September 2000 QST

By Dan Wissell, N1BYTThe OCR II ReceiverHere’s the radio a number of readers have been asking for:A simple, all-mode shortwave receiver based on thecombination of the popular SLR and OCR receiver designs.ince the introduction of the SLRS(shielded loop receiver) 1 and OCR(optically coupled regenerative) 2receiver designs in QST, I havebeen gratified by the overwhelmingly positiveresponse from builders of these receivers.Many builders have asked the samequestion: “How can I convert the receiverto cover a wider frequency range?” Independentlyconverting the SLR or the OCR tocover a broader frequency range poses designchallenges. Being a simple D-C design,it’s easy to make the SLR cover a broaderfrequency range, but this receiver is not suitablefor good AM shortwave reception. Onthe other hand, the OCR is by design anall-mode receiver, but it’s quite difficult tomake it cover a broader frequency range. Toanswer the question, I combined the SLRand OCR designs to produce an all-modemultiband (ie, 3.5 to 8.5 MHz) shortwavereceiver that I’ll describe here.The challenge I faced was designing areceiver that retains the qualities of bothearlier unique designs. For the SLR, thesequalities include its sensitivity and abilityto use a small loop antenna to reduce localnoise pickup. The OCR offers the extraordinaryperformance of the optically isolatedregenerative detector, providing all-modeoperation. The receiver presented here answersthe design challenge, yet containsabout the same number of components asused in either one of the other receivers.Because I’ve provided a means of usingsimple random-wire antennas and a tunedloopantenna, I’ve dubbed this receiver theOCR II. A PC board and kit of parts areavailable to speed construction. 3 I encourageyou to review the previous two QSTarticles to gain a greater insight into the evolutionof this design (see Notes 1 and 2).The Receiver CircuitOverviewRefer to Figure 1. The OCR II is basicallya single-conversion receiver with a1Notes appear on page 37.32 September 2000455-kHz IF. An incoming 3.5- to 8.5-MHzsignal is converted to the IF, amplified andpresented to the detector, which is an OCRoperating at a fixed frequency of 455 kHz.An audio preamplifier and a headphoneamplifier follow the OCR. This approachis similar to that employed by simple receiverdesigns of the 1950s and 1960s thatuse oscillating (regenerative) detectors at afixed IF. There is, however, no comparisonbetween the performance of those earlierdetectors and the better OCR!DescriptionAs in the SLR design, the receiver’s converteremploys an SA602 mixer, U1. L1 andBANDSET capacitor C9 control U1’s internaloscillator frequency. Tuning diode D1provides bandspread. The oscillator tunesbetween about 3 and 8 MHz. This providescoverage of about 3.5 to 8.5 MHz withoutthe need for a band switch. This tuning rangeincludes 80 and 40 meters and a number ofpopular shortwave bands. It’s possible tooperate the mixer at higher frequencies, butmore-complicated oscillator circuits arePHOTOS BY JOE BOTTIGLIERI, AA1GWneeded to achieve the required frequencystability. A preselector consisting of Q1 andrelated components precedes the converter.The preselector allows the use of simple wireantennas. T1 and C1 form a tuned circuitproviding receiver front-end selectivity thathelps minimize images. R2 at the gate of Q1reduces the T1/C1 tuned-circuit Q andsufficiently broadens the tuning so that avernier drive is not required with C1.The incoming-signal level can be attenuatedby R1, a 1-kΩ potentiometer. Providingattenuation control is important with theSA602 mixer. Overloading the mixer createsmany unwanted mixing products thatproduce considerable audio hash. The loopantenna used in the original SLR makes itvery difficult to overload the mixer. That’sone of the reasons the apparent sensitivityand selectivity of the SLR receiver designare so good.Broadband transformer T2 converts thesingle-ended low-impedance output of Q1into a fairly well balanced 3-kΩ input impedancerequired by the mixer. As I foundwith the SLR receiver, the SA602 works