Download Introduction to Fundamental Crystal Oscillators

Introduction to
Fundamental Crystal
Oscillators
This side will explain some different Crystal Oscillators.
All contribution to this page are most welcome!
Jump to :
 Introduction to Fundamental Crystal Oscillators 1 - 21 MHz
 Series Resonant Circuit
 Parallel Resonant Circuit
 Introduction to Overtone Crystal Circuits
 Third Overtone Oscillator Circuit
 Fifth Overtone Oscillator Circuit
 Ovetone Crystal Oscillator for up to 200Mhz
Introduction to Fundamental Crystal Oscillators 1
- 21 MHz
Crystal units manufactured by Quartslab Marketing Ltd for use over the frequency range 1 to 21
MHz use an AT cut quartz plate and operate in their fundamental mode. Crystal units may be
specified for operation in either a series or parallel resonant condition. This operating condition
should be correctly specified when ordering or the crystal unit will not operate on the correct
frequency. Typically, over the frequency range of 1 - 21 MHz there may be between 2 and 15
kHz difference between the parallel resonant and series resonant conditions, the series
resonant condition being lower in frequency.
If parallel resonance is required, it is necessary to specify a load capacitance for the Crystal.
This capacitance is not related to any capacitance present in the crystals or the circuit but refers
to the load capacitance used when crystals are measured in standard test sets. The preferred
value of capacitance is 30 pF but crystals can be manufactured for other values.
These circuits are designed for non-critical adjustment and operation with easily available
components. In each case, an output of at least 500 millivolts RMS is available across a load of
1000 ohms in parallel with 15 pF. The circuits are suitable for use with supply voltages from 5 to
10 volts DC. Over this voltage range frequency stabilities of 0.001% (10 ppm) should be
achieved.
Please note that where no load capacitance is specified for fundamental crystals we supply
them calibrated for a 30 pF load and although the circuit is shown as suitable for crystals at 0.95
MHz (950 kHz) we do not offer crystals below 1.5 MHz.
Series Resonant Circuit
R1, R2, R3, R4 See Below
C1, C2, C3, See Below
C4 0.01µF
C5 47pF
TR1 BC108 or similar
L1 Close wound with 37SWG enamelled wire on 7.62mm diameter former with Neosid F25 coil
Frequency of X1
R1 R2
MHz
R3
R4
C1
0.95 -1.65
68k 33k
*
2.2k .0047mf
1.6 - 2.5
68k 33k
*
2.2k .0047mf
2.5 - 4.0
68k 33k
4.0 - 6.0
15k 6.8k
6.0 - 10.0
15k 6.8k
10.0 - 15.0
15k 6.8k
15.0 - 21.0
15k 6.8k
560
ohm
560
ohm
560
ohm
560
ohm
560
1.5k .0047mf
1.5k
.001mf
1.5k
150 pF
680
ohm
680
100 pF
100 pF
C2
C3
L1
680
pF
680
pF
220
pF
270
pF
220
pF
220
pF
100
680
pF
680
pF
220
pF
270
pF
220
pF
220
pF
100
140
Turns
65 Turns
65 Turns
40 Turns
26 Turns
16 Turns
10 Turns
ohm
ohm
pF
pF
Parallel Resonant Circuit
R1 100K
R2 33K
R3 See below
C1, C2, C3 See below
C4 . 01µF
C5 47pF
VC 60pF Trimmer
TR1 BC108 or similar
Frequency of X1 MHz
0.95 -3.0
3.0 - 6.0
6.0 - 10.0
10.0 - 18.0
10.0 - 18.0
18.0 - 21.0
R3
3.3k
3.3k
2.2k
1.2k
1.2k
680 ohm
C1
220 pF
150 pF
150 pF
100 pF
100 pF
68 pF
C2
220 pF
150 pF
150 pF
100 pF
100 pF
33 pF
C3
not used
33 pF
33 pF
not used
not used
not used
Introduction to Overtone Crystal Circuits
With overtone operation of crystals, the overtone frequency is not an exact multiple of the
fundamental frequency and the circuit design must ensure that the crystal does in fact operate
at the overtone frequency.
In order to be certain of correct operation, it is preferable to specify series resonant crystals, and
to provide external circuit elements to prevent oscillation at the fundamental frequency. Where
parallel resonant overtone crystals are required a load capacitance must be specified
On following pages circuits are shown for 3rd overtone crystals 15 to 65MHz and 5th overtone
crystals 60 to 105 MHz operating in their series resonant mode. In both of these circuits with the
crystal short circuited, the oscillator should operate at or near the required frequency. With the
crystal in circuit L1 should be adjusted for either (a) minimum voltage across the crystal or (b)
for the exact frequency required. Ideally, these two points would coincide but they rarely will due
to the need for a manufacturing tolerance on crystal frequency.
If L1 is of incorrect size it is possible for the oscillator to operate on a different order of overtone,
for this reason it is important to accurately check the output frequency.
Under no circumstances should a tuned circuit at the crystal overtone frequency be included in
the collector circuit of TR1 as this configuration will result in oscillation not controlled by the
crystal. However it is possible to include a tuned circuit at that point which is twice or three times
the crystal frequency. It is then possible to extract from the collector the harmonics of the crystal
frequency.
Unless otherwise specified we supply 3rd overtone crystals between 21 and 60 MHz. 5th
overtone between 60 and 126 MHz and 7th between 126 and 175 MHz.
Third Overtone Oscillator Circuit
R1 10K
R2 4.7K
R3 470 Ohm
R4 560 Ohm
TR1 BF180 or similar
C1 C2, C3, C4, See Below
C5 0.001mF
L1 wound is on a 7.62 diameter former with Neosid F29 core
Frequency MHz C1
C2
C3 C4
L1
15 - 20
100 pF 100 pF 68 pF 33 pF 12 Turns 30SWG Close Wound
20 - 26
100 pF 100 pF 68 pF 33 pF 8 Turns 30SWG Close Wound
25 - 31
100 pF 68 pF 47 pF 33 pF 8 Turns 30SWG Close Wound
30 - 43
100 pF 68 pF 47 pF 33 pF 6 Turns 30SWG Close Wound
42 - 55
100 pF 68 pF 47 pF 33 pF 5 Turns 30SWG 6 mm long
48 - 65
68 pF 33 pF 15 pF 15 pF 5 Turns 30SWG 6 mm long
Fifth Overtone Oscillator Circuit
XL1 Ri L1
50-70 MHz 2.7 K 7 Turns 6mm long
60-85 MHz 2.7 K 5 Turns 5mm long
80-105 MHz 1.2 K 3 Turns 6mm long.
R2 470 ohm Cl l2pF
R3 330 ohm C2 18pF
R4 470 ohm C3 33pF
C4 1000 pF
VT1 8F180, SE1O1O or similar.
L1 Wound with 20 B & S enameiled wire on 7.62 mm diameter former with Neosid F29 Core.
Under no circumstances should a tuned circuit at the crystal overtone frequency be included in
the collector circuit of VT1, as this configuration will result in oscillation not controlled by the
crystal.
OVERTONE CRYSTAL OSCILLATOR FOR UP TO
200 MHz
Frequency
75
120
150
200
C1
8 pF
8 pF
5 pF
3 pF
C2 Ic (mA) RE (Ohms) RL (Ohms) Lp (mH)
100 pF
25
510
470
0.25
50 pF
25
390
300
0.1
25 pF
5
1.1k
600
0.08
20 pF
5
1.1k
600
0.05
The above diagram shows a true series resonant oscillator circuit suitable for use up to 200
MHz
It is recommended that transistors for use in this circuit have a high DC gain (H FE) and a low
base resistance (RBB). Also ensure that the transit frequency is at least ten times that of the
oscillator frequency.
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© Max Last modified on 16th Mars 2001.