IL28929A - Retractable slide rule apparatus for indicating tuning frequency-modulated signal transmissions - Google Patents

Description

Apparatus for tunin of transmissions FOR INDICATING TU OF SIGNAL TRANSMISSION Abstract of Disclosure Novel apparatus is disclosed for using simple direct or similar indicators to indicate incidental amplitude modulation and of conditions received FM signals and the the aid of brid circuits embodying transistors controlled by demodulator and rectifier present invention relates to apparatus for indicating tuning of signal sions to indicating the amount of incidental amplitude modulation that might create difficultie in the reception of signals so as to enable more accurate tuning or antenna orientation or similar While the techniques of the present as will later be made more are suited for more general the invention is adapted for utilization in indicating that a receiver designed for the reception of waves is tuned accurately to the desired signal and that the antenna for that receiver is properly oriented to receive the a receiver for waves consists of a pluralit of amplifying including a converter all taining selective circuits to select a band of frequencies containing the transmitted These amplifying stages are followed by limiter stages designed to remove the convert the frequency variations contained in the signal into a output voltage of audio and supersonic frequencies equal to those originally modulating the Practical receivers for signals contain all the building all of which have certain although considerable ingenuity has been applied in the past to improve their For the selective filters employed to filter out the desired band of frequencies do not have the ideal characteristic of constant attenuation for that band and infinite attenuation outside of that but rather have a filter characteristic which is a compromise between the ideal the tolerable plexity of the filter and the phase characteristics of the the limi er stages employed in a practical receiver do not remove all amplitude variations of the but rather reduce the effective amplitude modulation percentage by a substantial factor which generally decreases for low input voltages to the limiter The detector circuit for will show a generally lower sensitivity for modulation than for frequency The will be sensitive to amplitude ion to a reduced and the amount of the ultimately detected amplitude modula ion added to amount of frequency Any f will be affected by the limiter t e nd the re ount assed b the limiters and by by inaccurate tuning to the the signal is placed on the uation slope of the selective The variation in frequency due to frequency modulation will cause a variation in amplitude and therefore amplitude Another source of incidental amplitude modulation is the reception by the antenna of the signal from the transmitter by a multiplicity of The direct signal from the arriving at the receiving antenna will be added to the signal arriving at the receiving antenna by way of objects reflecting the signal for a single reflection is assumed and the difference in path length between direct and reflected signal is 3 Km with a frequency of nominally 100 MHz to a wave length of a maximum and a minimum i signal strength can occur every 100 An PM broadcasting station assigned to this frequency is customarily allowed to vary its carrier frequency over a total range of 150 one or two maxima and minima can occur within the band assigned that more than one signal reflection occur from electrical airplanes and other and the relative strength of these i reflected signals can vary the direction of arrival of the signal and any of the reflected signals is so that an antenna of sufficient directivity can discriminate against the unwanted directive antennas designed for the reception of signals have been adjusted in orientation so that the average signal strength as observed an indicating meter connected to read the limiter circuit input voltage was maximizedc This maximum only indicated that the sum of signal and tions was a maximum rather than the presence of one desired no indication was given which might result in the selection of a better antenna In order to have an indication of the amount of amplitude modulation present at the input of the limiter previous receiver designs have relied upon a visual presentation where the detected input voltage was applied after to the tion plates of a cathode ray tube such as an oscilloscope The user of an receiver so or with the aid of an external after considerable interpret the observed waveforms tune the receiver and the antenna With a cathode ray indicator of a widening the junction of the light and dark portion in accordance observable more difficult to of these employ direct amplifiers subject to considerable drift indicated light The amount of amplitude modulation which can be is further limited by the trace width or sharpness of the fluorescent pattern observed by In addition to the indicator tube requires a direct supply of substantially higher voltage than required for thereby necessitating a second power supply It is to the obviating of the complexities and difficulties of such indicators of the incidental amplitude modulation that the present invention is primarily In the present all of these disadvantages are In a direc meter of the type vised as a normal tuning indicator can be connected to serve as a simple and effective amplitude modulation cator and can also provide a measure of the combination of both A further object of the invention is to provide a novel tuning indicator of more general Other and further objects are hereinafter presente and are detailed in the The invention will now be described in connection with the accompanying 1 of which is a circuit diagram showing the invention in a preferred applied for use an broadcast receiver and In transistor Ql having an emitter electrode base electrode 3 and collector electrode is the first amplifier of such a obtaining its input from terminals 7 and the latter of which is shown connected ground Input terminal 7 is connected by way o capacitor to the base electrode 3 of transistor The intermediate output signal of the transistor Ql is obtained at collector electrode the primary of transformer the inductance of which is resonated with capacitor C3 to the intermediate The secondary winding of transformer Tl feeds the further stages of the requency amplifier circuit the output of which is at transformer The secondary winding of transformer T2 grounded at its lower end at the upper feeds the amplified intermediate signal to the limiter of terminals 13 and The same signal is also fed from the secondary winding of former T2 to a diode the automatic gain control DC operating current for transistor Ql is supplied from voltage source by way of the primary winding of transformer Tl connected to the collector electrode of transistor Ql and returned to ground G from the electrode through resistor Resistor R3 is bypassed for the intermediate frequency to ground by means of C bias for transistor Ql is also obtained from voltage source and by way of resistor connected to 1 is a resistor which is series with resistor R5 and the parallel combination of resistor R in series with the secondary winding of transformer forms the grounded portion of a voltage with resistor Rl being connected to the supply The direct voltage existing at the emitter electrode 1 of transistor is substantially lower for a silicon than the voltage determined at the base electrode 3 as a result of this voltage divider The operating current of transistor Ql then equal to the emitter voltage divided by the value of resistor An output voltage at the secondary winding of transformer T2 will be rectified by diode charging capacitor and thereby developin negative voltage at terminal with respect to ground This negative voltage will by divider action between resistors and result in a more negative voltage on the base electrode 3 of transistor Ql with consequently lower emitter voltage of transistor Ql and a lower emitter This lowered emitter current will then decrease the gain of transistor Ql thereby ing in automatic gain control of transistor The current through transistor Ql or the voltage on resistor R3 may be measured with a direct ment and can be used for an indication of relative signal level arriving at the 7 9 of transistor It has been found to have this current instrument indicate 0 current or 0 voltage in the absence of A meter Ml is therethrough to the emitter electrode 1 of transistor with the positive meter terminal connected to a voltage at conductor 19 substantially equal to the emitter voltage of transistor Ql in the absence of Resistor capacitor connected across the parallel tion form a network with a time constant typically i excess of one of a which tively prevents the intermedia envelope tions audio frequency rates reaching the base electrode 3 of transistor and thereby also prevents them from modulating the output signal of transistor These possible audio frequency are not applied to the indicating meter In the circuit of the positive terminal of meter is connected to the emitter electrode of a second transistor the current of which is returned to ground G by way of resistor The emitter current of transistor Q2 and the value of the emitter resistor R6 are chosen so that the voltage at the emitter electrode 21 is substantially equal to the emitter voltage at the emitter of transistor Ql in the absence of thereby resulting in the absence of current flow through resistor R13 meter and consequently resulting in a 0 meter indication The indicating meter Ml is thus nected in a bridge with transistors Ql and Q2 and emitter resistors R3 and R6 forming the four arms of the Transistor base electrode 23 and a collector electrode 25 further provided with a receiving collector Resistor form a voltage divider supply voltage and ground and at their terminal 27 supply base voltage and base current to the base 23 of transistor by of terminal resistor R9 and resistor envelope of the output signal is demodulated and detected by charging capacitor C and discharging The value of is chosen so that effectively the instantaneous negative peaks of the intermediate frequency output signal are obtained at terminal The time constant resistor and capacitor C is chosen so that substantially the audio and supersonic frequencies of interest present in the envelope of the intermediate frequency output signal are preserved at the terminal These audio and supersonic frequency travel from terminal 17 by wa of a switch 31 and a coupling capacitor to the base electrode 23 of the transistor The audio and supersonic frequency components present at the base electrode of transistor are then by transistor and are obtained at the collector electrode 25 A network consisting of bypass capacitor series with resistor R8 is connected in parallel ith emitter resistor For audio and supersonic resistor R6 and resistor are effectively connected in parallel from the emitter electrode 21 to ground thereby producing negative feedback in the amplification of transistor and thereby serving to adjust the audio and supersonic frequency gain of to be The audio and supersonic frequency signals present at the collector electrode 25 of transistor coupled way of capacitor C7 to a rectifier ing of diodes and charging capacitor C9 and ing resistor the demodulator or rectifier circuit effectively being returned to ground G for audio and supersonic signals from common terminal 27 by way of bypass capacitor A negative voltage is thereby developed at terminal 29 of the rectifier with respect to terminal 27 such that a more negative voltage is applied to the base electrode 23 of transistor reducing its emitter current and reducing direct voltage present at conductor with respect to The presence of any audio and supersonic envelope components in the output of the amplifier thus reduce the indication of meter giving the impression of an output signal of lesser strength than would be observed when switch 31 is opened with the same average output signal first indication of amplitude modulation nents present in the output signal of the frequency 11 is thus provided by any observed reduction in indication of meter Ml as switch is The time constant of the envelope signal demodula or circuit can be adjusted by choosing the value of capacitors C7 and and the value of resistor to show detector time constant to the rise time of syllabic rate of speech or similar to the len th of a short musical note or other intelligence across resistor will not be a steady but will in accordance with the rate of speech and i The mechanical time constant of a indicating meter is substantially similar to this rate a pulsating indication of meter Ml will be observed when the original signal is modulated with either speech or music and amplitude modulation components are present in the put of the amplifier A visual indication given of the relative presence of amplitude modulation and the necessary tunin and antenna orientation adjustments can be made by tuning for a maximum indication of meter Ml when switch 31 is The absence of all amplitude modulation components can easily be observed under mitter modulating conditions by opening and closing switch 31 and observing no change in indication of meter Resistor R13 is preferably chosen so that indicating meter Ml gives substantially indication when an equency signal of amplitude is fed to input terminals 7 and which in turn causes minimum emitter current to flow in transistor by automatic gain control resistor R8 is probably chosen so that the presence of amplitude cause the meter o return near zero indica or any other desired Consequen the tion switch 31 closed will show mately proportional the miminum of the envelope of the output signal and will follow that minimum as the effective amplitude modulation is e c rcu o us ma s us o wo transistors for the purpose of tuning indication and for amplitude modulation with transistor simultaneously serving the further purpose of i mediate amplification with automatic gain control of that It is to combine the two cating functions in a single transistor This is shown in 2 wherein transistor Q2 also receives automatic gain control voltage on its base terminal 23 by way of resistor R16 from rectifier diode pi connected to terminal For purposes of tuning indication or relative signal strength indication the audio and supersonic components present at terminal 17 sre filtered out by filter Rl6 and C6 connectable to ground G by way of switch Operating bias for the base electrode 23 of transistor is supplied from supply voltage by way of voltage divider resistors and connected to ground If an intermediate output voltage is present at the secondary winding of trans ormer diode Dl will rectify this voltage and produce a negative voltage at terminal thereby causing case electrode 23 of transistor to assume a more negative with a consequent reduction in emitter voltage of the Indicating Ml has its terminal connected to the po tive terminal connected to the junction of resistors R17 and R15 which form a voltage divider between supply voltage and ground The values of resistors R17 and R15 chosen amplifier at transformer T2 substantially zero meter indication is Here meter Ml is connected in a bridge circuit consisting of resistors and transistor as the four Resistors R17 and R15 also chosen so that the maximum expected output voltage causes a substantially full scale deflection of meter switch in the position as shown in the audio and supersonic frequency components present at terminal 17 are coupled into the base electrode 23 of transistor and are being then fed from collector electrode 25 by way of coupling capacitor C7 to the rectifier diodes and connected to produce a positive voltage at terminal This positive voltage is now by way of resistor R12 to the base electrode causing increased emitter current and emitter voltage and a consequent decrease in current through meter All component values for circuit of unless shown are chosen on the same basis as those the circuit of 1 and the circuit of 2 all the functions of the circuit of 1 except c tion of the i e automatic n The simple circuits described above therefore perform the functions of indication of relative signal indication of the relative amount of undesired amplitude modulation with indication from a single simple direct the indications of which are readily Receiver tuning and antenna adjustment are made using maximum steady meter indication as the tion of proper the apparatus for indicating tuning of frequency modulated signal transmission of the invention has been shown as adapted for connection to an broadcast tuner with transistor circuits operating in the frequency range for 88 to 108 it is not to restrict the application of the invention to the described purpose because much broader applications are Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended insufficientOCRQuality

Claims (1)

1. HAVING NOW particularly described and ascertained t e na ure o our said invention and dn what manner the same is to be performed, we declare that what we claim is : . .. . .

1. Apparatus for indicating the presence of r' amplitude -modula ion components indicative of inaccurate adjustments in reception of frequency-modulated input signals and the like comprising, means for receiving said input, signals, demodulating means connected to the receiving —means- o detect frequency components in said signals, direct-current indicator circuit means, peak-to-peak detector means, means for selectively- app.lying said frequenc components to the peak-to-peak detector means to