Download AN1882 The NE568A as a wideband FM modulator

Philips Semiconductors
Application note
The NE568A as a wideband FM modulator
AN1882
INTRODUCTION
Linearization of the Modulator
The NE/SA568A used as an FM modulator for frequencies up to
140MHz is described. This device is fabricated on 8GHz integrated
bipolar technology and operates at 5V. The VCO gain constant, KO,
is 4.2 x 10 rad/ volt-sec at a center frequency of 70MHz. This is
equal to approximately 670MHz per volt. The gain, KO, is calculated
from the magnitude of the frequency deviation as a function of the
differential voltage present at Pins 17 and 18. Figure 1 shows the
equivalent VCO circuit as used in this application.
In order to change the transfer function of the VCO-Phase Detector
block from an exponential small signal function to a linear
voltage-to-frequency converter, the output buffer amplifier signal is
fed back from Pin 14 to Pin 18 (inverting input) of the VCO (see
Figure 3). The output buffer amplifier has a gain of approximately
25dB. Pin 15 of the buffer amplifier allows for the addition of a shunt
oriented passive filter to the feedback loop. The filter -3dB
frequency is calculated based on a Pin 15 source impedance of
350Ω. A typical shunt lowpass filter consists of a 100pF capacitor
from Pin 15 to ground and has a break frequency of 4.5MHz.
The internal phase detector reference inputs are hard wired to the
VCO as shown in Figure 2. When used as a modulator the phase
detector error voltage outputs (Pins 19 and 20) are no longer tied to
the VCO inputs as with normal PLL operation. Instead these pins
become the output for the modulator.
x25
14
15
RFB
+
18
-5V
–
VMOD
350Ω
17
X25
14
VCO
+
PHASE
DETECTOR
IIN
OUTPUT
–
SR01128
Figure 3. Closed Loop Linearization
15
10mA
ICO
17
Modulator Frequency Deviation
4.8V
2x
The modulator frequency is determined by the frequency set
capacitor, CO, attached to Pins 4 and 5. The value of the frequency
set capacitor is calculated by the formula given below for FO equal
to 70MHz.
2x
MODULATOR
INPUT
CO = (0.0013 /fO) Farad; @ fO = 70MHz, kO = 0.0013
2mA
500µA
18
4mA
500µA
A linearizing DC bias current is fed into Pin 17 through an RF choke
and series resistor RIN as shown in Figure 4. This reduces the
harmonic content of the output signal while also increasing its
amplitude. The AC modulator output signal appears on Pins 19 and
20 superimposed on a DC common mode voltage 4.8V above
ground. In order to provide a low impedance ground referenced
output signal, a gain-of-two buffer amplifier (NE5539) is added. The
output impedance is 75Ω. Note that the buffer amp differential input
resistance is kept relatively high in order to reduce loading on the
internal differential stage (RS = 200Ω.) This also decreases the
harmonic distortion of the modulator output waveform. Harmonic
distortion and added sideband interference may be further reduced
by the addition of low pass filters around the buffer amplifier, but
caution is required due to the critical stability requirements of very
wideband amplifiers such as the NE5539.
4.8V
SR01126
Figure 1. VCO
5V
50Ω
20
100Ω
19
LF-1
LF-2
100Ω
VCO
VCO
4.8V
4.8V
11
LIMITER
10
2mA
SR01127
Figure 2. Limiter — Phase Detector
1996 Jan 29
1
Philips Semiconductors
Application note
The NE568A as a wideband FM modulator
AN1882
+5V
0.1µF
1.8kΩ
2.7kΩ
freq. adj.
2kΩ
2.7kΩ
+2.65V
16
2kΩ
1
7
+6V
.01µF
–
14
20
1kΩ
2kΩ
4.7µF
18
100pF
10µH
CL
NE568A
18kΩ
RFB
19
MODULATOR
OUTPUT
8
3
1 7
+
RFC
1kΩ
17
0.1µF
P
N
P
B
Y
P
RL
75Ω
.01µF
VMOD
INPUT
R
E
F
B
Y
P
RFC
10
I
N
B
Y
P
14
–6V
4
30kΩ
NE5539
CO
15 12 2
CLPF
3 13
5
CL = 82pF
RL = 100Ω
3.9kΩ
SR01129
Figure 4. NE568A FM Modulation
Next, a sine wave generator is set to FMOD and this signal fed into
the NE568A modulator. The modulator output signal is now
monitored with a spectrum analyzer as shown in the setup diagram
below (Figure 6). The input signal amplitude is increased from zero
amplitude upward until the 70MHz carrier spectral line begins to fall
(see Figure 7). As the amplitude is carefully increased, a definite
null will occur. Record the amplitude of the 2.9MHz generator at the
null. This is the required modulation input amplitude for operating at
a frequency deviation of 10% for any modulating frequency.
Calibration of the Frequency Deviation
The single-sided frequency deviation is a function of the amplitude
of the modulating signal and can be estimated by use of the graph in
Figure 5. Alternately, the exact modulation input amplitude required
for a given frequency deviation may be determined by use of the
standard Bessel function using the first carrier null method at the
assigned center frequency as described in the example below.
20
ATTEN
10dB
RL
0dBm
16
10dBm/
MKR -48.83dBm
70.00MHz
12
F PK
Figure 6. Carrier Null Test
8
ATTEN
10dB
RL
0dBm
10dBm/
MKR -48.83dBm
70.00MHz
START
22.50MHz
RBW 1.0MHz
VBW 1.0MHz
STOP 117.50MHz
SWP 50Ω
START
22.50MHz
RBW 1.0MHz
VBW 1.0MHz
STOP 117.50MHz
SWP 50Ω
4
0
0
0.333
0.667
1
Vn
VOLTS (RMS)
1.333
1.667
2
SR01130
Figure 5. Modulator Frequency Deviation vs Input Volts (RMS)
Example: The first carrier null Bessel constant, B, is 2.4048. Using
this relationship and the modulation index equation as shown below,
the modulator can be accurately calibrated to the desired deviation.
For a center frequency of 70MHz and a desired modulator peak
frequency deviation of 10% (±7MHz), the proper modulating signal
frequency is first calculated by the relationship,
FMOD = FDEV/B
= 7 MHz/2.4048
= 2.9 MHz
1996 Jan 29
Figure 7. First Carrier Null Spectrum at 70MHz
2
SR01131
SR01131
Philips Semiconductors
Application note
The NE568A as a wideband FM modulator
CONCLUSION
mVP-P Output vs Frequency in MHz
The NE/SA568A is capable of working as a linear FM modulator at
frequencies up to 140MHz by the addition of external feedback and
DC biasing circuitry. A wideband amplifier is also necessary to
provide a differential to single-ended gain stage for ease of interface
to the transmission media. Widely diverse technologies for signal
communications may consist of VHF coaxial cable for wideband
color video or infrared fiber optics links for still higher quality digital
RGB signal systems.
180
170
162
160
V P-P 150
70
-10 3 140
130
120
AN1882
150
110
114
100
60
72.5
85
110
fK
97.5
122.5
135
147.5
10 6
REFERENCES:
160
1. AN178, “Modeling the PLL”, Philips Semiconductors IC11
General-Purpose/ Linear ICs, 1995
SR01133
Figure 8. Output Level vs Frequency for CO = 18pF
2. Gardner, Floyd M., “Phaselock Techniques” ; John Wiley &Sons,
Inc.
Modulator Output versus Frequency
3. The Wiley Monograph Series on Electronic Circuits, 1966, New
York, London, Sydney.
The modulator output voltage amplitude as a function of frequency is
shown in the graph below (Figure 8). The timing capacitor is set to
18pF.
4. Hadley, Les , AN1881, “The NE568A Phase Locked Loop as a
Wideband FM Video Demodulator”, 1996; Philips
Semiconductors.
Millivolts p-p Output vs Frequency in MHz
The typical output waveform at 100MHz is shown in Figure 9 below.
1
23.4 mV
pos:
5.07
1.000:1
50
-10.40 ns
+width
(1) 4.896 ns
-400.0 ps
200 ns/div
frequency
9.600 ns
repetitive Trigger Mode
(1) 100.321 MHz
Edge
SR01134
Figure 9. Typical Output Waveform at 100MHz
1996 Jan 29
3