(2) SD sensitivity adjustment method

The SD sensitivity is determined by the voltage level of the two inputs to the comparator. Because the absolute value is

determined by the resistor as described above, no matter which level, the sensitivity can be adjusted through both levels.

However, from the standpoint of the stability of the system as a whole and given the possibility that the S-meter output will

be used for other purposes, it is best to make adjustments by adjusting the value on pin 9.

Because the S-meter I/O characteristics during a seek operation or a receive operation are basically as depicted in the diagram

below, so allowing for potential differences, the setting level for the adjustable range should be 25 to 60 dBµ.

(3) Support for the LM7000 (IF count method)

Implemented as shown in the figure at right.

15. Tweet Prevention Circuit

(1) Cause and countermeasures for Tweet distortion

The beat noise frequency characteristics for LA1137N is fr = 900 kHz and the corresponding circuit where it is generated are

shown below. The minimum signal-to-noise ratio for tuners using a reference input signal is 30 dB.

Mechanism behind the deterioration of the characteristics

The 900 kHz RF input signal from the antenna is amplified by the FET RF amplifier before passing through the RF

alignment stage to the mixer. If this signal is large, the varactor diodes can cause second-harmonic distortion of 1.8 MHz.

The 900 kHz and 1.8 MHz components each generate an IF signal; (450 +

α) kHz for the 900 kHz signal, and (450 – α) kHz

for the 1.8 kHz signal. These two signals interact to produce a beat frequency at 2

×α kHz.

Receive mode

Station seek mode

Antenna input level – dBµ

IF count control voltage (STRQ)

IF buffer signal

Ω

Antenna input – dBµ

Complete alignment (Maximum

beat noise at 900 kHz)

Complete alignment

(zero beat noise)

Tweet input

Signal Characteristics

LA1136N, 1136NM, 1137N, 1137NM

No. 3507-19/28