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General Technical Description...

    The DAVCO DR-30 is a double conversion superheterodyne receiver employing 38 solid-state devices in circuitry designed for maximum, uncompromising performance and versatility.

    The DR-30, in its standard form, gives full coverage of all amateur frequencies between 3. 5 and 50. 5 Mc., with provision for other coverages between 3.0 and 54 Mc. at the user's option. Three selectivity choices are provided for AM, SSB, and CW reception. Other standard features include a built-in T notch, a crystal calibrator, and a noise blanker.

    The DR-30 utilizes field-effect transistors in the RF stage and in the first mixer. The unusually low noise of these transistors provides exceptional weak-signal reception; the DR-30 sounds quiet between stations, and the unique capability of the field-effect transistor in resisting overloading and cross-modulation coupled with the high sensitivity gives superb performance.

    The variable selectivity is provided by three IF transformers, three ceramic transfilters, a 455 kc. crystal filter and a 2.1 kc. Collins mechanical filter. Switching is effected through switching diodes operated by DC from the front panel. Each selectivity choice is designed for exceptional skirt rejection and the optimum bandwidth for reception in each receiver mode.

    The noise-blanker is designed particularly for CW and SSB use, providing a dramatic improvement in the reception of CW and SSB under the most difficult conditions. The performance of the blanker permits virtual elimination of impulse noises reaching the receiver from vehicle ignitions, power lines, and electrical equipment. It is usually possible to extract extremely weak signals of perfect intelligibility even when noise causes very high S-meter readings. Operation is very simple: set the front-panel threshold control until noise disappears.

    Both the noise limiter and T-notch circuits are in the IF portion of the receiver. These circuits prevent the actuating of the AGC system by interference and 80 eliminate any desensitizing of the receiver by undesired signals or noise.

    The compactness of the DR-30 is achieved, in part, by the use of special toroid coils in the VFO and RF circuitry. For a given Q, a toroid is much smaller, requires less shielding, and is more stable them an ordinary coil.

    The DR-30 can be used with the chassis at either positive or negative ground . There is diode protection from an incorrect power supply connection. Muting provisions and built-in RF protection make it compatible with any existing equipment.

    Varying input voltages have no effect on stability. The panel lights can be switched off to reduce drain while operating from batteries. The audio output matches a wide variety of speaker and earphone impedances. Output jacks on the rear of the receiver give access to the crystal oscillators, the VFO, and the mechanical filter for use with companion transmitters or home-built equipment. A front-panel switch wired to the rear connection plug permits control of other accessories such as a phone patch or an antenna relay.

    The DR-30 is designed for mobile, fixed, and portable operation under the demands of continuous commercial service and under extreme environmental conditions.

General Technical Description

    The DAVCO DR-30 receiver is designed and manufactured to the standards reserved for scientific instrumentation. The same care and precision used in the production of optical and precision measurement equipment are employed throughout DAVCO manufacturing operations.

    The basis of the DR-30 is the solid, 3/16 inch thick aluminum extrusion used for the chassis. The extrusion is milled to provide holes, slots, and surfaces for the attachment of parts and sub-assemblies. The inherent rigidity of the extrusion contributes to the superb mechanical and frequency stability of the DR-30

    The subassemblies mounted to the chassis include

    (1) The wiring harness composed of the plug-in module connectors, the controls, and the between-module wiring All the wiring is TEFLON insulated so that servicing or modifications can be undertaken without fear of burnt insulation from soldering.

    (2) The main tuning drive for the DR-30 is an aluminum block milled to closer than one-thousandth of an inch tolerances. The drive mechanism couples directly from the tuning knob to the main tuning capacitor. Attached to the milled block are four precision-ground ball-bearing races in which the tuning shafts with their integral pinion gears rotate Also on the pinion shafts are the two 64-pitch spring-loaded split anti-backlash gear assemblies. The two-step reduction gives a tuning ratio of 81:1. A small flywheel on the first shaft is employed to achieve smooth tuning and to permit rapid band excursions.

    (3) A dial-drag knob located just below the main tuning knob allows the operator to apply an adjustable amount of friction to the tuning mechanism. The dial-drag is most useful in locking the dial for fixed-frequency applications and in mobile operation when the operator might accidentally jar the tuning knob. With no drag applied, tuning is virtually effortless and, even in band-scanning, operator fatigue is eliminated.

    (4) The dial pointer is driven by a cord wound on a threaded shaft. The thread of the shaft allows the cord to wind in only one way, and calibration cannot shift.

    Since the cord moves only the dial indicator, not the tuning capacitor, cord life is virtually unlimited.

    (5) The panels, sub-panels, and brackets which hold the controls and other parts are made of stainless steel.

    (6) The band-change switch part of the RF module assembly, is a top-quality ceramic unit.

    (7) All the active electronic circuitry of the DR-30 is mounted on subassemblies which plug into the main harness assembly. The modules slide into the slots in the chassis extrusion which guide them to the harness receptacles and hold them solidly. The circuit module boards are of glass-epoxy construction. Each of the nine modules is described in section VI of this manual.

    (8) The receiver is provided in a scratch-resistant case of enamel textured 1/16 inch aluminum but can be easily removed from the case for custom installation.

Theory of Operation...

    The DR-30 receiver 16 a double-conversion super-heterodyne unit with frequency coverage of 12 one-half megacycle bands in the range of 3.5 to 54 megacycles.

    Signals from the antenna input J1 pass through the RF GAIN CONTROL and thence to a low-impedance tap on the antenna input coil L1 . This high-Q toroidal inductor is tuned by the preselector capacitor to a frequency of approximately 6.5 Mc. to 17 Mc. For frequencies lower than this range, capacitors are connected across the toroid by the band switch S1; for higher frequencies, inductors are connected. The RF amplifier, Q1, is a field-effect transistor (Type K1504). The field-effect transistor is characterized by a high operating impedance, and is a voltage-operated device similar to a tube. This voltage-operation allows maximum pre-selector selectivity, since no compromise between power transfer and Q is necessary as would be the case with normal (bipolar) transistors. In addition, the transfer characteristics of the field-effect transistor approach very closely the theoretical square-law configuration, and performance under conditions which lead to cross-modulation and overloading in other devices is markedly superior. Since the field-effect transistor is a majority-carrier device, the shot-noise noted in bipolar transistors is absent; this field-effect transistor allows extremely low-noise operation even to the UHF region. The output from the RF amplifier is via a tuned circuit similar to the input circuit.

    The high frequency conversion oscillator, Q2, is operated in the fundamental mode from crystals up to 18 Mc., and as an overtone oscillator above 18 Mc. using the collector load as a resistor for fundamental operation and a tuned circuit for overtone. A different tunable collector circuits provided for each overtone crystal range. The first conversion oscillator is fed to an oscillator amplifier, Q4, which provides correct amplitude and impedance matching for the first mixer.

    Signals from the xtal oscillator and the RF amplifier are combined in the first mixer, The first mixer is also a field-effect transistor stage, so that the high performance in weak-signal reception and rejection of undesired effects made possible in the RF amplifier are not lessened, The crystal oscillator frequencies on each band are higher than the received signal, and the output range from the mixer is 2.405 to 2.955 Mc. This range is fed to a high-Q tuned circuit tracked to the VFO by one section of the main tuning capacitor, C2. A first IF amplifier, Q4a, follows this circuit.

    The VFO, or tuning oscillator, Q9, covers a range of 1.950 - 2.500 Mc. and utilizes a high-Q toroidal inductive element for extreme stability. Note that, because of the choice of xtal frequencies, when the VFO frequency goes "up", the received frequency goes "down". An amplifier-buffer stage (Q10) for the VFO provides isolation for the tuning oscillator, and the voltage supply to the VFO is regulated by a separate zener regulator.

    The second mixer combines the VFO and IF and has an output of 455 kc. Tunable IF transformers precede and follow a 455 kc amplifier (Q6), and several fixed-tuned ceramic 455 kc filter-resonators provide additional selectivity.

    The output of the 455 IF amplifier is fed to the noise limiter amplifier (Q7) which increases the amplitude and rise time of the noise pulses which reach it. Since the effective selectivity is still rather broad at this point, no significant noise pulse lengthening has taken place, as would be the case if the noise were not eliminated until after the highly selective IF filters. A noise amplifier-detector further processes the noise pulse, providing a pulse output which is used to turn "off" the switching diode gate (D3) for the duration of the noise pulse and this effectively mutes the receiver during this time. Noise pulses are thereby prevented from actuating the AGC circuits or reducing the receiver sensitivity. The ANL level control adjusts the bias on the gating diode and therefore the noise limiter threshold.

    Following the pulse gate several diodes, D4, 5, 7, 8, 10, make up a switching network which changes the signal path to the various selectivity paths; DC biasing is applied to the diodes to turn them on or off as required. The broad selectivity incorporates three ceramic 455 kc filter resonators plus three IF transformers; the 2.1 kc selectivity path uses the Collins mechanical filter, while the narrowest path for CW reception uses a 455 kc xtal filter in series with the mechanical filter. Other switching diodes are used in the circuit for connecting the mechanical filter to the connector jacks on the rear of the receiver for use with a companion transmitter or other units.

    One additional 455 kc IF amplifier (Q12) follows the filters, and precedes the rejection notch circuit. A back-panel screwdriver notch depth control is factory pre-set for best results. This notch is tunable across the IF passband and provides up to 60 db rejection of an interfering carrier. Since, like the noise limiter, the notch eliminates the interference before it reaches the AGC circuitry of the receiver, the AGC is not actuated by the interference but only by the desired signal. The receiver is, therefore, operating at the optimum sensitivity for the desired signal, not the QRM-QRN.

    Following the notch circuit is the AGC isolation amplifier, Q20, which prevents the AGC detector (Q21) from causing distortion in the other detectors. The AGC detector itself operates in a "bootstrap" arrangement to provide the fast-attack adjustable-hold characteristics which are desired. The output of the AGC amplifier varies with signal strength from full B- to 1 volt negative; this control voltage is fed via a voltage divider to the RF stage, the 1st IF amplifier, the 2nd mixer, and the 1st 455 kc amplifier, and is also sent to the S-meter. Hold action for the AGC is adjusted by changing the RD constant of the hold circuit.

    For CW and SSB reception, a crystal BFO-carrier generator signal (Q19), chosen in relation to the passband of the mechanical filter, is injected into the product detector (Q18). A circuit in the VFO shifts the frequency to compensate for the USB and LSB signal-frequency shift. For AM reception, a low-distortion AM detector (Q13) is employed.

    The function switch, in addition to shifting the VFO and the BFO crystals, selects the output of one or the other detectors, feeding it to the audio preamplifier and thence to the AG Gain control. An additional audio preamp and driver stage then provide the signal for the push-pull transformerless audio output circuit which is designed for low distortion and good communications quality.

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This Home Page was created by wa3key, Wednesday, September 23, 1998
Most recent revision Wednesday, September 23, 1998