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Fig. 1. Schematic diagram oj the converter. All resistors are hU+watt carbon. All bypass capacitors arc disk ceramic, Dipped silver mica capacitors arc preferred for capacitance values below 100 but disk ceramics arc acceptable unless otherwise specified. The 1 pL dipped silver mica capacitors are made by Cornell Dubilier. For best sensitivity* connect the 77588 source directly to a ground lug as at A. For better overload control, connect the 1/V191 (or IA'192) diodes across }1 as at CX, and then connect the 77S88 source to the 330 ohm (,005 in/-) bias network as at B.

amplification through the rf stage,

An electrically continuous circuit ground from input BNC receptacle to output BNC receptacle has been provided by size 18 bus wire. All mechanical connections of wire, lugs, screws, and even BNC receptacle base are soldered over to provide this electrically continuous ground circuit. This technique may seem redundant but, in low noise circuitry, it is not particularly wise to depend upon mechanical connections for electrical continuity- Ground currents from one circuit, in general, should not enter the ground circuits of another circuit. This ground current isolation is most easily achieved by using on y one grounding point for each circuit.

Transistor sockets are not used. The transistor leads are soldered directly to ceramic base standoff insulators (Useco 1460B). The transistors, barring electrical accidents, should never require replacing. Since the converter input is at dc ground, the TIS 88 should be well protected from dc overvoltages, The builder who is concerned with rf overvolfc&ges or an unusually strong signal can protect the input with the optional diode and bias circuit described in the schematic diagram. These optional cir cuits will produce a very slight decrease in converter sensitivity,

Ceramics slug tuned coil forms are used throughout Coil losses are most important in the input stage, A slight improvement in noise figure might be obtained by replacing the input coil and fixed capacitor with an air core coil and ceramic base air capacitor, The air core coi! should be of large size wire and can be enameled, silver-coated, or bare. Tinned wire is not recommended. The coils should be at least one diameter distant from other objects. However, air core coils are not actually necessary since the converter already has a lower noise figure (2 dh or less) than can ever be used at the fairly low frequency of 50 MHz with its high atmospheric noise level.

The %" nut and bolt in the mixer selection is used only to fill a hole in the chassis, This hole has been used to hold an rf gain control A 100 K or 1 Meg (audio) taper pot had been installed from mixer gate to ground and it provided several decades of good gain control. However, the mixer stage is now biased near cutoff, and this should provide enough overload control for all but the most unusual situations. No.

Inside the converter* rf stage on right; mixer in center; oscillator on left.

18 wire lias been used in many places merely to obtain high mechanical rigidity. This rigidity adds frequency stability to critical regions such as the oscillator and neutralization circuits.

Finallys the outside of the aluminum chassis can be rubbed with sandpaper or steel wool to give it a pleasing white satin appearance,

Alignment

First of all, it is important that the power supply or battery have the right polarity when plugged into the converter. A medium current diode, with a PJ.V, of 50 volts or more has been set in series with the power input to prevent damaging the transistors, The converter will operate well for voltages in the range of 10-25 volts,

Connect the converter output to the receiver with a shielded cable. Without a shielded cable, 7-11 MHz atmospheric noise may mask the converter signals. Next connect a signal generator to the converter input. Set the generator at 52 MHz, Set all tuning slugs at mid-range. Adjust the oscillator coil slug until the signal generator signal ''kicks" in. Then peak the mixer output. mixer input, rf output, and rf input, in that order. The oscillator coil may need additional peaking as the other coils are adjusted. Since the cascode rf stage is designed for high gain, it may be in oscillation at this point. Careful adjustment of the neutralization coil La should stop the oscillation and bring t ie desired signal in loud and clear. Neutralization of triode type receiver circuits can be difficult especially for the inexperienced builder. However, once

Parts Data

SM—Dipped silver mica.

Li—turns No. 24 en am. wire closewtrand on Vi in. diumt uined form. Tup 1% turns from tfrnL

end, (Miller 4500-3). La—10 uh, molded r.fi choke {Milter £>£30-44), L*—Slug-tuned, 9.9 to 15.0 uh. (Miller 4506). Li, Ls. Lfl, Lt, Li.—SIuk-tuned, „44 to -Tii uli, (MiUer 4501).

L*—turns No. 22 in^ufated wire wound around Lp*

Winding direct ion Sfiuie La, RFCi, RFCa—22 uKt molded r.f. choke (Miller 928052},

CHi—Silicon diode (optional), 50 P.LW minimum, 200

Ct—15 ti> 130 pf. rnicsi |>adder (Arco Klmenco 302), Ji. Jj—BNC receptacle«;

XT A L—4/1.000 MHz, third-overtane, series resona \\t crystal.

the neutralization technique is learned and the circuit stabilized, the low noise performance of the triode stage more than compensates for the tedium of neutralization. Tapping of the bare finger on the rf stage output coil should produce a loud clunk in the receiver speaker and a jump in the S-meter. While so tapping the finger, the neutralization coil should be adjusted for smallest clunk in the speaker and smallest jump in the S-meter. This will be the approximate neutralization point, and the stage should be stable. Remove the signal generator and replace it with an antenna. The atmospheric noise can be used for the fine adjustments of all previously peaked circuits. A 50 ohm noise generator can be used to obtain the very lowest noise figure.

At VHF frequencies, tuned circuits are quite sensitive to component geometries. The geometry of the builder's circuit may differ somewhat from that of the author's so, to properly cover the 50 to 54 MHz range, it may be necessary to slightly change some of the values of the capacitors in the various tank circuits. It may also be necessary to change the value of the RFC La used in the neutralization circuit,

If a TIS 34 with an extremely small gate-to-source cutoil voltage is used for the mixer, the local oscillator injection level may be too high. This will cause cross-moduhv tion distortion and a profusion of clunks and bleeps across the band, This could be remedied by changing the values of the coupling capacitors Cn and C^. However, for most cases, it should be enough to merely try other TIS 34's until the distortion ceases.

The converter is designed for 50 ohm and output impedances. If the receiver used does not have a 50 ohm input, the number of turns of L> should be adjusted for maximum gain

I he measured converter noise figure is about 2 db or less. The measured converter gain is about 30 db, Further reductions in noise figure could be obtained by the selection of an especially quiet TIS 88, and by the use of a very high Q coil-capacitor combination at the converter input. However, a 2 db noise figure is more than adequate for six-meter work.

Since the converter gain is high, an unusually strong signal may cause the converter to overload the communications receiver used as the if amplifier. This overload problem can be avoided by placing an attenuator between the converter output and receiver input.

I would like to thank Texas Instruments, Inc. tor the sample TIS 88 transistors used in the development of this converter.

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