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LA1787M
? Hall detection
The Hall detection function detects the level of the pin 36 quadrature input signal and then applies peak detection
to that result. The result is output from pin 33. This circuit has three effects.
(1) It assures that muting will be applied for weak inputs with an antenna input of under 5 dBμ. The amount of
attenuation is referenced to an antenna input of 60 dBμ, fm = 1 kHz, and a 22.5 kHz dev output, and is variable
from 10 dB to 40 dB when there is no input. Thus one feature of this circuit is that the weak input noise
attenuation and the –3 dB limiting sensitivity for over 5 dBμ inputs can be set independently.
Hall Detection Output — Antenna Input Characteristics Fig.25
5
Area muted by Hall detection
4
3
2
1
0
–20
–10
0
10
20
30
Antenna input — dB μ
(2) When the pin 36 quadrature input is a saturated input, the pin 36 noise level (Va) is detected and a peak-hold
function is applied to pin 33 (Vb) for locations rapid field strength variations and severe multipath occurs for
fields that result in an antenna input level of over 5 dBμ.
36
33
0
Va
0
Vb
+
0.1 μ F
A12083
Fig. 26
(3) Unique features
One unique feature of the LA1784M is that if there are adjacent stations such that f 1 = 98.1 MHz and f 2 =
97.9 MHz, a search operation will not stop at 98.0 MHz. Since V AFC = 0 V and V SM = 3.6 V at 98.0 MHz in
the situations shown in figure 27 and 28, even though Hall detection would normally not operate and SD would
be high, in this IC the Hall detection circuit will operate, V Mute will be set to 1.2 V (over 0.7 V) and the SD
signal will go low, thus preventing incorrect stopping of the search.
No. 6655-35/54
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