Intermediate Frequency Amplifier Research Articles

Tuner for Complete UHF-TV Coverage without Moving Contacts

A tuner covering the uhf television band continuously and having essentially constant selectivity over all of this band is described. A unique feature is that the tuner employs no moving contacts in the tuned circuits, and so avoids the well-known contact troubles. The circuit consists of a double-tuned preselector, a crystal mixer, a triode oscillator, and a cascode-type first intermediate stage. The two preselector circuits and the oscillator employ balanced parallel-rod constructions, each tuned with a movable capacitor element having surfaces shaped to give a linear displacement-frequency characteristic. Shielded compartments are provided for the three tuned circuits. The tuner input matches a 300-ohm antenna, and the output is suitable for feeding the conventional "40-mc" intermediate-frequency amplifier of a television receiver. Measurements of the performance show the noise figure to be 15 db or better over the band, the image rejection at least 43 db, and the transadmittance (which is a measure of sensitivity) 11 db, or better, above 1 millimho. Oscillator signal at the antenna terminals averages approximately 43 db below 1 volt.

Design of unequal-Q double-tuned transformers

THERE are many instances, in the application of double-tuned transformer coupling, where the primary and secondary Q factors are unequal. When coupling from a crystal converter to the grid of the first intermediate-frequency amplifier, for example, the primary Q is likely to be much lower than the secondary Q. A similar situation exists, if the primary is tuned, when coupling from a transmission line to the grid of a radio-frequency amplifier. An intermediate-frequency interstage transformer, with a damping resistor only across the secondary, may have a high primary Q and low secondary Q.

The design of a superheterodyne receiver for television

The paper deals with some of the factors affecting the complete specification and design of radio- and intermediate-frequency amplifiers for use in television receivers. The work from which this material has been drawn covers receivers for both home and export markets.The field includes the choice of an intermediate frequency and the determination of the exact frequency response of the sound- and vision-channels. A method of obtaining the recommended features is described with reference to the performance of mass-produced apparatus.In addition, the questions of automatic gain control, noise requirements and multi-channel tuning are discussed. In conclusion, the authors suggest future developments.

Universal Design Curves for Intermediate-Frequency Amplifiers with Particular Reference to Phase and Amplitude Distortion and Their Compensation

The accurate assessment of distortion in intermediate-frequency amplifiers and the possibilities of compensation assume importance for the development of frequency-modulated very-high-frequency multi-channel equipment with its concomitant wide bandwidths. The present purpose is to furnish graphical data for the evaluation of phase and amplitude distortion arising from various types of coupling network including compensating combinations, to demonstrate a graphical procedure for accurate design, and to summarize the analysis on which the graphical treatment is based. Sets of nomograms and curves provide for the assessment of performance as represented by gain times bandwidth, and for the interpretation of equation parameters in terms of circuit parameters.

New Techniques for Electronic Miniaturization

The general approach to the problems of miniaturization is outlined with special emphasis on problems caused by the high operating temperatures. In addition to the general techniques, both conventional and printed circuit, three models of miniature wide-band, high-gain radar-type intermediate-frequency amplifiers (having similar characteristics) are described. A discussion of production methods and techniques is also included.

The Demodulation of a Frequency-Modulated Carrier and Random Noise by a Discriminator

With the aid of an idealized representation of the discriminator, Middleton has shown that a frequency-modulated carrier remains intelligible for smaller carrier strengths when narrow-band f-m is used and the limiter is omitted than with a limiter and/or wide-band f-m. It is, therefore, of some interest to treat the demodulation process as it actually occurs, in the absence of a limiter. Thus, the discriminator has been taken to consist of two selective circuits, both fed by the output of the intermediate-frequency amplifier, but peaked at different frequencies, feeding rectifiers whose outputs are subtracted. Each half of the device is treated in the manner used by Rice to determine the result of passing random noise and a sine wave through a rectifier; however, there is a correlation between the noise voltages fed to the two rectifiers. The signal output and the spectral distribution of the noise output are obtained first for quadratic rectification, then in the general case, which is then specialized to linear rectification. The results are applied to a case of rectangular i-f noise spectrum, and the signal-to-noise ratio is determined for the cases of narrow-band and wide-band f-m. These results are found to be very much like those for the idealized representation of the discriminator; all are compared, along with amplitude modulation, in Table I. The optimum signal-to-noise ratio for f-m without a limiter is found to obtain with narrow-band f-m when the discriminator is designed for no wider a band than necessary; this optimum signal-to-noise ratio differs very little from that for a-m.

A Flat-Response Single-Tuned I. F. Amplifier

An intermediate-frequency amplifier, providing double-tuned response using single-tuned circuits with negative feedback, is described. Particular attention is centered on the problems arising in the case where relatively narrow pass bands are wanted.

Intermediate-Frequency Gain Stabilization with Inverse Feedback

Increased gain stability and gain-bandwidth product result from the use of inverse feedback in an intermediate-frequency amplifier. Improvement in gain stability is related to the number of cascaded stages, the stage gain, and the magnitude of the feedback. A circuit is described which uses feedback over a pair of cascaded stages. Generalized selectivity curves for this feedback couple axe shown, and the design procedure is outlined. A description of an experimental amplifier concludes the paper.

Considerations in the Design of a Radar Intermediate-Frequency Amplifier

The intermediate-frequency amplifier of a microwave radar receiver is commonly required to provide approximately 100 decibels amplification in a bandwidth of 1 to 10 megacycles, centered at frequencies in the 30- and 60-megacycle regions. Meeting such requirements involves the use of five to ten amplifier stages of the highest efficiency that can be suited to production methods. In addition, the noise figure of the radar intermediate-frequency amplifier is a significant contributor to the over-all radar receiver noise figure, and must therefore be maintained at an absolute minimum. By examining a particular intermediate-frequency-amplifier design (one providing an over-all bandwidth of 10 megacycles centered at 60 or 100 megacycles), this paper discusses qualitatively the theoretical problems involved in such a design and gives data of practical importance to the engineer attempting to build a similar amplifier. Measured characteristics of approximately fifty amplifiers are summarized to illustrate the end results achieved.

Intermediate-Frequency Amplifiers for Frequency-Modulation Receivers

In order to obtain the maximum benefits from the frequency-modulation system of broadcasting, it is necessary to give special attention to the selectivity and symmetry of the receiver intermediate-frequency-amplifier channel. Voltage feedbacks must be reduced to a minimum in order to obtain good results in mass production without resorting to some sort of stagger tuning. Selectivity and stability formulas, stabilizing methods, and methods of aligning double-tuned transformers are discussed.

Selective Demodulation

A method of demodulation is proposed in which the output current of the demodulator is a linear function of the input voltage, while at the same time provision is made for producing the necessary product terms which will result in demodulation. Demodulation is brought about by integrating the product of the instantaneous value of the modulated wave by the instantaneous value of a wave having the same frequency and phase as the carrier. Where this method of demodulation is used it is proposed that two carriers in quadrature on the same frequency may be employed, reducing the bandwidth to that required for single-sideband transmission. It is suggested that the required linear demodulation characteristics may be obtained through the use of "electron-coupled" demodulators. Theoretical considerations indicate that, when demodulation of this type is employed, selectivity ahead of the demodulator may be dispensed with, the signal-to-noise ratio is improved, greater economy of spectrum space is obtained, the number of tubes required is materially reduced through the use of a common intermediatefrequency amplifier for a number of channels, and any impairment due to the instability of the carrier or oscillator frequency is reduced. As an example of the possible application of the principles outlined, a hypothetical eight-channel transmission system is described.

Considerations in the Design of Centimeter-Wave Radar Receivers

A review of the radar duplexer and receiver, as developed during the war, is presented. Attention is devoted to the principles of operation and typical circuit arrangements employed in the duplexer, the crystal converter, the local-oscillator injection circuits, the intermediate-frequency amplifier, and the automatic-tuning unit. Emphasis is placed on methods found advantageous in the 1-centimeter and 3-centimeter wavelength regions. The interrelation between the various receiver components in determining the over-all receiver noise figure is shown analytically, and typical performance numbers are given.

A Note on Noise and Conversion-Gain Measurements

The development of microwave receivers with a low-gain converter as the first stage has made it desirable that the noise level and the conversion gain be determined independently of the following intermediate-frequency amplifier. Often this converter is a crystal with a conversion gain less than one. This paper discusses some of the general procedures that have been used to measure microwave-converter noise levels and conversion gains.

Capacitance-Coupled Intermediate-Frequency Amplifiers

This paper discusses the performance characteristics of capacitance-coupled intermediate-frequency-amplifier circuits. Double-ended loading is preferred to single-ended loading in order to satisfy practical tolerance requirements. An approximate method is treated for the design of capacitance-coupled attenuating traps for a television intermediate-frequency amplifier. Performance specifications for such an amplifier are considered. Performance characteristics of an amplifier built to conform to the specifications as outlined are shown.

Electronic Frequency Stabilization of Microwave Oscillators

Two circuits for use to control the frequency of a microwave oscillator by an external high Q cavity are described. One of the circuits uses a microwave equivalent of the frequency discriminator, in conjunction with a d.c. amplifier. The other uses the cavity in a special circuit that provides an intermediate-frequency signal that is a measure of the difference between the frequencies of the oscillator and cavity. This allows the use of an intermediate-frequency amplifier. The resulting stability of the oscillators is such that audible beat frequencies can be produced between two oscillators at 10,000 Mc/sec. The resultant signal can be frequency modulated at audiofrequencies, with stabilization acting throughout the modulation cycle. A technique by which the frequency-stabilization systems could be used to investigate, with high resolution, the structure of microwave absorption spectra is suggested.

Characteristics of Vacuum Tubes for Radar Intermediate Frequency Amplifiers

The desired characteristics for vacuum tubes for use in broad-band intermediate frequency amplifiers are primarily high transconductance, low capacitances, high input resistance, and good noise figure. These characteristics determine the frequency bandwidth, amplification, and signal-to-noise ratio attainable with such an amplifier. The maximum operating frequency is generally limited by the input resistance of the tube which decreases as the frequency is increased, and, in some cases, by the tube noise which increases with increasing frequency. Three other characteristics which are also important are small physical size, low power consumption, and ruggedness. The present paper describes how these characteristics are related to the performance requirements for intermediate frequency (IF) amplifiers used in radar systems and shows how the requirements were met in the design of the Western Electric 6AK5 Vacuum Tube.

Transmission of Television Sound on the Picture Carrier

Several pulse methods for the transmission of television sound on the picture carrier during the line-blanking intervals are analyzed from the points of view of signal-to-noise ratio, audio fidelity, and transmitter and receiver design. The advantages of duplex transmission are: (1) elimination of a separate sound transmitter; (2) elimination of the ambiguity and difficulty which may occur when a standard frequency-modulated sound signal is tuned in; (3) freedom of the audio output from the type of distortion which occurs in frequency-modulated receivers as a consequence of excessive drift of the frequency of the local oscillator; and (4) improvement of the phase characteristic of the picture intermediate-frequency amplifier resulting from elimination of trap circuits. The highest audio-modulation frequency in duplex systems must not exceed one half of the line-scanning frequency. This is a disadvantage under the present television standards which specify a line frequency of 15,750 cycles per second. With the exception of pulsed frequency modulation, the signal-to-noise ratios of sound in duplex systems are not so great as the ratio offered by the transmission of a standard frequency-modulated carrier. The comparison is subject to the condition that the amplitude of the frequency-modulated carrier is 0.7 of the peak amplitude of the duplex carrier. The signal-to-noise ratio of a pulsed frequency-modulated signal may equal the ratio of a standard frequency-modulated signal up to a critical distance from the transmitter, but is less at greater distance.

A pulse field-strength measuring set for very high frequencies

The paper describes a portable equipment for the measurement of the field strength of pulse and continuous-wave signals in the frequency bands 20–30 and 40–650 Mc/s (wavelengths 10–15 and 0.46–7.5 m). The equipment consists essentially of a receiver, in which are incorporated calibrated signal-frequency and intermediate-frequency attenuators and an output meter, and a cathode-ray output-indicator unit. The field strength is measured by adjustment of the attenuators for a standard output, which for pulse signals is read on the cathode-ray tube and for continuous-wave signals on the meter. A half-wave dipole aerial is used, and the initial calibration of the standard output in terms of the field strength at the aerial is carried out by a radiation method. The noise voltage of the first circuit of the intermediate-frequency amplifier is used as the reference voltage for setting up the gain of that amplifier, and an internal oscillator supplies the reference voltage for setting the gain of the indicator unit.The minimum measurable field strength varies with frequency between 3 and 500 μV/m on short pulse (< 2 microsec) signals, and between 1.5 and 250 μV/m on long-pulse and continuous-wave signals. The maximum field strengths which can be measured are 140 db above these minimum values. The accuracy of relative measurements on any one frequency is within ± 0.5 db, and that of absolute measurements is within ± 2 db. Within these limits of accuracy the measurements are independent of pulse width for values greater than 0.5 microsec. The recurrence frequency range is 25–100 000 c/s.Various types of field-strength measurement which have been made with the equipment are described, and they illustrate its wide range of application.

Band-Pass Bridged-T Network for Television Intermediate-Frequency Amplifiers

Bridged-T networks offer great economy in television intermediate-frequency amplifiers for sharp attenuation of the associated and adjacent sound channels. A simple design method was obtained by the use of the equivalent lattice. By the same method, general formulas were obtained for the phase, attenuation, and delay characteristics. Two designs are given to illustrate the convenience of the method.

The Design of an Intermediate-Frequency System for Frequency-Modulated Receivers

With a possibility that the present frequency-modulation wave band may be increased in width it has become imperative that an intermediate-frequency amplifier operating at a frequency higher than that most commonly in use at present, 4.3 megacycles, be developed. Stability both as to performance and permanence of adjustment govern the choice of frequency and restrict this choice to a definite maximum value. Design data are given in this paper for an amplifier operating on 8.25 megacycles which satisfies the stability conditions, and gives the required performance.