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參數資料
| 型號： | LA7555 |
| 元件分類： | 接收器 |
| 英文描述： | AM/FM, AUDIO/VIDEO DEMODULATOR, PDIP24 |
| 封裝： | SDIP-24 |
| 文件頁數： | 10/11頁 |
| 文件大小： | 355K |
| 代理商： | LA7555 |
LA7555
No.2927–8/11
Fig.10-1
11. AF amplifier
As shown in Fig.11-1, this is an AF amplifier equipped with an NFB pin. An audio power amplifier can be easily
configured by use of this NFB pin. The gain of the power amplifier can be controlled by the ratio of R1 and R2.
Fig.11-1
VCO adjustment methods
The following two methods are available for adjusting the VCO.
1. APC voltage offset adjustment method
a. Connect a digital voltmeter to pin 20 (APC filter pin ) (Fig.1).
b. Lower the IF input level and connect the IF AGC (pin 13) to GND.
Measure the voltage on pin 20 at this time.
c. Raise the IF input level to the desired value. Free the IF AGC from GND (internal AGC). Adjust the VCO so
that the voltage (V2) on pin 20 is equal to V1.
Note : When adjusting by this method, use a DVM (digital voltme-
ter) with high input impedance. At Zi=10M
Ω, the adjustment
error is approximately 20kHz, The error (
ΔV) with the DVM
connected is.
ΔV (mV)= V (1–
)
The change in VCO free-running frequency (
Δf) according to
ΔV is Δf (kHz)=ΔV× β
where
β=VCO control sensitivity (kHz/mV)
R1
Z+R1
2. Direct reading method
a. Lower the IF input level and adjust the IF AGC (pin 13) to between GND and about 4V (VCO free run).
b. Monitor the carrier frequency leaking from pins other than the VCO coil, or pattern, chassis, and adjust the
VCO coil to obtain the desired frequency (fp).
Fig.1
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