Download BH1417F Stereo PLL FM Transmitter Design Guide Introduction

BH1417F Stereo PLL FM Transmitter Design Guide
Introduction
For the designer of FM stereo transmitter, the Rohm Semiconductors BH1417F
provides a stereo modulator for generating stereo composite signals and an FM transmitter for
broadcasting. The stereo modulator generates a composite signal which consists of the MAIN,
SUB, and 38 kHz pilot tone signal. The FM transmitter radiates FM wave on the air by
modulating carrier signal with audio composite signal. BH1417F FM transmitter employ direct
modulation technique with its simple configuration oscillator to generate direct FM signal. With
a proper understanding of the oscillator design considerations, one can put BH1417F into work
quickly.
Colpitts LC Tank Oscillator
BH1417F provide a simple configuration oscillator which is one part of a phase-locked
voltage-controlled oscillator (VCO) which should be able to cover the frequency range of FM
band(88-108 MHz). As shown in BH1417F data sheet, LC resonator for Colpits oscillator is
composed of tunable RF coil from some manufacturer. The purpose of using this kind of
tunable inductor is that perhaps it helps to adjust the resonant frequency of the
oscillator(voltage controlled oscillator) to operate all over the FM band(88-108MHz). That is, it
need manually tuning during the process of assembly and testing.
In fact, we can replace this tunable RF coil with fixed value SMT inductor. However,
with the new LC tank resonator, the VCO can still operate covering the frequency range of FM.
The simplicity of the BH1417F oscillator enable to design in many configurations other than
Colpitts. While the oscillator is quite simple, but oscillator design isn’t. In this article, we are
going to focus only Colpitts circuit as in the data sheet. Figure 1 shows the basic block diagram
of Colpitts oscillator. Note that this is block diagram, not circuit diagram, so the apparent ‘shot’
through the coil from the output to ground is not a problem(there is no DC).
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Fig. 1 Colpitts oscillator.
The oscillating frequency is tuned by the resonance between inductor L1 and the series
combination of capacitors C1 and C2. In actual oscillator as in BH1417F data sheet, there will
also be a tuning capacitor in parallel with L1, and all the capacitance used in resonance will
consist of the tuning capacitance, plus C1 and C2 in series. This capacitive voltage
divider(C1/C2) is the feedback network in which the output signal of the amplifier is fed back to
the amplifier’s own input by passing through the feedback network. One can distinguish
particular characteristic of Colpitts oscillator by identify the tapped capacitive voltage
divider(C1/C2) in parallel with inductor,L1(parallel resonance).
The gain or transfer function β of feedback network(total capacitance and inductance)
must achieve the criteria for oscillation, as following
1. Feedback voltage must be in-phase(360°) with the input voltage.
2. The closed loop gain βAopen loop must be unity(1).
Then, when these conditions are met the circuit will oscillate. Furthermore, the Q of the LC tank
will affect the upper frequency limits of oscillation, the higher the Q the higher the frequency.
The feedback, of course, depends on the Q of the tank. If the Q is too low, the oscillator will
not start. It is generally accepted that minimum amount of feedback should be used. The
Colpitts oscillator will exhibit a small tuning range since the fixed feedback capacitors limit
variable capacitance range, however, the Colpitts has good frequency stability with proper
components.
Figure 2 shows the Colpitts oscillator circuit, the BJT is used as an amplifier in common
collector configuration and biased with resistors R1 and R2. The feedback network consist of a
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tapped capacitive voltage divider(C1/C2) in parallel with inductor, L1 and capacitor,C4 and
indespensible couple capacitor,C3 in series. All these elements perform parallel resonance for
the Colpitts oscillator.
Fig. 2 Negative resistance Colpitts oscillator
If the oscillator tends to oscillate parasitically in another frequency region(VHF/UHF). This is
spurious frequency, then put in a snubber resistor(R3) to solve this problem. This can be
occurred since the BJT used has sufficient gain at another some VHF frequency in which
parasitic and distributed L-C elements generate the correct phase shift and meet all criteria to
oscillate at unintentional frequencies. A resistor(10 to 47Ω) will usually get rid of the problem,
as in BH1417F data sheet, a 33 Ω resistor is used to suppress the spurious frequency(Fig. 10).
The Colpitts oscillator in Fig. 2 is called negative resistance oscillator circuit because
when we look into the base terminal of the circuit, the resistance in this terminal will exhibit
negative value. If proper value of all components(C1-C4 and L1) are selected this circuit will
has better stability of 10 ppm over a wide temperature range.
In BH1417F data sheet, One can set the oscillating frequency by adjusting inductor L1
which is slug-tuned coil, by this way, it helps to tune the resonant frequency of the voltagecontrolled oscillator(VCO) to operate all over the FM band(88-108 MHz). In fact, We can
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replace this tunable RF coil by fixed value SMT inductor. However, with the new LC tank
resonator, the VCO can still operate covering the frequency range of FM.
Voltage Controlled Oscillator
The Colpitts oscillator can be modified to be voltage-controlled oscillator by adding a
voltage variable capacitance diode(varactor) in series with a capacitor(C4) that already shunt
with the inductor as shown in Figure 3.
Fig. 3 Voltage-controlled oscillator
The capacitance value of the varactor will vary depending on the voltage across the varactor ‘s
terminal. So, voltage variations will result in changes in overall L-C tank resonant frequency. By
this method, we can tune the oscillating frequency by applying the DC voltage pass through a
resistor(10 kΩ) to the varactor. The highest frequency and the lowest frequency that the
oscillator can operate will directly depend on the maximum and minimum values of the varactor.
In BH1417 data sheet, This Colpitts voltage-controlled oscillator is employed as a VCO
for phased-lock loop circuit which is a frequency synthesizer and is capable of producing very
accurate, high quality signal. Also, this Colpitts voltage-controlled oscillator is directly used as
FM modulator by input the audio composite signal to varicap diode(varactor). This technique is
called direct FM modulation since the audio signal is applied directly to the oscillator. The
oscillating frequency will deviate from its center frequency according to the instantaneous audio
ac amplitude voltage across the varactor. BH1417F uses this simple and direct modulation
technique to generate FM signal.
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Fig. 4 L-C components for BH1417F
Figure 4 shows the L-C tank resonator for Colpitts oscillator of BH1417F IC. We
recommend to use 0805 size for all components. The circuit is connected to pin 9, the base
terminal of internal transistor with a 33 Ω snubber resistor, the same as in the data sheet. The
capacitive coupling use the same value 33pF as well as in the data sheet. The slug-tuned coil
is replaced with 68 nH chip inductor and varicap diode is in series with 100 pF chip capacitor.
The audio composite signal is added with DC voltage from loop filter(PLL) before input to the
Colpitts VCO through a 10 kΩ resistor.
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