Simple BFO UsingaCeramic Filter

Here is another beat frequency oscillator, using a Undertheseconditions,asactually , which we have just developed. Although simpler shown infigure2, we were able to achieveafrequencycoveragefrom than our previous design (October, 1969), it may be used 452 to 458KHz. as a fixed frequency oscillator; as a two frequency device, This circuit was subsequently adapt- switched for upper or lower sideband reception; or as a ed for use in the 240 Communications continuously variable oscillator covering a fairly wide range. Receiver, described in January, Febru- ary and March of this year. In use, it has come up to all expectations. How- ever,ithad meanwhile come to our ByIanPogson notice that the type SFB-455A single cell were still available and that the earlier report had no founda- In September. October and Novem-ordinary IF winding. Even tioninfact. The need for a simple ber last year, we ran a series of articles withthe100pF shunt capacitorstillBFO is always with us and we thought describingsomeinterestingapplica- acrossthe winding, a significant up-it a good idea to take another look at tionsof Murata ceramicresonators.ward frequency shift could be thepossibilityof developing a BFO The second part of the series, whichachieved. By adding extra capacitancearound the single cell. appearedinOctober, described howacross the combination, the frequency We therefore turned back to figure theseresonators couldbe made tocould be shifted by about the same1. Although not satisfactory as it was, oscillate and used as a beat frequencyamount inthe opposite direction. itseemeda logical place to start. By oscillator. The stability proved to be Although encouraging, we felt that changingit tothecircuitshown almost as good as a crystal with thethe amount of shift so far obtained in figure 3 we were able to adjust the extraadvantagethatthefrequencystill left something to be desired. Sub-frequency by means of the5-55pF could be varied over a sufficiently wide sequently,itwas foundthatifthetrimmer. The coverage ranged from rangefor upper and lower sidebandfunctionsof terminals1-2and 3.4448.1KHz,withthetrimmerfully reception. were reversed, a considerable increasemeshed, to 453.5KHz with the Perhapsitmay beinterestingto in frequency shift could be obtained.almost out and just before oscillation recall just how this all came about andIt was also found desirable to increasebecame unreliable. This was astart to follow it through and see how thethe top capacitive coupling to 180pF.but, although the frequency could be present unit emerged. The initialin- vestigation centred around the single celltypeSFB-455A. Thecircuitis shown infigure1.This circuit was successful,inthatthe resonator did oscillate. A further check showed that the frequency inthiscase could be changed from 450KHz, with the series trimmer shorted,to 456.8KHz, with 10% thetrimmeradjustedtothepoint where any further reductionresulted inunreliable oscillation. The possible change in frequency of 6KHz could beveryuseful,althoughtheactual rangecovereddidnotstraddlethe

likely pass band of a practical IF strip. vasioier11141)4'. Inevaluating thelikelyusefulness ofthiscircuit,we were furtherin- fluenced by a report, current at that time, that production of this particular resonator was to be discontinued. All components can be clearly seen in this picture of the complete These facts led us to consider the BFO unit. Note the ceramic resonator adjacent to the Philips trimmer. double units, type SFD-455B. For a + 9V basic circuit we turned to the success- madelow enough, it could not be made fulcrystaloscillatorwhich we had developed in our own laboratory and high enough. described in September 1966. (File No. We recalled that the introduction of 8/DT/16). This circuit was used very I- OUTPUT inductive reactance into the circuit of successfullyinanumber ofsubse- figure 2 gave a high shift in frequency. quent projects. From this we evolved By introducing inductance between the the circuit shown in figure 2 although, junction of the 5-55pF trimmer and initially, without the coil and capac. the resonator, and earth, we noted a significant upward shiftinfrequency. tors in the base of the first . Presto. This was theanswer. More Overall, results were so encouraging specifically, by leaving the trimmer at that we decided to investigate means the lower capacitance setting previous- of shifting the resonant frequency so lymentioned, and by adding anin- that it could be adjusted to either side ductance of 680uH, we noted an upper of the pass band of the IF filter. shiftto 471.7KHz. In the face ofit Duringtheinvestigation,several then,itseems reasonableto assume importantpointsemerged.Itwas that we can effect a maximum shift of foundthatquitealargefrequency from 448.1KHz to 471.7KHz. shift could be achieved, ±-1KHz or so, Figure Further checks showed that the value by adding inductance and capacitance, of the inductance had considerable in- insuitableproportions,tothebase Figure 1:This is the circuit fluence on the amount of upper shift. Valuesbetween and circuit of thefirsttransistor.Rather basisfor about2.5mH fortuitously, the optimum value of in- which formed the 470uH are the most useful, the lower ductance turned out to be about 1mH, later development of the BFO inductance making a higher shift pos- near enough to the inductance of an as presented. sible. ELECTRONICS Australia, September, 1970 77