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A complete Eldico SSB installation.

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Fig. 8. Ad audio phase-thilt network.

Fig. 9. Two r.f. phase-shiit networks. A "Knight-Kit” communication* receirer.

prove the linearity of these stages. By thus improving thc linearity, the tubes can be driven harder to obtain even morę signal output. One popular meth-od of feedback is to feed r.f. from the piąte of the finał stage to the cathode of the driver, thus improving the linearity of both stages.

An additional advantage of linear amplification is the reduction of harmonie radiation. And for best results, distortion of these stages should be kept below 0.1%. At least one manu-facturer of SSB eąuipment uses servo tuning to make surę the amplifiers remain at the point of maximum-signal tuning.

Single-sideband equipmcnt is rated as to peak-envelope-power. abbreviated •‘p.e.p.” This is the r.m.s. power de-vcloped at the peak of the modulation cycle. In SSB, the instantaneous power undergoes a much larger variation than in conventional AM transmission, so the peak rating gives a better idea of power than would the average.

The power rating can be mcasured in a dummy load with an oscilloscope. Peak voItage can be determincd, then multiplied by .707 to obtain the r.m.s. va)ue. The '‘p.e.p.” then equals the sejuare of the r.m.s. voltage divided by the resistancc of the dummy load. Peak-envelope-power can also bo de-fined in terms of input, and is then equal to the peak swing of piąte cur-rent of the finał amplifier multiplied by the d.c. piąte voltage.

Receivers

Receivers for SSB reception must be comparable in quality to the trans-mitter with regards to stability, band-width, and linearity. Double, and even triple, i.f. conversions are used in some designs to obtain the selectivity charac-teristics needed. Receiver tuning is probably the biggest disadvantagc en-eountered in SSB because of the re-quired accuracy. However, most of the newer equipment overcomes this dis-advantage admirably.

So that demodulation can takc place, the carrier must be generated locally in each receWer. This means that an oscillator operat i ng at the carrier fre-queney must be in the receiver and its output added to thc sideband being received, otherwise there would be no demodulation bccause there would be no signal to heterodyne with the sideband in the second detector stage.

Bear in mind that the sideband which

Hammarlund HQ170 amateur-band receirer.

t was transmitted is equal in frequenr-* to the carrier plus or minus the aud^:o. But in itself it is only a single fre-quency and if applied to the second detector by itself would produce no usable output. After the carrier is added, the two signals can beat to-gethor to form the audio difference frequency, a replica of the original modulating signal.

Using a superhet receivcr we can inject the carrier at an intermediate frcquency into one of the i.f. stages. Or. a frequency equal to that of the original carrier can be generated and introduced at the receiver antenna in-put terminals. Actually it could be re-inserted at any point between the antenna and the second detector. but the two points listed are those which aro most often used.

Receivers containing a beat-frequen-cy oscillator (b.f.o.) can usc this stage to generate the rcquired carrier. elimi-nnting the necd for an extra stage. The b.f.o. signal should be at least twice the amplitudę of the SSB signal, at the insert ion point, to prevent overmodu-lation and distortion.

Front-end insert ion is considcred to be superior to i.f. insert ion because it facilitates tuning, is morę easily con-trolled. and does not require as good receiver stability. In a double-comcr-sion receiver, carrier insert ion at the high intermediate frequency is better than at the Iow i.f. for somewhat the same reasons.

To tune in a lower sideband. the in-serted carrier frequency must be above the incoming sideband. for tuning in an upper sideband the carrier must be lower. Amplitudę of the re-inserted carrier can be varied by changing re-ceiver coupling or by varying oscillator amplitudę. When interference is heavy, inereasing the amplitudę of the re-inserted carrier usually gives better results.

Filters, some of them adjustable, are often used in the i.f. stages to improve selectivity where needed. Even morę selectivity oeeurs as a result of using a slicer after the last i.f. stage. This is a phasing-type unit, similar to that used in the transmitter <except in re-verse) which also demodulates the SSB signal while eliminating undesired signals.

No matter how it is generated. the re-inserted carrier still rcquires ex-treme stability. If different from the [Continued on pttye 130)

National Radio NC303 amateur receirer.

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