging impedance comprises a series arrangement of a first 
resistor and an inductance, bridged by a first control resistor, and the 
parallel impedance of a second control resistor connected in series with a 
parallel arrangement of a second resistor and a capacitance, the value of 
the first control resistor being smaller and of the second control 
resistor being greater at higher temperatures than at lower temperatures. 
By applying these measures a constant overall flat attenuation 
characteristic is achieved in a simple manner, for each temperature within 
the minimum and maximum cable temperatures occurring in operation, at a 
constant input and output impedance of the variable attenuation correction 
circuit. 
For a correct equalization of the cable attenuation variation the 
attenuation to be added in the variable attenuation correction circuit 
should have a frequency-dependent variation corresponding to the 
frequency-depending attenuation variation characteristic for the type of 
the relevant cable. Starting from a cable whose temperature coefficient 
is, for example, 0.2%/.degree.C. and the apparent length varies between 
100 m and 116 m at temperatures varying between, for example, -20.degree. 
C. and 60.degree. C., the cable is then apparently extended to a length of 
120 m for any cable temperature between -20.degree. C. and 60.degree. C. 
when the minimum attenuation of the variable attenuation correction 
circuit corresponds to, for example, 4 m cable length. The fixed 
attenuation correction circuit must then have a frequency-dependent 
attenuation variation which is opposite to that of the relevant cable of a 
length of 120 m. 
An attenuation variation of the variable attenuation correction circuit 
corresponding to that of the characteristc attenuation of the relevant 
type of cable can be realized by a proper choice of the mutual values of 
the first and second resistor, first and second control resistor, 
inductance and capacitance, incorporated in the above-mentioned preferred 
embodiment. 
In practice the variable and the fixed attenuation correction circuit 
according to the invention also introduces a frequency-independent 
residual attenuation which is, however, not relevant for the equalization 
of the cable attenuation. 
A further preferred embodiment of an attenuation equalizer according to the 
invention, whose first and second control resistor comprise first and 
second PIN-diodes respectively, is characterized in that the control 
circuit comprises a control input, first and second control outputs each 
being connected to one of both PIN-diodes. An adjustable current is 
connected to the control input, as well as a series arrangement of first 
and second pn-semiconductor junctions connected to a fixed current source. 
The series arrangement is arranged in parallel with the series-arranged 
base-emitter junctions of first and second transistors whose collectors 
are connected to the first and the second control outputs respectively. 
The adjustable current source is connected to both the emitter of the 
first transistor and the base of the second transistor. 
The use of this measure results in a control circuit wherein the product of 
the currents at the first and the second control output remains constant, 
while the mutual value of these currents can be adjusted by means of a 
control signal at the control input. In this way a constant product of the 
resistance values of the first and second PIN-diodes, functioning as 
control resistors, is guaranteed, so that the input and the output 
impedance of the variable attenuation correction circuit has the same 
value at any setting of the resistance values of the PIN-diodes.

FIG. 1 shows an attenuation equalizer 50 comprising input terminals 4, 5 
and output terminals 8, 9. The attenuation equalizer 50 comprises a fixed 
attenuation correction circuit 1, connected in cascade with a variable 
attenuation correction circuit 2 via terminals 6, 7. The variable 
attenuation correction circuit 2 is connected to first and second control 
outputs 11 and 12 of a control circuit 3. 
The fixed attenuation correction circuit 1 comprises a bridged-T network 
having in the series branch between the input terminal 4 and the terminal 
6 two series-arranged identical resistors 13 and 14 bridged by a fixed 
bridging impedance comprising a parallel arrangement of a series resonant 
circuit 16, 17 and a resistor 15. In the parallel branch the fixed 
attenuation compensator circuit has a fixed parallel impedance comprising 
a parallel arrangement of a resistor 18 and an inductance 19 arranged in 
series with a parallel resonant circuit 20, 21. One end of the series 
branch is connected between the two resistors 13 and 14 and the other end 
to the input terminal 5, which is connected to ground. 
The variable attenuation correction circuit 2 also comprises a bridged-T 
network having in the series branch betweem the terminal 6 and the output 
terminal 8 two series-arranged identical resistors 22 and 23, bridged by a 
variable bridging impedance comprising a series arrangement of an 
inductance 25 and a first resistor 24, the series arrangement being 
arranged in parallel to a first PIN diode 26 which functions as a first 
control resistor. Diode 26 has in the anode and the cathode lead 
respective d.c. blocking capacitors 27 and 28. The anode of the first PIN 
diode 26 is also connected to ground via an HF blocking inductance 29 and 
the cathode is connected to the second control output 12 of the control 
circuit 3. The parallel branch of the variable attenuation correction 
circuit 2 comprises a variable parallel impedance provided with a parallel 
arrangement of a second resistor 30 and a capacitor 31, this parallel 
arrangement being arranged in series with a d.c. blocking capacitor 32 and 
a second PIN diode 33 which functions as the second control resistor. The 
anode of the second PIN diode 33 is connected to ground, while the cathode 
is connected to the first control output 11 of the control circuit 3. 
The resonant frequencies of the series resonant circuit 16, 17 and the 
parallel resonant circuit 20, 21 of the fixed attenuation correction 
circuit 1 are higher than the maximum signal frequency in the frequency 
range of the signal applied via a cable to the input terminal 4. This 
frequency range may, for example, be between 40 and 300 MHz. In this 
frequency range the series resonant circuit 16, 17 is of a capacitive and 
the parallel resonant circuit 20, 21 of an inductive character, which 
manifests itself in a high attenuation for the lower signal frequencies 
and a low attenuation for the higher signal frequencies. A proper rating 
of the values of the elements of the bridging and the parallel impedances 
results in a desired attenuation variation which, as regards the frequency 
dependency, should be opposite to the sum of the cable attenuation 
variation at the highest prevailing cable temperature and the attenuation 
variation of the variable attenuation correction circuit 2 occurring at 
this cable temperature and in an input and output impedance which is equal 
for any signal frequency which corresponds to the resistance value of each 
of the resistors 13 and 14. 
The bridging and parallel impedance respectively of the variable 
attenuation correction circuit 2 is, on the contrary, inductive and 
capacitive, respectively, in said frequency range for any value of the 
first and the second PIN diode. For any setting of the two PIN diodes this 
results in a higher attenuation for the higher signal frequencies and a 
lower attenuation for the lowest signal frequencies, so that cable 
attenuations can be simulated with this variable attenuation correction 
circuit. The frequency dependency of the attenuation variation of the 
variable attenuation correction circuit 2 increases when the first PIN 
diode 26 is adjusted to a higher resistance value, and decreases when the 
diode 26 is adjusted to a lower value. The frequency dependence of the 
circuit 2 increases when the second PIN diode 33 is adjusted to a lower 
resistance value and decreases when the diode 33 is adjusted to a higher 
resistance value. To maintain a constant input and output impedance of 
this attenuation correction circuit the product of the impedance values of 
the bridging and the parallel impedance should remain equal to the square 
of the resistance value of each of the two resistors 22 and 23. So a 
decrease in the resistance value of the first PIN diode 26 must be 
accompanied by an increase in the resistance value of the second PIN diode 
33 and vice versa. 
By a proper rating of the elements 24 and 25 of the bridging impedance and 
of the elements 30 and 31 of the parallel impedance as well as of the 
control currents for the PIN diodes 26 and 33, attenuation variations are 
obtained within this current control range, which correspond to those of a 
cable of the cable type connected to the input terminal 4 for lengths 
within a given length-variation range. 
At higher cable temperatures at which the cable length is apparently 
longer, the variable attenuation correction circuit 2 should be adjusted 
to an attenuation variation corresponding to a shorter cable length than 
at lower cable temperatures, at which the cable length seems to be 
shorter. The attenuation correction circuit 2 simulates a cable having a 
negative temperature coefficient. 
In practice the minimum attenuation of the variable attenuation correction 
circuit 2 with a still cable-like variation corresponds, to the 
attenuation of the cable, connected to the input terminal 4, at a given 
minimum length. Consequently, also at the maximum cable temperature, a 
given, minimum attenuation of the variable attenuation correction circuit 
2, is added to the cable attenuation. 
Besides this minimum, still cable-like attenuation the variable attenuation 
correction circuit 2 also introduces a residual attenuation which is equal 
and frequency-independent at each setting of the PIN diodes 26 and 33. For 
the equalization of the cable attenuation variation this residual 
attenuation is of no importance. 
The cascade arrangement shown in the drawing on the one hand effects the 
elimination of the temperature dependency of the cable attenuation in the 
variable attenuation correction circuit, and on the other hand effects the 
elimination of the frequency dependency of the cable attenuation in the 
fixed attenuation correction circuit 1, at least for the greater part. A 
correct equalization of the cable attenuation variation occurs with an 
attenuation variation of the fixed attenuation correction circuit 1 as 
mentioned above. The frequency-independent residual attenuation also 
introduced by this fixed attenuation correction circuit 1 is of no 
importance for said equalization. 
The control currents for the PIN diodes 26 and 33 are supplied by the 
control circuit 3 via the second and first control output 12 and 11 
respectively. 
The control circuit 3 comprises a control input 10 which is connected to 
the base of a transistor 47. This transistor 47 is connected by means of 
its collector to a supply line and by means of its emitter via a resistor 
48 to ground. The emitter is also connected to the base of a transistor 
45. This transistor 45 is connected by means of its emitter via a resistor 
46 to the supply line and by means of its collector to the base of a 
transistor 41, the emitter of a second transistor 40 and, via a series 
arrangement of a decoupling capacitor 49 and a HF blocking inductance 50, 
to the first control output 11. The junction between the decoupling 
capacitor 49 and the blocking inductance 50 is connected via a HF 
shortcircuiting capacitor 51 to ground and also to the collector of the 
first transistor 41. The collector of the second transistor 40 is 
connected via a HF blocking inductance 44 to the second control output 12 
and, via a decoupling capacitor 43, to its base, this base being connected 
to the collector of a transistor 34 and to both the base and the collector 
of a transistor 38. This transistor 38 is connected by means of its 
emitter to both the base and the collector of a transistor 39. The 
emitters of the transistors 41 and 39 are both interconnected and 
connected to the supply line via a Zener diode 42, arranged in the inverse 
direction. The transistor 34 is connected by means of its emitter to 
ground via a resistor 36 and by means of its base to ground via a zener 
diode 35 as well as to the supply line via a resistor 37. 
The transistor 34 which functions as a fixed current source supplies a 
collector current I.sub.o, the value of which is determined by the value 
of the resistor 36. 
For transistors which are assumed to be ideal, the collector-emitter 
current varies exponentially versus the base-emitter voltage. 
In the control circuit 3 the sum of the base-emitter voltages of the 
transistors 40 and 41 is equal to the sum of the base-emitter voltages of 
the transistors 38 and 39. 
If it is assumed that the transistors 38-41 are identical and ideal, i.e. 
that their collector currents vary exponentially with their base-emitter 
voltages, the product of the collector-emitter currents of the transistors 
40 and 41, being the second and first control current respectively, will 
be equal to the product of the collector-emitter currents of the 
transistors 38 and 39, being I.sub.o.sup.2. So, if the first control 
current at the first control output increases, the second control current 
at the second control output decreases and vice versa. The second control 
current, that is to say the collector-emitter current of the transistor 40 
can be varied by means of a variable current source consisting of 
components 47, 48, 45 and 46 and, simultaneously, the first control 
current, that is to say the collector-emitter current of the transistor 
41. 
A temperature-dependent control voltage supplied to the input 10 also 
appears at the emitter of the transistor 45 and determines the value of 
the second control current by means of the value of the resistor 46. 
In a practical embodiment the resistor 36 is implemented as a potentiometer 
to be able to control also the collector current I.sub.o of the transistor 
34, which may be advantageous for adapting the attenuation equalizer to 
different kinds and lengths of cables. The components of an attenuation 
equalizer and an associated control circuit according to the invention may 
have in practice the following values: 
______________________________________ 
Values Values 
Resistors 
Values Capacitors 
(F) Inductances 
(H) 
______________________________________ 
13,14, 75.OMEGA. 
27,28 10n 29,44,50 
5.mu. 
22,23, 32,49, 
24,30 51 
15 280.OMEGA. 
17 8p2 
18 20.OMEGA. 
20 5p6 
36 24k.OMEGA. 
31 22p 
34k.OMEGA. 
37 15k.OMEGA. 
43 4n7 
46 820.OMEGA. 
48 18k.OMEGA. 
______________________________________ 
The RCA integrated circuit CA 3046 is used for the transistors 40, 41, 45, 
38 and 39. 
The PIN diodes 26 and 33 are of the type IN5957 and the zener diodes 35 
and 42 of the type BZY 79, and the type BZX79 respectively. The 
inductances 25 and 19 respectively are formed by winding 0.63 mm thick 
copper wire 41/2 turns around a core of 5 mm and 41/2 mm diameter 
respectively. The inductances 16 and 21 respectively are formed by winding 
0.5 mm thick copper wire 11/2 and 21/2 turns respectively around a core of 
3 and 21/2 mm diameter respectively. 
FIG. 2a shows the attenuation variation of a cable at a cable temperature 
of 20.degree. C. by means of a curve 60, at a cable temperature of 
60.degree. C. by means of a curve 61 and at a cable temperature of 
-20.degree. C. by means of a curve 62. 
FIG. 2b shows by means of a curve 60' the required cable-like attenuation 
variation of a variable attenuation correction circuit according to the 
invention at a cable temperature of 20.degree. C., by means of a curve 61' 
at a cable temperature of 60.degree. C. and by means of a curve 62' at a 
cable temperature of -20.degree. C. Curve 63 shows the residual 
attenuation of the variable attenuation correction circuit. 
FIG. 2c shows by means of a curve 64 the required attenuation variation of 
a fixed attenuation correction circuit. A curve 64' represents the sum of 
the attenuation variations in accordance with curves 60 and 60', which is 
equal to the sum of the attenuation variations according to the curves 61 
and 61' and that according to the curves 62 and 62'. 
As regards its frequency dependency curve 64 is the opposite of the curve 
64' so that, when adding the attenuations according to these curves, a 
flat attenuation variation according to a curve 66 is obtained. The 
frequency-independent residual attenuation of the fixed attenuation 
correction circuit is shown by means of a curve 65 and is of no importance 
for the equalization of the different cable attenuation variations, which 
also applies to the residual attenuation of the variable attenuation 
correction circuit.