LA1781M_09 Datasheet, PDF (31/48 Pages) Sanyo Semicon Device – For Car Radios Single-Chip Tuner IC

LA1781M
• 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 5dBμ. The amount of
attenuation is referenced to an antenna input of 60dBμ, fm = 1kHz, and a 22.5kHz dev output, and is variable from
10dB to 40dB when there is no input. Thus one feature of this circuit is that the weak input noise attenuation and
the -3dB limiting sensitivity for over 5dBμ 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 -- dBμ
(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 5dBμ.
36
33
Va
Vb
+
0.1μF
0
0
Fig. 26
(3) Unique features
One unique feature of the LA1781M is that if there are adjacent stations such that f1 = 98.1MHz and f2 =
97.9MHz, a search operation will not stop at 98.0MHz. Since VAFC = 0V and VSM = 3.6V at 98.0MHz 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, VMute will be set to 1.2V (over 0.7V) and the SD signal
will go low, thus preventing incorrect stopping of the search.
No.6038-31/48