MC13175 Datasheet, PDF (9/17 Pages) Motorola, Inc – UHF FM/AM TRANSMITTER

MC13175 MC13176
For C = 0.47 µ;
  then, R1 = t1/C = 33.8
  dthus, R2 = t2/C = 0.283
10–3/0.47
10–3/0.47
10–6 = 72 k
10–6 = 0.60 k
In the above example, the following standard value
components are used,
C = 0.47 µ; R2 = 620 and R′₁ = 72 k – 53 k ~ 18 k
(R′₁ is defined as R1 – 53 k, the output impedance of the
phase detector.)
Since the output of the phase detector is high impedance
(~50 k) and serves as a current source, and the input to the
frequency control, Pin 6 is low impedance (impedance of the
two diode to ground is approximately 500 Ω), it is imperative
that the second order low pass filter design above be
modified. In order to minimize loading of the R2C shunt
network, a higher impedance must be established to Pin 6. A
simple solution is achieved by adding a low pass network
between the passive second order network and the input to
Pin 6. This helps to minimize the loading effects on the
second order low pass while further suppressing the
sideband spurs of the crystal oscillator. A low pass filter with
R3 = 1.0 k and C2 = 1500 p has a corner frequency (fc) of
106 kHz; the reference sideband spurs are down greater
than – 60 dBc.
Figure 14. Modified Low Pass Loop Filter
Pin 7 18k
1.0k
R′₁
620 ^R2
0.47 C
R3
C3
Pin 6
1500p
VCC
Hold–In Range
The hold–in range, also called the lock range, tracking
range and synchronization range, is the ability of the CCO
frequency, fo to track the input reference signal, fref • N as it
gradually shifted away from the free running frequency, ff.
Assuming that the CCO is capable of sufficient frequency
deviation and that the internal loop amplifier and filter are not
overdriven, the CCO will track until the phase error, θ_e
approaches ±π/2 radians. Figures 5 through 8 are a direct
  measurement of the hold–in range (i.e. ∆fref N = ±∆fH
2π). Since sin θ_e cannot exceed ±1.0, as θ_e approaches ±π/2
 the hold–in range is equal to the DC loop gain, Kv N.
 ±∆ω_H = ± Kv N
where, Kv = KpKoKn.
In the above example,
±∆ω_H = ± 27.3 Mrad/sec
±∆fH = ± 4.35 MHz
Extended Hold–in Range
The hold–in range of about 3.4% could cause problems
over temperature in cases where the free–running oscillator
drifts more than 2 to 3% because of relatively high
temperature coefficients of the ferrite tuned CCO inductor.
This problem might worsen for lower frequency applications
where the external tuning coil is large compared to internal
capacitance at Pins 1 and 4. To improve hold–in range
performance, it is apparent that the gain factors involved
must be carefully considered.
Kn = is either 1/8 in the MC13175 or 1/32 in the
Kn = MC13176.
Kp = is fixed internally and cannot be altered.
Ko = Figures 9 and 10 suggest that there is capability
Ko = of greater control range with more current swing.
Ko = However, this swing must be symmetrical about
Ko = the center of the dynamic response. The
Ko = suggested zero current operating point for
Ko = ±100 µA swing of the CCO is at about + 70 µA
Ko = offset point.
Ka = External loop amplification will be necessary
Ka = since the phase detector only supplies ± 30 µA.
In the design example in Figure 15, an external resistor
(R5) of 15 k to VCC (3.0 Vdc) provides approximately 100 µA
of current boost to supplement the existing 50 µA internal
source current. R4 (1.0 k) is selected for approximately
0.1 Vdc across it with 100 µA. R1, R2 and R3 are selected to
set the potential at Pin 7 and the base of 2N4402 at
approximately 0.9 Vdc and the emitter at 1.55 Vdc when error
current to Pin 6 is approximately zero µA. C1 is chosen to
reduce the level of the crystal sidebands.
Figure 15. External Loop Amplifier
30µA
Phase
Detector
Output
30µA
VCC = 3.0Vdc
12
C1
1000p
R1
R3
68k
4.7k
R4
1.0k
R5 15k
1.6V
6
2N4402
7
R2 33k
5, 10, 15
50µA
Oscillator
Control
Circuitry
MOTOROLA RF/IF DEVICE DATA
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