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Part: AN456
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Philips Semiconductors
Application note
Using LC oscillator circuits with Philips microcontrollers
Author: William Houghton
There are two basic types of LC oscillators, Colpits and Hartley. The Colpits is the two capacitor type shown in Figure 1. It is generally favored over the Hartley (shown in Figure 2). This is because of the simplicity of requiring only one inductor, which are generally more expensive and difficult to obtain than capacitors C2 and C1. Feedback is determined by the ratio of C2 and C1 (for our application, this is approximately 2:1). The center frequency for Figure 1 is given by: f+ (1 L 1) (12 C1 ) 1 C2) P
AN456
Table 1 contains measured frequencies for various combinations of inductor and capacitor values. The test circuit (Figure 1) was a Philips P87C750PBPN, using the internal inverting amplifier. The results compare well with the frequency predicted by the above formula at low frequencies. At higher frequencies, however, the gain of the amplifier becomes a significant factor, as do the internal capacitances of the device. The inductor identified as 10T0.16 is an air core coil 10 turns on a 0.16 diameter form using 26 gauge magnet wire close wound. The inductors used in this test were TOKO types 5K, 5PH, 5P 5mm coils (adjustable ±5%). If the frequency is not critical, then a fixed inductor would be suitable. TOKO produces a range of inductors from 1µH to 1mH in the 144LY series. These are available from Digi-Key.
L1
Table 1. Inductor and Capacitor Chart
FREQUENCY (MHz)
OUTPUT
INDUCTANCE (µH) 680 470
C1 = 100p C1 = 47p C2 = 220p C2 = 100p 0.8 0.95 1.3 1.9 2.4 2.8 4 5.9 7.2 8.7 12.8 19 24 28 35 1.1 1.3 1.9 2.7 3.3 4 5.7 8.3 10.2 12.3 18 27 33 39 47
C1 = 22p C2 = 47p 1.5 1.8 2.6 3.7 4.6 5.6 7.8 11.4 14 16.7 25 37 46 55
C1 = 10p C2 = 22p 2 2.4 3.3 4.8 6 7.4 10 15 18 21.5 31 50
C1
C2
SU00617
220 100 68
Figure 1. Colpits Oscillator
C1
47 22 10 6.8
OUTPUT
4.7 2.2
L1
L2
1 0.68
SU00618
0.47 10T0.16
Figure 2. Hartley Oscillator
Table 2. Ceramic Capacitor Temperature Characteristics
EIA CODE M7J COG U1G P2G R2H S2H T2J U2J P3K Y5P Z5V TEMPERATURE CHARACTERISTICS P100 NPO N075 N150 N220 N330 N470 N750 +120/40PPM ±30PPM ±30PPM ±40PPM ±40PPM ±60PPM ±60PPM ±120PPM COLOR CODE Red/Violet Block Red Orange Yellow Green Blue Violet Orange/Orange Yellow Green
N1500+ 500/ 0PPM ±10% C from 30°C to +85°C +22/85% C from +10°C to +85°C
1995 May 05
1
Philips Semiconductors
Application note
Using LC oscillator circuits with Philips microcontrollers
AN456
TEMPERATURE COMPENSATION
Using a 6.8µH inductor, 47p and 100p capacitors gives a frequency of approximately 10MHz at room temperature. As the temperature changes, so do the values of both the capacitors and inductor. There is also an effect with the microcontrollers changing gain, however, this is small compared with the capacitors and inductor. This variation is shown in Figure 3. These results were obtained using NPO capacitors. NPO capacitors are zero temperature coefficient (nominally). Also available are other temperature coefficients (see Table 2). By trial and error we can pick one of these temperature coefficients which will compensate for the inductor. Just about all inductors have a positive temperature coefficient. Using an N750 for the capacitors C1 and C2, the following results were obtained (Figure 4). This a large improvement on the uncompensated design.
AIR WOUND INDUCTORS
The most expedient source of inductors are air wound (wind them yourself). Although probably not suitable for production, these offer the designer an infinite variation of inductance values. The table below gives a few useful values for use in microcontroller oscillators. OSCILLATOR FREQUENCY (MHz) (with C1 = 47p; C2 = 100p) 47 32 27 23 21
NUMBER OF TURNS (T) 10 15 20 25
INDUCTANCE (µH)
0.3 0.68 1 1.5 1.8
10.2
30
10.1
Frequency (MHz)
All were close-wound with 26 gauge magnet wire on a 0.19" diameter form. Greater than 30 turns requires a permanent form.
10.0
START UP
9.9
9.8
9.7 0 20 40 60 80
Temperature (°C)
SU00619
Although an LC oscillator will start much quicker than a crystal oscillator after power is applied, this effect is not apparent in this application. Although the Q expressed as rate of change in impedance versus frequency of a crystal is very high, the losses can be quite large (resistive). This can lead to starting problems (given a low gain amplifier). With an LC oscillator the losses are much lower than with a crystal, thus the LC oscillator will start more reliably than a crystal oscillator.
Figure 3. Frequency Variation Using NPO Capacitors
SUPPLY VOLTAGE EFFECTS
Figure 5 depicts a Supply Voltage versus oscillator frequency graph for the test circuit shown in Figure 1, using C1 = 47p, C2 = 100p and L1 = 6.8µH.
10.2
10.1
Frequency (MHz)
10.074
10.0
Frequency (MHz)
10.070
9.9
10.066
9.8
10.062
9.7 0 20 40 60 80
10.058 4.50
4.75
5.00
5.25
5.50
Temperature (°C)
SU00620
Supply Voltage (Volts)
SU00621
Figure 4. Frequency Variation Using N750 Capacitors
Figure 5. Supply Voltage vs. Frequency
1995 May 05
2
Others parts begin by an
AN-1 AN-2 AN-3 AN-4 AN-5 AN-6 AN-7 AN-8
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