US2341240A - Frequency discriminator network - Google Patents

Frequency discriminator network Download PDF

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Publication number
US2341240A
US2341240A US353028A US35302840A US2341240A US 2341240 A US2341240 A US 2341240A US 353028 A US353028 A US 353028A US 35302840 A US35302840 A US 35302840A US 2341240 A US2341240 A US 2341240A
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frequency
circuit
terminal
output
network
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US353028A
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John D Reid
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RCA Corp
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RCA Corp
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D3/00—Demodulation of angle-, frequency- or phase- modulated oscillations
    • H03D3/02—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal
    • H03D3/06—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal by combining signals additively or in product demodulators
    • H03D3/08—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal by combining signals additively or in product demodulators by means of diodes, e.g. Foster-Seeley discriminator

Definitions

  • This invention relates to the detection of Hequency-modulated impulses, and has for its principal object the provision of an improved apparatus and method of operation for providing frequency-modulation detection and frequencyresponsive controlling potentials.
  • Frequency variation detector systems or frequency discriminator networks are disclosed, for example, in the United States patent to Conrad 2,057,640 or Seeley 2,121,103.
  • the frequency discriminator network comprises two tuned signal circuits, one tuned above and the other tuned below a carrier or center frequency, and connected with suitable rectifiers back-to-back to provide cumulative response to the frequency variation of an incoming wave, and difierential response to the amplitude variations on said wave.
  • suitable rectifiers back-to-back to provide cumulative response to the frequency variation of an incoming wave, and difierential response to the amplitude variations on said wave.
  • Frequency discriminator networks of this type are used in connection with automatic frequency control (AFC) means for superheterodyne receiver oscillators and in detectors for frequency modulation receiving systems.
  • AFC automatic frequency control
  • the said output voltages are made equal and opposite at the mean or center frequency, thereby providing substantially zero D. C. (direct current) or output voltage at said frequency.
  • a high or intermediate frequency signal-conveying channel for a signal-receiving system such as the intermediate frequency amplifier of a superheterodyne receiver
  • an output circuit including a two-terminal frequency discriminator network providing a single shunt path across the channel and including a parallelresonant circuit and means providing a capacity in series therewith effective to render the shunt path series-resonant, together with rectifier means and output circuits therefor connected with said shunt path at suitable points for deriving controlling and output potentials therefrom resulting from the frequency variation of an applied signal.
  • a frequency-modulation detector it is desirable, as is well-known, to cause th peaks of the frequency-response characteristic thereof to be maintained relatively close together for maximum sensitivity and selectivity and, accordingly, it is a still further object of this invention to provide a frequency discriminator network wherein a single tuned signal path, seriesand parallelresonant to differing frequencies, may include. as the series capacity element thereof for series resonance, the inherentinterelectrode capacity of a tube or rectifier associated with the circuit for deriving control potentials therefrom.
  • a discriminator network is herein provided in which the peaks of the output characteristic are separated in frequency by an amount determined by the interelectrode capacity of a tube associated with the circuit, thus permitting a maximum sensitivity in response to frequency variation.
  • AFC automatic volume control
  • limiter means may be provided preceding the discriminator networks and detector.
  • AVC automatic volume control
  • Figure 1 is a schematic circuit diagram of a frequency discriminator network embodying the invention and providing selective tuning means for a radio signal receiving system
  • Figure 3 is a further schematic circuit diagram showing a portion of the circuit of Fig. 2 modified to provide symmetrical AVC potentials
  • Figure 4 is a schematic circuit diagram of a frequency discriminator network embodying the invention in connection with an intermediate frequency amplifier of a superheterodyne receiver, and modified to include a minimum of circuit elements,
  • Figures 5, 6 and 7 are graphs showing curves representing certain operating or response char: acteristics of a frequency discriminator network of the type shown in Figs. 1, 2 and 4.
  • the circuit 8 and ground or chassis l2 represent the high and low potential sides respectively of the signal-conveying channel of the system.
  • a frequency discriminator network I is coupled to the signal-conveying channel for receiving and converting frequency-modulated or variable frequency signals into amplitude-modulated signals, and to derive from the latter audio frequency or control potentials for further utilization in the signaling system.
  • the frequency discriminator network I5 is coupled at its high potential terminal IE to the circuit 8 through a suitable coupling capacitor ll of relatively low capacity, for preventing the transmission of audio frequency hum potentials from the amplifier 9 to the network I5, and is coupled at its high potential output terminal I8 through a suitable audio frequency coupling capacitor ill to an audio frequency system comprising a volume control device 20, an audio frequency amplifi r 2
  • the audio frequency system and the low potential terminal 24 of the frequency discriminator network l5, as indicated at 23 and 25, respectively, are provided with a common circuit return path which preferably is the chassis or ground. With this arrangement, the low potential side I2 of the signal-conveying channel is also common to the low potential side of the discriminator network and of the audio frequency system.
  • the discriminator network I5 includes a tuned circuit or section 26 comprising a tuning inductance 21 and shunt capacity means therefor provided by two series-connected capacitors 28.
  • a midtap 29 between the capacitors is connected to ground at 30 through a resistor element 3
  • this provides a balanced output circuit for the discriminator network and substantially zero D.
  • D. C. potentials of opposite sense are obtained between terminals 18 and 24, providing a resultant output voltage of variable amplitude and polarity between the output terminals I 8 and 24.
  • F0 is varied at an audio frequency rate, then at a-f, voltage may be obtained between terminals l8 and 24, of a frequency equal to the rate of variation of Fe, and of an amplitude proportional to the deviation from the mean frequency Fe.
  • a single shunt path is provided by the frequency discriminator network between the terminals 16 and 24 across the signal-conveying channel of the system, the said path including the tuned circuit or section 26 and the series capacitance at 31.
  • the circuit 23 provides a parallel-resonant element for the discriminator network and resonates at a frequency above the mean frequency of the discriminator network and of the si nal channel of the system.
  • the tuned circuit 26 is inductively reactive below the mean frequency and is tuned to series resonance below said mean frequency by the series capacity at 31. These frequencies are above and below 'the mean frequency by equal amounts.
  • the frequency diflerence between the upper and lower resonances is determined by the capacitive reactance at 31 and the inductive reactance of the circuit 26. It may be pointed out that the eflect of a series capacitance, as at 31, for a shunt-tuned circuit, as at 26, is to place the series resonance of 26-31 at a lower frequency than the parallel resonance of 26, and
  • the frequency difference of the resonances is substantially the frequency diii'erence in resonanceof 26 which would be obtained by adding 31 in shunt thereto. Therefore, this difference is a minimum when the tube capacity alone at 31 is provided as the series resonating capacity for any given value of inductance at 21.
  • the D. C. circuit through the rectifier device 38 may be traced from the terminal l6 through the device 38 to the terminal 18, thence through the output resistor section 39 to the terminal 4! and returning through the lead 42 and choke coil 43 to the terminal 35 and through the coil or inductance 21 back to terminal I6.
  • the D. C. path through therectifier 36 may be traced from the terminal 35 through the recuser as to the terminal :4, theme through the resistor section 40 to the terminal 4! and returning through the lead 42 and choke coil 43 to the terminal 35, thus making the rectifier 36 shunt -connected across the output resistor 46 and the voltage source existing between the terminals 35 and 24, while the rectifier 33 is shuntconnected across the output resistor 39 for D. C. potentials and across the voltage source existing between the terminals 96 and 2t for'r-f and i-f potentials.
  • the choke coil 33 is an 1-1 choke coil provid-
  • the signal voltage existing at the terminal l6 or the plate of diode 38 with respect to the low potential terminal 24 is a minimum at series resonance of the circuit with the capacity 31, as indicated at 50 on the voltage response or frequency discriminator S-shaped curve 5! of Fig. 5, which is drawn with respect to the usual impedance-coupling response characteristic 52 of voltage output with respect to frequency, and
  • the signal voltage existing between the terminal or the plate of diode 38 and the terminal 24 is a minimum at parallel resonance of the circuit 26, as indicated at 54 on the frequency discriminator or S-shaped curve 55 of Fig. 6, and is a maximum at series resonance of the circuit 26 with the capacity 31, as indicated by the peak 56 on the curve 55.
  • Figs. 5 and 6 it will be noted that the curve 5
  • the curves of Fig; 6 also indicate the D. C. potential appearing across resistors 39 and 40, as well as the rinput potential at points l6 and 35 with respect to ground.
  • the r-j input across the diode 38 causes D. C. current to flow in the resistor 39.
  • the 1-) input-across the diode 36 causes D. C. current to flow in the resistor 4
  • the cathodes ar positive and the center tap 4
  • and 55 each include center portions 50--53 and 5456 which are ubstantially linear and provide positive and negative slopes intersecting at 51 at a frequency of the order of the mean frequency, substantially midway of said center portions as indicated in Fig. 6.
  • Fig. 7 shows the voltage between output terminals l8 and 24 with respect to frequency.
  • the D. C. voltage I8 to 24 will increase until the frequency F1 is reached and terminal I8 becomes negative with respect to the terminal 24. From F1 to F0 it will decrease and will be zero at F0, and from F0 to F2 it will increase, and terminal 18 will be positive with respect to ground. -From frequency F2, going higher in frequency, the output voltage decreases.
  • the band width of the discriminator or frequency variation detector must be equal to the signal band width, such as F1 to F2 or Fm to F2 in Fig. 7. v
  • the ratio of the capacities at 31 and in shunt with inductance 21 determine the distance between the peaks F1 and F2 of the voltage response curves shown in Figs. 5, 6 and 7. Since the capacity at 31 is that which exists across the electric discharge device or rectifier 36, it is substantially a minimum and, therefore, it will be seen that, with this circuit, the distance between the peaks at F1 and F2 and the mean frequency F0 may be equalized by adjusting the tuning of the circuit 26 as by varying the capacity in shunt with the inductance 21.
  • the band width of the discriminator or frequency variation detector must be equal to that of the incoming signals. Hence, a shunt capacity as at 8
  • may be tuned over a relatively wide frequency range, fo example, from a center or mean frequency F0 to a center or mean frequency Fa, substantially without varying the amplitude of the peaks indicated at BI and 62 for the new low and high frequency limits F4 and F5.
  • the inductance 21 may be provided with a movable tuning core 63, preferably of ferromagnetic material and controlled by suitable means, such as a tuning dial 64, for varying the tuning of the circuit 26-31 and of the system, through a. predetermined tuning range as established by the band pass amplifier 9.
  • the circuit 26-31 may provide the sole tuning means for the receiving channel. It is obvious, however, that the ampliher 9 or other input system may be variably tunable, if desired.
  • variable inductance 2 provided with a movable ferromagnetic core may, for example, have 16 turns of No. 30 wire on a one-half inch form and the capacitors 28 may have a value of 200 mi. (microfarads).
  • the capacity at IT may be of the order of 200 mmf. (micromicrofarads) with a coupling resistor of 2200 ohms at I I, while the output resistor sections 39-40 ma have a value each of 100,000 ohms, shunted by capacitors 41 of 250 mmf.
  • the output impedance at 20 is preferably relatively high and of the order of 1 to 2 megohms.
  • the tubes 36 and 38 may be of the type commerciall known as the RCA 6H6 double diode.
  • Fig. 2 the circuit arrangement is similar to that of Fig. 1, and like circuit elements are designated by the same reference numerals.
  • the discriminator network includes the rectifier devices 36 and 38, connected to high and low potential terminals l6 and 35 and to output terminals 24 and I8, across resistor elements 39 and 40 shunted by capacitors 41, as in the cirlt of Fig. 1, and the center tap 4
  • variable resistor element 10 is provided in the lead 42 and the parallel-resonant circuit 26 comprises a high frequency inductance II and variable shunt capacitor 12. Signals are applied to the discriminator network through the coupling capacitor I! from a variable resistor I3 connected with the output circuit 14 of an amplifier or limiter stage 15 having signal input terminals 16 and 11.
  • is provided in shunt with the rectifier 36, thus being an addition to the internal electrodal capacity of the tube 36.
  • variation of this capacity may serve to shift the low frequency peak 85 of the curve 58-59-40 to a lower value as indicated at 86, an increase in capacity serving to shift the peak in the direction of the lower frequency.
  • variation of the band width of the response of the network may be provided by the capacitor 8
  • variable resistors l3 and 10 respectively, does not materially alter the sharpness of the peaks 50 and 54 of Fig. 6, and therefore the return traces 56 and 60 of the curve 58596
  • Variation of the resistor 13 causes a variation in the input impedance or loading of the discriminator network and controls the rise of the high frequency peaks 53 and 81 of Figs. 5, 6 and 7, while the variation of the output load by variation of the resistance 10 controls the rise of the low frequency peaks 56 and 85 of Figs. 6 and 7.
  • the impedance or resistance value of the D. C. return path through circuit 42 and the resistor 10 must be relatively low, or the audio frequency output voltage will be reduced, for the reason that resistor 10 is effectively in series with the resistors 39 and 49 in the output circuit. Therefore, the resistor 14 should be a. small percentage of the resistance of the elements 39 and 40.
  • each rectifier may be used; that is, either rectifier may be omitted, if the normal D. C. outputvoltage in its output resistor section is maintained and the capacitor 8
  • AVC potentials may be derived from the circuit by utilizing the resistor of Fig. 2 and the choke coil of ⁇ Fig. 1 in series in the return circuit lead 42' from the center tap M to the intermediate terminal 35, the resistor being indicated at 82 and the choke coil at 83, with the AVCconnection taken at a point between the two elements through a filter resistor 88 for the AVG lead 86.
  • the resistor 62 is provided with an audio frequency bypass capacitor 84, thus reducing the impedance to a-f signals. Since the bypass 64 also reduces the impedance at the signal frequency, the choke coil 83 is provided to prevent shunting of the r-f or 1-! voltage appearing across the diode rectifier 36.
  • the AVC voltage thus obtained is symmetrical with respect to F and may be substantially constant over the frequency range l b-Fa.
  • the discriminator network l5 comprises the two rectifier devices 36 and 36 connected with the terminals I 6 and 35 and with the terminals 08 and 24. Between the terminals I 8 and 24, the output resistor sections 39 and 46 are serially connected with a single bypass capacitor 65 in resonates at the higher frequency.
  • the tuning ofgthe circuit 26 is adjusted to provide the resonant peaks on opposite sides of the mean frequency of the network, the spacing of the peaks being determined by the ratio of the capacity 9'! to the capacity 31.
  • the signal supply for the discriminator is taken from an 1-! amplifier stage Hill which may also be the limiter stage of the H amplifier, having input terminals I M and having a tuned output circuit I62 coupled to the discriminator circuit i5 through a coupling capacitor I63.
  • a shield elemer t; i 04 may be provided to prevent stray capacity o shunt therewith, thus eliminating the choke coil I and one bypass capacitor of the preceding circuits, and the center tap H is connected through a direct lead connection 90 with the terminal 35.
  • this connection may be returned to any portion of the circuit between the terminal 35 and the terminal i6 and is shown connected to the terminal 35 only by way of example as being a preferred arrangement of the return' connection for the D. C. path through each rectifier.
  • the output terminal I8 is connected through an output filter 9
  • is provided to attenuate or de-emphasize a-f signals in the upper end of the audio frequency range to compensate for the higher audio frequency boost or increase provided in the usual signal at the transmitter.
  • the tuned parallel-resonant circuit 26 comprising an inductance winding 96 and shunt capacitor 9'8, the inductance being suitably adjustable as by means of a movable tuning core 68.
  • the capacity existing across the electrodes of' the tube 36 is indicated at 31 and may be utilized as the capacity element for tuning the shunt path through the circuit 26 from the terminal it to the terminal 24, to resonance at the lower frequency, while the shunt-tuned circuit 26 magnetic coupling between the circuits 26 and I02.
  • the effect of the tuned circuit I02 across the signal channel, indicated at I05 and H16 as the low and high potential sides thereof respectively, is to raise the amplification or to increase the sensitivity of the amplifier stage preceding the discriminator network and to further increase the selectivity or sharpness of the return lines 56-60 of the response characteristic of the network shown in Fig. 7.
  • the circuit of Fig. 4 is other wise the same as that of the preceding figures and has the advantage of not onlyhigher gain or sensitivity but also improved selectivity with fewer circuit elements.
  • the circuit is particularly adapted for use as the second detector of a. superheterodyne receiver for'frequency modulation signal reception.
  • the voltage between the terminals I 8 and 24 is made substantially the same by connecting between said terminals the bypass capacitor 95 which also forms part of the filter iii.
  • the voltage at the terminal I 6 with respect to ground is minimum at the resonant frequency provided by the series resonance of the circuit 26 and the capacity 31. It is a. maximum at that terminal with respect to ground, at parallelresonance of the portion 26 of the circuit 26-31.
  • the voltage at the terminal 35 with respect to ground is at a minimum at parallel resonance of vention may provide a single shunt path across a signal circuit or transmission channel of a receiving system including therein inductance and shunt capacity means providing parallel reso-' nance above a mean frequency, and means, preferably including the electrode capacity of an electric discharge device, forming a series capacity in the circuit resonating with the inductive reactance of the parallel-resonant portion to tune the network below said mean frequency, thereby providing a frequency discriminator network While the invention has been described in its several preferred embodiments as particularly adapted for the detection of frequency-modulated signals, it is obvious that it may be.
  • the network provided is essentially of the two-terminal type, including but one tuned circuit and a minimum of circuit elements,
  • a frequency variation detector comprising a two-terminal network connected across said circuit, and means in said network providing parallel resonance at a higher frequency and series resonance at a lower frequency by an equal difference with respect to said predetermined frequency.
  • a circuit responsive to a band of frequencies centered about a carrier wave the frequency of which is modulated in accordance with a signal, a network connected across said circuit providing a single signal path tuned for parallel resonance above and series resonance below the carrier frequency by an equal frequency difference, and rectifying means connected with said network for deriving therefrom signal and control potentials proportional to the frequency variation of said wave.
  • a frequency discriminator network including series and parallel-resonant elements in a single shunt signal path providing a voltage-frequency output characteristic having positive and negative peaks, and means in said network for causing said peaks to be separated in frequency above and below a mean frequency by predetermined equal amounts and for establishing a predetermined band width and sensitivity in said network.
  • a frequency discriminator network including a parallelresonant circuit in a single shunt signal path providing a voltage-frequency output characteristic having positive and negative peaks, a rectifier in said path having inherent capacity providing series resonance with said circuit below a predetermined frequency, means in said network for causing said peaks to be separated in frequency by a predetermined amount above and below said frequency, means for varying one of said parallel-resonant elements to vary the tuning of said network over a predetermined frequency range, and means for maintaining the amplitude of said peaks and the band width of said network substantially constant.
  • a two-terminal frequency discriminator network having a high potential terminal and a low potential terminal and comprising a parallel-resonant circuit including inductance and shunt capacity means connected to the high potential terminal,
  • rectifier means having inherent capacity providing series resonating capacitance for said network in series with said parallel resonant circuit between said circuit and the low potential terminal, said parallel-resonant circuit being tuned to a frequency higher than a predetermined reference frequency by a desired frequency value, and said capacitance tuning said parallelresonant circuit to a frequency lower than said reference frequency by said desired frequency value.
  • a band pass amplifier having an output circuit, a frequency discriminator network providing a single shunt signal path across said output circuit, a parallel-resonant circuit in said shunt path, capacitance means in series with said parallel-resonant circuit effective to render the shunt path series resonant, and means for variably tuning said parallel-resonant circuit through a predetermined frequency range within the pass band of said amplifier.
  • a frequency variation detector comprising a pair of input terminals upon which are impressed frequency modulated carrier energy having a predetermined mean frequency, a twoterminal network connected between said input terminals, said network consisting of a tuned circuit parallel resonant to a frequency above said means frequency, and a capacitative element series resonating the inductive reactance of said tuned circuit to a frequency below said mean frequency by a substantially equal frequency value, and utilizing means coupled to opposite terminals of said two-terminal network.
  • a parallel resonant input circuit tuned above the mean frequency of said energy by a predetermined value, a pair of opposed diodes, like electrodes of the diodes being connected to respective opposite sides of said parallel resonant circuit, a resistive load impedance connecting the remaining like electrodes of the diodes, a conductive connection from an intermediate point on the load impedance to one side of the input circuit, means establishing a point of the load impedance at an invariable alternating potential, and a capacitative element, in series between said input circuit and said point, series tuning said input circuit to a frequency less than said mean frequency by substantially said predetermined value.

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US353028A 1940-08-17 1940-08-17 Frequency discriminator network Expired - Lifetime US2341240A (en)

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FR876095D FR876095A (fr) 1940-08-17 1941-10-17 Réseau discriminateur de fréquence

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2488585A (en) * 1945-05-29 1949-11-22 Rca Corp Frequency modulation receiver
US2561089A (en) * 1948-01-30 1951-07-17 Rca Corp Frequency modulation ratio detector
US2561149A (en) * 1945-09-10 1951-07-17 Rca Corp Frequency modulation detector circuits
US2686871A (en) * 1949-03-22 1954-08-17 Sol L Reiches Piezoelectric cyrstal as a frequency discriminator
US2700103A (en) * 1946-05-15 1955-01-18 Selove Walter Balanced-detector circuit
US3049667A (en) * 1959-01-20 1962-08-14 Collins Radio Co Filter circuit
US4272726A (en) * 1979-07-20 1981-06-09 Rca Corporation Differential FM detector with series tuned filter

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2488585A (en) * 1945-05-29 1949-11-22 Rca Corp Frequency modulation receiver
US2561149A (en) * 1945-09-10 1951-07-17 Rca Corp Frequency modulation detector circuits
US2700103A (en) * 1946-05-15 1955-01-18 Selove Walter Balanced-detector circuit
US2561089A (en) * 1948-01-30 1951-07-17 Rca Corp Frequency modulation ratio detector
US2686871A (en) * 1949-03-22 1954-08-17 Sol L Reiches Piezoelectric cyrstal as a frequency discriminator
US3049667A (en) * 1959-01-20 1962-08-14 Collins Radio Co Filter circuit
US4272726A (en) * 1979-07-20 1981-06-09 Rca Corporation Differential FM detector with series tuned filter

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