44
5. Features
• Quartz PLL Frequency Synthesizer for maintaining high accuracy of reception in frequencies of 100 kHz spacings. ・AIIelectronic controlled manual tuning in addition to a 7・stationpreset tuning convenience. • Phase Locked Loop (PLL) discriminator for high performance and elimination of interference， unaffected by
vanatlons In envlronment or In aglng. • Surface Acoustic Wave (SAW) filter to insure ideal transferring characteristics and superb selectivity for hi-fi reception.
• Automatic Pilot Signal Canceller circuit and negative feed backed decorder employed in the MPX demodulator IC for obtaining extremely low distortion during FM stereo reception. ・2Dual Gate MOS FETs and 2 double tuned circuits employed to gain a high performance in receiving various levels of input signals.
• Anti-birdy filter with defeat switch employed to eliminate noise interference during FM stereo reception. ・7-segmentindicator shows both the receiving frequency and the preset number， and 5 LED indicators for showing input signal level to the antenna terminal
• R EC LEVEL calibrator is a convenience for adjusting to a highly accurate recording level. ・Coaxialantenna connector for hookup to an FM aerial with correct impedance.
6. Explanation of New Technology
6・(1)Synthesizer Circuit
The word "synthesizer" has become shomewhat popu-larized with the increase of what is known as "electronic music" produced by a "music synthesizer". Likewise， the term "frequency synthesized tuner" is becoming more and more popular with the increasing number of such tuners which have appeard on the market. As the term "frequency synthesized" suggests， those tuners incorporate a frequency synthesizer circuit， which can be constructed in various ways. The T・3030frequen-cy sγnthesized FM tuner employs a PLL (Phase-Locked Loop) synthesizer in which a closed loop containing a reference crystal oscillator is constructed， thereby provid-ing a highly stable and accurate reception.
Fig. 1 shows a block diagram of the frequency sunthe-sizer circuit employed in the T-3030.
Station Select Input Data
1/R
1/P
Pre-Scaler
Fig.1
To
-4-
The VCO (voltage controlled oscillatorl in the PLL circuit functions as a local oscillator and is comtructed as a resonance circuit consisting of a variable capacitance diode. The VCO oscillating frequency varies according to the DC output voltage from the low-pass filt~r. The frequency to be received is determined by the mixer wh ich accepts the VCO output. The DC output voltage of the low-pass filter is also applied to the variable capa-citance diodes of the antenna and RF tuning circuits for
them to be tuned in to a signal freqency corresi>onding to this voltage. The VCO output frequency far exceeds the upper limit of frequencies that the progra市 mablecounter can accommodate. A prescaler is， therefore，
inserted between the VCO and the programmable counter to step down the VCO output frequenl;y until
it permits the counter to operate with a sufficiently high accuracy. The programmable counter is a circuit
for dividing the VCO output frequency by a divider wh ich differs for different selected station data so t河atthe divided frequency always equals that of the reference frequency wh ich enters the phase comparat<o r. The reference oscillator employs a crystal for obtaining a stable and accurate output frequency. The accur週cyandstability of the received frequency depends upon the accuracy of this reference oscillator， which is， in this sense， considered to be the most important corrJ>onent in this circuit. The output frequency of the reference oscillator is stepped down through frequency iJividers until it reaches a frequency which is most suit， ble for phase comparison. The outputs of the progranmable counter and the reference oscillator enter the phase comparator， which produces an error voltage if t~ere is a difference between these two signals. When theil phases coincide with each other and an error voltage is n~ longer produced， the PLL loop is stabilized and its outp~ t takes a steady state. The phase comparator output contains various components including the signals to be co帆 paredand other h igh and low frequency components. Hto wever，
since only the DC component is to be applied to the
T・3030No.2449
VCO， a low-pass filter is provided to obtain frequencies lower than predetermined value. The time constant of this low司passfilter is one of the factors which determines the time required for the entire loop to reach its locked state. Therefore， if a time constant large enough to completely suppress the AC components is selected， it takes some time for reception to be stabilized. On the contrary， however， if too small a time constant is se-lected， some difficulties are encountered while reception can be rapidly stabilized; i.e. much of the ACcomponent leaks out into the VCO， where it is converted into FM signals and enters the mixer. This means that these FM signals are demodulated as well as the received signals and are present in the final demodulated signal output， causing the signal-to-noise ratio to deteriorate. For th is reason， the low-pass filter is another key com-ponent in the design of this circuit. Summ ing up the principle of the frequency syntheseizer circuit， the reception frequency is determined by the frequency division ratio of the programmable counter and stabilized when the two inputs to the phase com-parator equals each other. Consequently， the re-lationsh ip between these parameters can be expressed by the following equation:
fr fo . .(1)
R P x N
Where fr = oscillating frequency of the reference os-
sillator，
fo = oscillating frequency of the VCO， R = frequency division ratio of the binary counter
(frequency dividers)， P = frequencydivision ratio of the pre-scaler， and N = frequency division ratio of the programmable
counter.
In an actual tuning operation， the reception frequency is deter灯、inedby varying the frequency division ratio of the programmable counter N. Therefore， rearranging the equation (1)， we obtain:
_ fo ~ R . .(2)
fr P
In this connection， fo is the input to the mixer from the VCO which functions as a local oscillator and， therefore，
must always be a frequency which is higher than the received frequency by a value corresponding to the 1 F frequency (10.7 MHz). In the T-3030， 3.6 MHz is chosen for fr， 360 for R and 10 for P. As a result， the phase
・3.6MHz_ 3向00・日comparison fr叩 e附 IS-'36O"--:3百HZ= 10 k
In order to enable tuning into frequencies from 87.6 MHz to 108 MHz in 100 kHz steps， the following relationship
between the tuned frequency fa， VCO frequency fo and the frequency division ratio of the programmable counter N is required as determined by the equation (2).
T-3030 No.2449
fa f。 N
87.6 MHz 98.3 MHz 983 87.7 MHz 98.4 MHz 984 87.8 MHz 98.5 MHz 985
108.0 MHz 118.7 MHz 1187
Tuning is， therefore， performed by entering a tuning data which specifies the value of N.
The DC output voltage supplied from the low-pass filter to the antenna and R F tuning circuits varies as shown in this diagram in relation to the reception frequency since
the VCO output is specified at intervals of 100 kHz. The tracking of each tuning circuit is adjusted so that maxi司
mum sensitivity is obtained at each operating point.
制コaHコO
』@】
F=H也
的凶国内出
EoJho∞四国H
一O〉υ。 Tuning Frequency -一一一一咽 Fig.2
The accuracy and stability of reception is dealt with in the followi ng. 1 n equation (1). R， P and N are predeter-mined and not subject to variations as long as the frequency divider stages of the corresponding blocks function properly in their counting operations. Con-sequently， the accuracy and stability of reception; namely， those of the VCO frequency， can be replaced with the accuracy and stability of the reference oscillator frequency， as suggested by the following equation:
P x N fo ・fr ・・・・・・・・・・・・・ ， . . . .(3)
R
For example， when tuning in a frequency of 90.0 MHz，
we obtain:
10 x 1007 fo (90.0+ 10.7) =一一一一一一一 fr(3.6) ~ 28fr 360 -， ，_._， . ---，
Of the result 28fr， fr is the oscillating frequency of the highly accurate and stable crystal oscillator ar司dalso is
established as a one-25th of the received frequency. This means that is can be safely said that the accuracy of the crystal oscillator is tantamount to thョtof the received frequency， resulting in an extremely stable and accurate reception.
-5-
6・(2)Surface Acoustic Wave (SAW) Filter
A resonance circuit consisting of a coil and a capacitor， usually called "IFT"， or a ceramic filter which utilizes the thickness oscillation of a ceramic plate is commonly used as a filter which determines the selectivity of the I F stage of a tuner. However， in light of the demand for increased performance in tuners， research has been con-ducted towards developing improved filters. The surface acoustic wave filter is just such a filter resulting from this research. The illustration below depicts the internal structure of th is filter.
Input Output
+-+ー+----..且且. 三士三コ
Fig.3
As can be seen from the illustration a set of "comb-like" electrodes are arranged on either end of the piezoelectric material_ These two sets of electrodes serve as receiver and transmitter of signals. When an AC signal is applied to one set of oppositely positined electrodes which serves as a transmitter， this set of electrodes tend to attract each other. This results in mechanical strain produced on the surface of the pie-zoelectric material at intervals corresponding to those of the teeth of electrodes_ This series of strains become vibrations that propagate in the direction towards the set
of electrodes serving as a receiver since the teeth of the electrodes are equispaced. When these vibrations reach the receiving set of electrodes， the output signal appears at both terminals of this set of electrodes since the receiving set of electrodes are also ofthis equispaced con-figuration. The highest output level is obtained at a signal corresponding to the intervals the teeth of electrodes. For this reason， selectivity characteristic can be obtained， since the delay time of the output signal depends on the propagation speed of vibrations along the piezoelectric material; being constant regardless of signal frequencies. In other words， the frequency vs. delay characteristic is flat. Since "frequency modulation" is to render the loudness of audio signals as variations of frequencies， FM signals vvill be distorted if the delay time does not change I inearly according to frequency. The delay time per unit of frequency variations is called a group delay. If the group delay characteristic is flat， no distortion is caused. The diagram below shows the relation between distortion and the group delay.
-6-
When Group Delay is flat
When Group Delay is not flat
auV 93
nv a
'L
e
m
T
-EE
・E
・-EEEι..
， Delay l
Advance ， ，
、、 ーー『宇ぐ、 l
/ジク |Ou刷 tSig問lI_~ I Time Lapse
ヨThedotted line↑ξ'
shows the wave I -:: from of input ，
Center Frequency signal. C~nter Frequencv
Fig.4
The reason why a filter is inserted in the I F stage is to pass desired signals alone without passing unwanted signals belonging to unnecessary frequency bands. In short， the purpose of a filter in the I F stage is to provide a certain degree of selectivity. Therefore， it is desirable that the amplitude characteristic is flat in the required frequency band， but is subject to steep atlenuation outside that band. On the other hand， if the group delay characteristic is considered on its own， an ideal is a linear relationship between the group delay and fr~quency. The diagram below showns an ideal filter charecteristic.
Center Frequency
Ideal Amplitudl 〆/'Characteristics
十Greater attenuation is desirable.
./ ~eal_Gro ':l p.Deay Characteristics
ド一一」Required Band Width Fig 5
However， at present， it is impossible to obtain such an ideal filter. Constant effort has been made todevise a filter which is as near as possible to an ideal filtEr'. In the conventional I FTs and ceramic filters， a内 plitudecharacteristic is incompatible with group delay c!~racterristic. Therefore， it was difficult to obtain a filtEi having a sufficiently good characteristic. However， it isフossibleto obtain with a surface acoustic wave filter， a~ almost ideal characteristic， because in this case， improvE11ent of the amplitude characteristic is sufficient， withoに havingto consider the group delay characteristic. In an actuality， with a surface acoustic wave面Iter，as well as waves being propagated along the surfac~ of the piezoelectric material， some waves (bulk waves) pi netrate inside the material.
T・3030No.2449
These waves do not reach the receiving electrodes directly， but passing through the material to the opposite side， reflect from the opposite terminal， and thereby arrive at the receiving electrodes. Other waves， reflected from the receiving electrodes， reflect from the transmission electrodes again， and finally reach the receiving electrodes in this manner (triple transit echo). These particular type of waves are undesirable and various means of suppressing them have been employed. As a result， the filter em-ployed in the T-3030 has the following excellent char.
acteristic.
Group Delay -20
-30
-40
-50
Frequency (kHz) Fig.6
6・(3)PLL FM Detector Circuit
CD-4 demodulators employ a PLL (Phase Locked Loop) circuit as a subchannel FM signal detector circuit. Con-cerning the principle of operation， the PLL circuit em-ployed in the T・3030has entirely the same parameter as that of the CD-4. However， the PLL circuit of the FM tuner deals with considerably higher frequencies.
Carrier Maximum De-frequency deviation e円、phasis
-6 dB/oct CD-4 30 kHz Approx. :1:10 kHz from 800 Hz
to 6 kHz
FM tuner 10.7 MHz :1:75 kHz 50μsec or 75μsec
Since the carrier frequency is higher than 10 MHz， con-siderable difficulties must be dealt with in the PLL circuit of the FM tuner， compared with the CD-4 demo-dulator _ As shown in Fig. 7， the PLL circuit is a loop basically consisting of a phase comparator， a low-pass
filter and a. voltage controlled oscillator.
T・3030No.2449
Fig.7
-7-
It utilizes the operation of the loop which tends to align the phase of the output signal to that of the input signal. Therefore， the PLL circuit features excellent stability by absorbing variations of characteristics that are com-monly due to temperature fluctuations and time lapse. The response of the loop to the FM input signal will be explained below. As shown in Fig. 8， when no FM input signal is present， the low-pass filter operates at its center frequency (free-runsl， i.e. at point A， producing an output voltage of士ov.
+
Error Voltage
Lock Range
Capture Range
Fig.8
However， when an FM input signal is present， the ouput voltage takes a positive or negative value cormsponding to the difference of the frequency of the FM input signal and the center frequency. FM signal is a sig日 Iwhose frequency changes according to the amplitude of an audio signal. Output voltages along the slope ~f the line shown in Fig.8 are obtained according to input frequen-cies， thus providing FM detection. The typical require-
ments of the detector circuit in a hトfituner are low distortion and good S/N ratio. To improve the per-formance of the detector circuit， a good linearity (differ-ential gain characteristic) is first required. However， ingeneral if good linearity is sought over a wid~ frequen-cy band， a demodulated output level， namely，S level of S/N ratio tends to lower. As a result， though lIon-linear distortion decreases， S/I¥ratio deteriorates. I n short， low distortion is incompatible with good S/N ratio. To obtain a low distortion， a high S/N ratio and wde tuning capability; a wide dynamic range of the deteclor circuit is required. This PLL detector circuit uses varicus devices to acquire a wide dynamic range which was iれ possiblefor conventional PLL detector circuits to attai~ at such a high frequency of 10.7 MHz. 1. To decrease noise to the lowermost limit，the phase
comparator， the output amplifier and the V:O have a special arrangement， and at the same tine， special elements are emロloyed.Above all， special at::ention is paid to obtain a low-noise. VCO.
2. To realize low distortion， the VCO has a esonance circuit consisting of twin variable capacitan: e diodes having a specially wide variable range. Th砲事ediodes affect distortion directly and this partic¥1 ar com-bination of a specially selected diode pair is ，dopted.
