27:
734:
These are also troublesome in crowded bands. They allow through unwanted signals that are fairly close to the expected one, but because of the random quality of these phase-noise 'skirts', the signals are usually unintelligible, appearing just as extra noise in the received signal. The effect is that
551:
reference frequency sources. Crystal-controlled oscillators are more stable than inductively and capacitively controlled oscillators. Their disadvantage is that changing frequency (more than a small amount) requires changing the crystal, but frequency synthesizer techniques have made this unnecessary
543:
There are disadvantages to digital synthesis such as the inability of a digital synthesiser to tune smoothly through all frequencies, but with the channelisation of many radio bands, this can also be seen as an advantage in that it prevents radios from operating in between two recognised channels.
792:
yield lower short term phase noise. Then lower frequency (and so lower cost) oscillators phase locked to a digitally divided version of the master clock deliver the eventual VFO output, smoothing out the noise induced by the division algorithms. Such an arrangement can then give all of the longer
768:
Because of the stability of the reference used, digital oscillators themselves tend to be more stable and more repeatable in the long term. This in part explains their huge popularity in low-cost and computer-controlled VFOs. In the shorter term the imperfections introduced by digital frequency
511:
Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal. The advantages include smaller designs, lack of moving parts, the higher stability of set frequency reference oscillators, and the ease with which preset frequencies can be
498:
can be used to control the junction capacitance. The varactor bias voltage may be generated in a number of ways and there may need to be no significant moving parts in the final design. Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low
532:) techniques to decide when to open an audio output channel and alert users to incoming communications. Pre-programmed frequency agility also forms the basis of some military radio encryption and stealth techniques. Extreme frequency agility lies at the heart of
662:
Ideally, for the same control input to the VFO, the oscillator should generate exactly the same frequency. A change in the calibration of the VFO can change receiver tuning calibration; periodic re-alignment of a receiver may be needed. VFO's used as part of a
527:
in that the control computer could alter the radio's tuned frequency many tens, thousands or even millions of times a second. This capability allows communications receivers effectively to monitor many channels at once, perhaps using digital selective calling
612:
techniques) and a phase detector, a control-loop can be set up so that the VCO's output is frequency-locked to a crystal-controlled reference oscillator. The phase detector's comparison is made between the outputs of the two oscillators after
793:
term stability and repeatability of an exact reference, the benefits of exact digital frequency selection, and the short term stability, imparted even onto an arbitrary frequency analogue waveform—the best of all worlds.
617:
by different divisors. Then by altering the frequency-division divisor(s) under computer control, a variety of actual (undivided) VCO output frequencies can be generated. The PLL technique dominates most radio VFO designs
586:
Early channelized communication radios had multiple crystals - one for each channel on which they could operate. After a while this thinking was combined with the basic ideas of heterodyning and mixing described under
342:
oscillator is that only a small portion of the radio receiver (the sections before the mixer such as the preamplifier) need to have a wide bandwidth. The rest of the receiver can be finely tuned to the IF frequency.
701:) can result in increased noise or two signals detected where there should only be one. Additional components can be added to a VFO to suppress high-frequency parasitic oscillations, should these be present.
704:
In a transmitter, these spurious signals are generated along with the one desired signal. Filtering may be required to ensure the transmitted signal meets regulations for bandwidth and spurious emissions.
738:
The effect of VFO phase noise on a transmitter is that random noise is actually transmitted either side of the required signal. Again, this must be avoided for legal reasons in many cases.
445:
where the value of at least one of the passive components is adjustable under user control so as to alter its output frequency. The passive component whose value is adjustable is usually a
773:), and the susceptibility of the common quartz standard to acoustic shocks, temperature variation, aging, and even radiation, limit the applicability of a naïve digital oscillator.
258:
212:
384:
335:. If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter.
325:
295:
154:
114:
746:
Digital or digitally controlled oscillators typically rely on constant single frequency references, which can be made to a higher standard than semiconductor and
638:. Since in general the tuning range is also inversely proportional to Q, these performance factors generally degrade as the VFO's frequency range is increased.
719:
When examined with very sensitive equipment, the pure sine-wave peak in a VFO's frequency graph will most likely turn out not to be sitting on a flat
164:
information. Depending on the receiver design, the IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (
461:
The variable capacitor is a mechanical device in which the separation of a series of interleaved metal plates is physically altered to vary its
1009:
529:
646:
Stability is the measure of how far a VFO's output frequency drifts with time and temperature. To mitigate this problem, VFOs are generally
267:
signal and its unwanted image (the mixing product of opposite sign above), the mixer output will also contain the two original frequencies,
735:
what should be a clean signal in a crowded band can appear to be a very noisy signal, because of the effects of strong signals nearby.
952:
917:
874:
846:
802:
667:
frequency synthesizer have less stringent requirements since the system is as stable as the crystal-controlled reference frequency.
994:
989:
750:-based alternatives. Most commonly a quartz crystal based oscillator is used, although in high accuracy applications such as
751:
601:
478:
409:
generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for
575:
1004:
999:
564:
347:
465:. Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning.
940:
414:
654:
to correct for the frequency drift of the VFO allowing for both wide tuning range and good frequency stability.
581:
83:
157:
442:
889:
784:, tend to combine multiple different references, and in complex ways. Some references like rubidium or
20:
591:
above. Multiple crystals can be mixed in various combinations to produce various output frequencies.
430:
217:
171:
969:
614:
548:
417:, and variable frequency audio generators used in musical instruments and audio test equipment.
26:
948:
913:
870:
842:
762:
676:
664:
651:
647:
594:
353:
332:
401:, VFOs are often used to tune the frequency of the output signal, often indirectly through a
754:
513:
491:
300:
270:
129:
89:
686:
574:
function are stored in digital memory. These are recalled at the right speed and fed to a
536:
techniques that have gained mainstream acceptance in computer wireless networking such as
533:
524:
517:
387:
121:
117:
71:
67:
62:
can be tuned (i.e., varied) over some range. It is a necessary component in any tunable
944:
789:
426:
983:
812:
484:
31:
785:
758:
631:
and spectral purity. All of these factors tend to be inversely proportional to the
410:
331:
combinations of the input signals. These undesired signals are rejected by the IF
781:
728:
720:
714:
628:
462:
398:
51:
747:
632:
402:
339:
165:
161:
55:
350:, the VFO is tuned to the same frequency as the incoming radio frequency and
807:
446:
391:
59:
727:' in the signal's timing will mean that the peak is sitting on 'skirts' of
681:
A plot of a VFO's amplitude vs. frequency may show several peaks, probably
682:
635:
500:
474:
450:
328:
495:
770:
724:
841:. Newington, Connecticut: American Radio Relay League. Chapter 10.
537:
487:
406:
63:
25:
571:
560:
The electronic and digital techniques involved in this include:
214:
or more commonly, the difference frequency (down-conversion),
490:
exhibits capacitance. Since the width of its non-conducting
405:
process similar to that described above. Other uses include
160:(IF) signal that can be processed downstream to extract the
777:
627:
The quality metrics for a VFO include frequency stability,
494:
depends on the magnitude of the reverse bias voltage, this
839:
The ARRL Handbook for Radio
Amateurs, Sixty-Eighth Edition
891:
Generating frequency chirp signals to test radar systems
16:
Oscillator whose frequency can be tuned over some range
788:
provide higher long term stability, while others like
523:
It is also possible for the radio to become extremely
356:
303:
273:
220:
174:
132:
92:
86:, the incoming radio frequency signal (at frequency
378:
319:
289:
252:
206:
148:
108:
935:Clark, Kenneth K. & Hess, Donald T. (1978).
930:
928:
731:at frequencies either side of the desired one.
386:Hz. Demodulation takes place at baseband using
832:
830:
828:
689:with some other incoming signal and produce a
66:transmitter and in receivers that work by the
867:Communication Receivers Principles and Design
685:related. Each of these peaks can potentially
547:Digital frequency synthesis relies on stable
8:
937:Communication Circuits: Analysis and Design
650:to a stable reference oscillator. PLLs use
19:"VFO" redirects here. For other uses, see
860:
858:
578:where the required sine wave is built up.
361:
355:
308:
302:
278:
272:
241:
225:
219:
195:
179:
173:
137:
131:
97:
91:
425:There are two main types of VFO in use:
824:
70:principle. The oscillator controls the
570:Enough data points for a mathematical
7:
126:with the VFO output signal tuned to
776:This is why higher end VFO's like
338:The advantage of using a VFO as a
14:
803:Numerically controlled oscillator
74:to which the apparatus is tuned.
609:
972:. Microwaves & RF Magazine.
968:Hittite Microwave Corp (2009).
837:Larry D. Wolfgang, ed. (1991).
600:Using a varactor-controlled or
512:stored and manipulated in the
1:
1010:Wireless tuning and filtering
970:"Compact PLLs Integrate VCOs"
769:division and multiplication (
605:
602:voltage-controlled oscillator
479:voltage controlled oscillator
253:{\displaystyle f_{IN}-f_{LO}}
207:{\displaystyle f_{IN}+f_{LO}}
44:variable frequency oscillator
765:are as of 2018 also common.
588:
576:digital-to-analog converter
556:Digital frequency synthesis
520:in the design in any case.
263:In addition to the desired
1026:
712:
674:
604:(VCO) (described above in
582:Direct frequency synthesis
472:
449:, but could be a variable
415:time domain reflectometers
397:In a radio frequency (RF)
348:direct-conversion receiver
18:
941:San Francisco, California
912:, John Wiley & Sons,
565:Direct digital synthesis
379:{\displaystyle f_{IF}=0}
84:superheterodyne receiver
780:transmitters locked to
38:1969, with external VFO
995:Communication circuits
990:Electronic oscillators
908:Holt, Charles (1978),
865:Rohde, Ulrich (1988),
380:
321:
320:{\displaystyle f_{LO}}
291:
290:{\displaystyle f_{IN}}
254:
208:
158:intermediate frequency
150:
149:{\displaystyle f_{LO}}
110:
109:{\displaystyle f_{IN}}
39:
675:Further information:
443:electronic oscillator
381:
322:
292:
255:
209:
151:
111:
29:
947:. pp. 216–222.
441:An analog VFO is an
354:
301:
271:
218:
172:
130:
90:
21:VFO (disambiguation)
910:Electronic Circuits
742:Frequency reference
697:(sometimes spelled
552:in modern designs.
503:and non-linearity.
615:frequency division
549:crystal controlled
483:A reversed-biased
376:
317:
287:
250:
204:
146:
106:
40:
1005:Electronic design
1000:Radio electronics
763:Rubidium standard
755:cellular networks
723:. Slight random '
677:Spurious emission
665:phase-locked loop
652:negative feedback
595:Phase locked loop
1017:
974:
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923:
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852:
834:
693:response. These
633:tuning circuit's
516:that is usually
514:digital computer
492:depletion region
457:Tuning capacitor
388:low-pass filters
385:
383:
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869:, McGraw Hill,
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790:hydrogen masers
744:
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534:spread spectrum
525:frequency-agile
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68:superheterodyne
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945:Addison-Wesley
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795:
743:
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713:Main article:
710:
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672:
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648:"phase locked"
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954:0-201-01040-2
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921:
919:0-471-02313-2
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876:0-07-053570-1
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850:
848:0-87259-168-9
844:
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813:Tuner (radio)
811:
809:
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800:
796:
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787:
786:cesium clocks
783:
779:
774:
772:
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759:atomic clocks
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749:
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658:Repeatability
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485:semiconductor
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411:oscilloscopes
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32:amateur radio
28:
22:
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775:
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761:such as the
745:
737:
733:
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703:
698:
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683:harmonically
680:
661:
645:
626:
559:
546:
542:
522:
510:
507:Digital VFOs
482:
460:
440:
424:
403:heterodyning
396:
345:
340:heterodyning
337:
327:and various
264:
262:
122:
82:In a simple
81:
47:
43:
41:
35:
34:transmitter
897:, IFR corp.
782:atomic time
729:phase noise
721:noise-floor
715:Phase noise
709:Phase noise
629:phase noise
623:Performance
463:capacitance
437:Analog VFOs
399:transmitter
116:) from the
52:electronics
30:A Heathkit
984:Categories
819:References
748:LC circuit
610:analog VFO
473:See also:
392:amplifiers
56:oscillator
808:Resonance
642:Stability
447:capacitor
235:−
162:modulated
72:frequency
60:frequency
797:See also
691:spurious
636:Q factor
606:varactor
518:embedded
501:Q factor
475:varactor
469:Varactor
451:inductor
329:harmonic
725:jitters
699:spuriae
589:purpose
496:voltage
431:digital
118:antenna
78:Purpose
951:
916:
873:
845:
771:jitter
695:spurii
671:Purity
618:today.
608:under
427:analog
333:filter
58:whose
54:is an
895:(PDF)
597:(PLL)
567:(DDS)
538:Wi-Fi
488:diode
421:Types
407:chirp
346:In a
123:mixed
64:radio
50:) in
36:circa
949:ISBN
914:ISBN
871:ISBN
843:ISBN
752:TDMA
572:sine
477:and
429:and
413:and
390:and
297:and
687:mix
530:DSC
168:),
120:is
48:VFO
986::
943::
939:.
927:^
857:^
827:^
778:RF
757:,
540:.
453:.
433:.
394:.
265:IF
260:.
42:A
957:.
851:.
528:(
374:0
371:=
366:F
363:I
359:f
313:O
310:L
306:f
283:N
280:I
276:f
246:O
243:L
239:f
230:N
227:I
223:f
200:O
197:L
193:f
189:+
184:N
181:I
177:f
142:O
139:L
135:f
102:N
99:I
95:f
46:(
23:.
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