146:. The first type of transmission was generated using a spark, since the spark fired at around 1000 times a second (when the telegraph key was pressed). The resulting damped waves (ITU Class B) could be received on a basic crystal set employing a diode detector and an ear phone as a spark rate tone. It was only with the introduction of tube transmitters that were able to create streams of continuous radio frequency carrier, that a BFO was required. The alternative was to modulate the carrier with an audio tone around 800 Hz and key the modulated carrier to permit use of the basic diode detector in the receiver, a method used for medium frequency (MF) marine communications up to 2000 (emission type A2A). Radio transmission using tubes started to replace spark transmitters at sea from 1920 onwards but were not eliminated before 1950.
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17:
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219:| which sounds like a tone in the receiver's speaker. During the pulses of carrier, the beat frequency is generated, while between the pulses there is no carrier so no tone is produced. Thus the BFO makes the "dots" and "dashes" of the Morse code signal audible, sounding like different length "beeps" in the speaker. A listener who knows Morse code can decode this signal to get the text message.
446:
generator. By using crystal and adjustable frequencies higher than the audio frequency desired, a wide tuning range can be obtained for a small adjustment in the variable oscillator. Although the beat-frequency oscillator can produce an output with low distortion, the two oscillators must be very stable to maintain a constant output frequency.
157:
simply sounds like "clicks". Sometimes, when the carrier pulses are strong enough to block out the normal static atmospheric "hiss" in the receiver, CW signals could be heard without a BFO as "pulses" of silence. However this was not a reliable method of reception. In order to make the carrier pulses
273:
were invented, the first CW receivers used a wheel with electrical contacts around its rim, spun at a high speed by a motor, to interrupt a current to create a radio frequency signal to beat with the incoming radio signal. This example, at the
Tuckerton transatlantic receiving station in New Jersey
445:
Another form of beat-frequency oscillator is used as an adjustable audio frequency signal generator. The signal from a stable crystal-controlled oscillator is mixed with the signal from a tuneable oscillator; the difference in the audio range is amplified and sent as the output of the signal
226:(TRF) receivers in the 1910s-1920s, beat with the carrier frequency of the station. Each time the radio was tuned to a different station frequency, the BFO frequency had to be changed also, so the BFO oscillator had to be tunable across the entire frequency band covered by the receiver.
424:
By varying the BFO frequency around 44000 (or 46000) Hz, the listener can vary the output audio frequency; this is useful to correct for small differences between the tuning of the transmitter and the receiver, particularly useful when tuning in
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against the IF signal in the mixer stage of the receiver. Any drift of the local oscillator or the beat-frequency oscillator will affect the pitch of the received audio, so stable oscillators are used.
241:, modern BFOs which beat with the IF need only have a constant frequency. There may be a switch to turn off the BFO when it is not needed, when receiving other types of signals, such as AM or
245:. There is also usually a knob on the front panel to adjust the frequency of the BFO, to change the tone over a small range to suit the operator's preference.
437:
For single sideband reception, the BFO frequency is adjusted above or below the receiver intermediate frequency, depending on which sideband is used.
115:
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134:. The different length pulses of carrier, called "dots" and "dashes" or "dits" and "dahs", are produced by the operator switching the
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Separate BFO oscillators were manufactured for receivers that didn't have them; a Rohde und
Schwarz STI4032 from 1944.
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297:= 45000 Hz. That means the dits and dahs have become pulses of a 45000 Hz signal, which is inaudible.
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68:
51:) transmissions to make them audible. The signal from the BFO is mixed with the received signal to create a
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421:= 44000 or 46000 Hz produces the desired 1000 Hz beat frequency and either could be used.
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16:
83:
524:
486:
485:
Paul
Horowitz, Winfield Hill "The Art of Electronics 2nd Ed." Cambridge University Press 1989
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233:
receiver the different frequencies of the different stations are all translated to the same
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56:
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To make them audible, the frequency needs to be shifted into the audio range, for instance
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of the receiver. This signal is mixed with the IF before the receiver's second detector (
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frequency which is heard as a tone in the speaker. BFOs are also used to demodulate
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412:, such as the radio's speaker, which cannot vibrate at such a high frequency.
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86:. What he called the "heterodyne" receiver was the first application of the
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audible in the receiver, a beat frequency oscillator is used. The BFO is a
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The beat frequency oscillator was invented in 1901 by
Canadian engineer
75:
listeners; they are almost always found in communication receivers for
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that was suppressed at the transmitter. BFOs are sometimes included in
63:(SSB) signals, making them intelligible, by essentially restoring the
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553:
260:
252:
15:
550:. Integrated Publishing, Electrical Engineering Training Series.
307:= 1000 Hz. To achieve that, the desired BFO frequency is
186:). In the detector the two frequencies add and subtract, and a
510:, United States National Bureau of Standards, 1932 page 691 ff
408:= 89000 or 91000 Hz, is unneeded. It can be removed by a
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stage of the receiver, this creates two other frequencies or
548:
265:
One of the first crude examples of a BFO, the
Goldschmidt
466:
The ARRL Handbook for Radio
Amateurs Sixty Eighth Edition
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165:that generates a constant sine wave at a frequency
198:range results at the difference between them:
429:(SSB) voice. The waveform produced by the BFO
40:used to create an audio frequency signal from
508:Bureau of Standards Journal of Research Vol 7
8:
504:An Improved Audio Frequency Generator RP367
149:Since the pulses of carrier have no audio
122:, information is transmitted by pulses of
79:, which often receive CW and SSB signals.
464:Larry Wolfgang, Charles Hutchinson (ed),
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324:is mixed with the BFO frequency in the
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116:International Telecommunication Union
102:(CW) radio transmission, also called
7:
523:, Holt, Rinehart and Winston, 1972,
521:Principles of Modern Instrumentation
385:| = 1000 Hz, is also known as the
14:
274:in 1917, created a 40 kHz signal.
130:which spell out text messages in
20:Add-on 455 kHz homemade BFO board
153:, a CW signal received by an AM
519:Frank Spitzer, Barry Howarth,
222:The first BFOs, used in early
1:
317:When the signal at frequency
282:signal, and the receiver's
138:on and off rapidly using a
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278:A receiver is tuned to a
108:wireless telegraphy (W/T)
30:beat frequency oscillator
172:that is offset from the
69:communications receivers
579:Electronic oscillators
574:Communication circuits
476:-9, pages 12-29,12-30
314:= 44000 or 46000 Hz.
284:intermediate frequency
275:
258:
235:intermediate frequency
174:intermediate frequency
114:and designated by the
21:
269:. Before vacuum tube
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224:tuned radio frequency
163:electronic oscillator
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362:difference frequency
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259:
84:Reginald Fessenden
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594:Radio electronics
589:Electronic design
120:emission type A1A
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544:"Radiotelephone"
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160:radio frequency
104:radiotelegraphy
100:continuous wave
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61:single-sideband
45:radiotelegraphy
36:is a dedicated
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502:E. G. Lapham,
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474:978-0872591684
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128:carrier wave
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330:heterodynes
271:oscillators
229:Since in a
184:demodulator
136:transmitter
124:unmodulated
90:principle.
568:Categories
493:, page 898
450:References
441:Other uses
360:|. The
280:Morse code
267:tone wheel
192:heterodyne
151:modulation
132:Morse code
88:heterodyne
73:short wave
53:heterodyne
42:Morse code
38:oscillator
531:, page 98
194:) in the
142:called a
468:, ARRL,
378:−
346:|, and |
339:−
326:detector
94:Overview
249:Example
65:carrier
527:
489:
472:
140:switch
126:radio
431:beats
369:audio
305:audio
290:) is
239:mixer
203:audio
196:audio
106:, or
24:In a
558:ARRL
525:ISBN
487:ISBN
470:ISBN
332:: |
57:beat
28:, a
419:BFO
404:bfo
402:+ F
383:BFO
371:= |
358:BFO
344:BFO
312:BFO
217:BFO
205:= |
170:BFO
118:as
110:or
98:In
55:or
34:BFO
32:or
570::
556:,
546:,
506:,
457:^
400:if
398:(F
396:,
389:.
376:IF
364:,
353:+
351:IF
337:IF
322:IF
295:IF
288:IF
243:FM
212:-
210:IF
180:IF
49:CW
560:.
416:f
406:)
380:f
373:f
366:f
355:f
348:f
341:f
334:f
319:f
309:f
302:f
292:f
286:(
214:f
207:f
200:f
190:(
177:f
167:f
47:(
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
