US2255668A - Frequency-modulation receiver - Google Patents

Description

Sept. 9, 1941. w. R, KOCH 2,255,668

FREQUENCY-MODULATIN RECEIVER Filed June 14, 1940 Patented Sept. 9, 1941 UNITED STATES PATENT CFFICE FREQUENCY-MODULATION RECEIVER Winfield R. Koch, Haddonield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 14, 1940, Serial No. 340,448

(Cl. Z50-20) 10 Claims.

lated signals for use in amplitude-modulated signal receiving systems at a relatively lower frequency.

The invention relates particularly to a converter for frequency-modulated signals to provide amplitude-modulated signals, and has for its further object to provide an improved method and means for converting wide-band frequencymodulated signals to relatively lower frequency signals such as normal high frequency amplituale-modulated signals for reception on an amplitude-modu'lating signal receiving system.

It is also an object of the present invention to provide adapter means as a remote control unit, for converting high frequency-modulated signals into lower frequency amplitude-modulated signals for reception on an amplitudemodulated signal receiving system.

In accordance with the invention, a high frequency signal is converted to provide sum and difference frequency signals, one of which is i passed through a discriminator network and the other of which is mixed with the converted carrier from the discriminator network to provide a lower R. F. signal 'for amplitude-modulation reception at a frequency of the order of that of the conversion oscillator. The converted signal may be tuned `in on a broadcast receiver coupled to the output in any suitable manner.

It is also an object of the present invention to provide an improved system for converting higher frequency-modulated signals at a relatively high frequency to amplitude-modulated signals at a relatively lower frequency, which eliminates the tuning of other than the input circuits to cover a predetermined variable tuning range.

This involves the mixing of Yan incoming signal to a relatively low frequency signal 'which is then converted to amplitude modulation.

The invention will further be understood from the following description when considered in connection with the accompanying drawing and its scope is pointed out in the appended claims.

In the dra-Wing Figure 1 is a schematic diagram of a frequency-modulation `receiving system embodying the invention,

Figure 2 is a graph showing a curve illustrating an operating characteristic of the circuit of Fig. 1, and l Figure 3 is a vschematic .circuit diagram of a modification of the system of Fig. 1 showing the circuits of a converter unit, also embodying the invention.

Referring to Fig. l, an adapter for frequency modulation conversion may include high frequency signal supply means such as dipole antennas 5 and 6 connected to a high frequency selector or R.'F. tuning unit l having tuning control means 8, and being coupled to a converter or ,first detector indicated at 9. The converter is supplied with oscillations, preferably at a frequency of the order of the frequency range of the broadcast or other receiver of the amplinude-modulation type to be used in conjunction with the adapter system. 'I'he oscillator is indicated at l0 and is provided with tuning control means indicated at ll for tuning with the selector 1 through a predetermined frequencymodulation signal receiving range.

In the present example, the broadcast frequency range is assumed to be the present standard range and accordingly the oscillator may be assumed to be tunable through that range, for example, to a frequency of l. mc. with an incoming frequency-modulated sig-nal wave selected at 40 mc. and with a maximum frequency swing of .l mc. There will be present in the output circuit l2 lof the converter several signals including the sum and difference frequency signals and the oscillator and incoming signals, that is, 39 and 41 mc. and 40 and 1 mc., respectively, in the present example.

In accordance with the invention, one of the signals, preferably the main high frequency signal wave of 4.0 mc., is passed through a frequency discriminator network i3 to derive a signal output voltage in an output circuit I4 which varies in amplitude with variation in frequency modulation between 39.9 mc. and 40.1 inc. as a result of the .1 mc. frequency modulation on the carrier "Ihis output voltage is `indicated between points l5 and IB on the discriminator output voltage curve I1 of Fig. 2.

Another of the signal frequenciesavailable in the output circuit I2 is selected through a bandpa'ss I. F. selector indicated at 20 also connected with the output circuit l2, and having an output circuit 2| connected, with the output circuit I4, to a mixer stage or second detector 22 wherein the resultant 1 mc. signal wave is provided by the beating of the two signal waves from the discriminator and band-pass selector.

The resultant signal wave is of constant frequency and varying amplitude for the reason that both the discrirninator signal and the bandpass .signal are derived from the same signal source. Therefore, they vary in synchronism, 1 mc. apart at all times, as determined by the converter oscillator frequency.

The resultant signal wave is selected by a tunable R. F. selector indicated at 23 and is supplied t-o a radiating loop 24 or other suitable means for coupling to the amplitude modulation receiver. In this case radiation is provided to a pickup loop 25 on the broadcast or other receiver 26 which is tuned by means of a tuning control 21 to that frequency.

The R. F. selector 23 is provided with a bandpass characteristic permitting the modulation to pass through without appreciable attenuation and in this case is assumed to be plus or minus 10 kc. as indicated, which is the maximum audio frequency swing of practical use at present.

In this system, the oscillator lil, the high frequency input selector 1, and the broadcast or R. F. selector 23 must be tuned, together with the receiver 25, for each differing signal desired. Hence, it is preferable to provide unitary tuning control means for such controls in any suitable well-known manner.

To simplify the tuning operation, the system may be modified as shown in Fig. 3 to require Athe tuning of two circuits by unitary control means. This involves the addition of a first converter and main oscillator together with a bandpass selector and limiter preceding the converter and broadcast oscillator of Fig. 1.

In Fig. 3, the band-pass I. F. selector which corresponds to that of Fig. 1 is shown at 30 and preferably is of the wide-band type comprising two tuned coupled circuits. These circuits may be of the well-known I. F. coupling transformer type, located between the converter 3l and the signal mixer 32, corresponding respectively to the converter 9 and mixer 22 of the circuit of Fig. 1.

Each of these stages preferably includes two control grids. In connection with the band-pass selector 3l), there is provided a signal input grid 33 in the mixer tube 32 and a second input grid 34 connected with a frequency discriminator circuit 35 which is similar in characteristic to that of Fig. 2 except that the maximum output occurs at the higher frequency. This is also connected in circuit between the converter 3l and the signal mixer as shown.

Frequency-modulated signals are applied to the converter 3l from a signal collector such as a dipole antenna 31 through a first converter stage 38 having an R. F. tuning circuit 39 coupled to the collector 31 and having an output circuit 4l) coupled through a band-pass selector lil of the wide band type, with the signal input grid 42 of the converter 3i.

A limiter comprising a pair of biased diodes 43 and 44 are connected across the input circuit of the grid 42 to remove amplitude modulation on the signal and to prevent excessive noise and signal voltages from reaching the converter 3|. Any suitable limiter may, however, be'provided for this purpose.

Both the tube 38 and the tube 3l are provided with oscillator grids 45 and 46 to which are connected, respectively, oscillator circuits 41 and 48, the former being tunable by variable means such as a tuning capacitor i) preferably in unison with the input circuit 39 which includes a similar variable tuning capacitor 5|.

The band-pass coupling circuit 4I is similar to that at 30 in that it involves the tuned primary and secondary circuits of an interstage coupling transformer and has wide band-pass characteristics. In the circuit shown, it is designated as the first I. F. band-pass selector, as distinguished from the second I. F. band-pass selector at 3U. Likewise, the converter 38 is designated as the first converter as distinguished from the second converter 3 l.

The operation of the circuit described is as follows: Assuming a 40 mc. signal being received through the high frequency selector 39 and applied to the rst converter with a maximum swing modulation) of .1 mc., the main oscillator 41 may be tuned to 45 mc. thereby to produce a resultant relatively low intermediate frequency which is more readily adapted for use in a discriminator circuit. In the present example, this will be a frequency of 5 mc. plus or minus .1 mc. The limiters 43 and 44 are adjusted to prevent excessive signal and noise voltages from reaching the second converter fi. The 5 mc. signal applied to the second converter mixes therein with a fixed oscillator frequency derived from the circuit 48 which may be setk at 1.5 mc., or within the range of the tuning of the amplitude-modulated signal receiver to be used with the adapter system, as described in connection with Fig. 1. The second oscillator i5 therefore referred to as the broadcast oscillator, and this circuit may be xed in frequency as is the frequency of the discriminator- 35, and the second I. F. band-pass selector 35 following the second converter.

The signal derived from. the second converter is the sum of the two signals applied thereto and results in a 6.5 mc. I. F. signal plus or minus .1 rnc. as indicated, which is passed through the second tuned selector to the signal mixer 32 while the 5 mc. signal with the modulation thereon is converted to both frequency and amplitude-modulation and applied also to the mixer, resulting in a 1.5 mc. amplitude-modulated signal at the output circuit BE) of the signal mixer.

The latter circuit is coupled to the utilization means or receiver through a tuned circuit El which may include the primary of an output couplingtransformer having a secondary 62 to which is connected the output terminals 63. In certain installations, the circuit 6l may become the output or loop circuit for the adapter, the winding 64 therein forming the radiating loop of the adapter unit.

I claim as my invention:

1. The method of converting frequency-modulated radio signals at a relatively high frequency to amplitude-modulated signals at a lower frequency, comprising selecting a frequency-modulated high frequency signal, converting said high frequency Ysignal by mixing with oscillations in a desired tuning range for amplitude-modulated signal reception, converting said frequency-modulated high frequency signal to an amplitudemodulated signal, mixing said last named converted signal with the converted high frequency signal to provide a relatively low intermediate frequency signal having amplitude modulation, and selecting said last-namedV signal.

2. The method of converting a frequency-modulated signal wave to an amplitude-modulated signal wave which comprises selecting a high frequency signal wave with frequency modulation, converting said signal wave to provide sum and difference frequency signal waves, converting one of said last-named signal waves to an amplitudemodulated wave, mixing said amplitude-modulated wave with another of said converted signal waves to provide a high radio frequency amplitude-modulated signal wave, and selecting said last-named signal wave.

3. The method of converting a high variable frequency signal to amplitude modulation, which comprises selecting said signal, applying to said signal oscillations to convert said signal into sum and difference frequency signals, converting said high variable frequency signal to an amplitude-modulated signal, beating said amplitudemodulated signal with one of the converted frequency signals to provide a high frequency amplitude-modulated signal, and selecting said lastnamed signal at a frequency of the order of the frequency of the conversion oscillations.

4. The method of converting frequency-modulated radio signals at a high frequency to amplitude-modulated signals at a relatively lower frequency, comprising selecting a frequency-modulated high frequency signal, converting said high frequency signal by mixing with selected oscillations in the desired amplitude modulation low frequency range, converting said high frequencymodulated signal to an amplitude-modulated signal, mixing said last named converted signal with the high frequency conversion signal to i provide a relatively low frequency signal having amplitude modulation, and selecting said lastnamed signal.

5. A frequency modulation signal converter system comprising in combination a tunable high frequency signal selector, a converter coupled thereto, a tunable oscillator having a predetermined relatively low frequency tuning range, a band-pass intermediate frequency selector coupled to said converter for deriving a signal at the conversion frequency, a frequency-discriminator network for converting the first-named signal to an amplitude-modulated signal, means for mixing said conversion frequency and lastnamed signals to provide a relatively low frequency signal, and a tunable radio frequency selector coupled to said signal mixer for deriving therefrom an amplitude-modulated signal within said first-named tuning range.

6. A frequency modulation signal receiving system for amplitude modulation reception comprising a tunable high frequency signal selector, a converter coupled thereto, a tunable oscillator coupled to said converter and having a tuning range within the tuning range of desired amplitude-modulated signals, a relatively wide bandpass intermediate frequency selector coupled to said converter for deriving therefrom a signal at the conversion frequency, a frequency-discriminator network for converting the high frequency carrier signal to an amplitude-modulated signal, means for mixing said last-named signals to provide a relatively low intermediate frequency signal, and a tunable radio frequency selector coupled. to said signal mixer for deriving therefrom an amplitude-modulated signal Within said first-named tuning range.

'7. In a frequency-modulation signal receiving system for amplitude-modulation reception, the combination of means for selecting a high frequency signal at a variable frequency, means for heterodyning said signal to a relatively low frequency signal, a band-pass selector for selecting the resultant signal, a second converter, an oscillator coupled to said second converter operative in a tuning range of the order of the tuning range of desired amplitude-modulated signals, a second band-pass selector and a frequency-discriminator network coupled to said second converter, means coupled to said second band-pass selector and frequency-discriminator networks for mixing signals therefrom, and means for selecting the output of said signal mixer means.

8. In a frequency-modulation signal receiving system for amplitude-modulation reception, the combination of means for selecting a high frequency signal at a variable frequency, means for heterodyning said signal to a relatively 10W frequency signal, a band-pass selector for selecting the resultant signal, means connected with the output of the band-pass selector for limiting the voltage amplitude of said signal, a second converter, an oscillator coupled to said second converter operative in a tuning range of the order of the tuning range of desired amplitude-modulated signals, a second band-pass selector and a frequency-discriminator network coupled to said second converter, means coupled to said second band-pass selector and frequency-discriminator networks for mixing signals therefrom, and means for selecting the output of said signal mixer means.

9. In a radio signal receiving system, the combination of variably tunable means for selecting and converting a high frequency variable frequency signal to a relative low intermediate frequency, a first intermediate frequency band-pass selector coupled to said first-named means for selectin.U said low intermediate frequency signal, means coupled to said band-pass selector for converting said intermediate frequency signal including a fixed tuned oscillator operating at a frequency within the tuning range of a desired amplitude-modulated signal receiving range, a second intermediate frequency wide-band selector coupled to said second converter for deriving therefrom a beat frequency signal, a frequency-discriminator network coupled to said second converter for deriving the low frequency intermediate carrier wave and converting said wave into an amplitude-modulated signal, and a signal mixer for converting said last-named signal and said beat frequency signal to a low frequency amplitude-modulated signal within said desired tuning range.

l0. In a radio signal receiving system, the combination of variably tunable means for selecting and converting a high frequency variable frequency signal to a relative low intermediate frequency, a rst intermediate frequency band-pass selector coupled to said first-named means for selecting said low intermediate frequency signal, means coupled to said band-pass selector for converting said intermediate frequency signal including a fixed tuned oscillator operating at a frequency within the tuning range of a desired amplitude-modulated signal receiving range, a second intermediate frequency wide-band selector coupled to said second converter for deriving therefrom a beat frequency signal, a frequency-discriminator network coupled to said second converter for deriving the low frequency intermediate carrier wave and converting said wave into an amplitude-modulated signal, a signal mixer for converting said last-named signal and said beat frequency signal to a low frequency amplitude-modulated signal within said desired tuning range, and utilization means coupled thereto including an amplitude-modulated signal receiving system.

WINFIELD R. KOCH.

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