TDA7514 STMicroelectronics, TDA7514 Datasheet - Page 66

• Manufacturer Part Number: TDA7514
• Description: SINGLE-CHIP FM/AM TUNER
• Manufacturer: STMicroelectronics

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TDA7514
5.2.2
An IF noise blanker circuit is present to detect and remove impulse noise especially at weak field strength
levels. The noise spikes are detected at the output of the first mixer and noise cancelling takes place in
the second mixer. The necessary delay in the signal path is provided by the IF1 ceramic filter.
The detection circuit consists of an expressly-built IF1 limiter and Smeter detector (not the same used for
FM Smeter generation), followed by a pulse detector. The sensitivity of the noise blanker is programmable.
The IF NB is desensitized at high field strength (using the filtered Smeter) and the desensitization thresh-
old can be programmed.
5.2.3
After channel selection by means of the IF2 ceramic filter, the AM signal is amplified by a very high gain
(up to 70 dB, programmable) linear amplifier, and demodulated by an integrated quasi-synchronous de-
tector. The phase information for the detector is derived from the IF2 limiter (shared with the FM signal
path) whose input is the IF2 amplifier input pin. The same IF2 limiter is used to generate the AM Smeter
information since it uses a narrow band signal and is not subjected to AGC.
The IF2 amplifier gain is controlled by the AGC2 loop.
The IF2 limiter output is also used to generate the input signal for the AM IF counter.
5.2.4
The demodulated signal is buffered on the same output pin used for the FM MPX signal and is subjected
to the action of the soft mute circuit as well (programmable threshold/slope). The controlling signal for this
function is the filtered Smeter.
5.2.5
Station detection in AM is done evaluating the field strength and the IF2 signal frequency position.
The field strength detector (sensing the filtered Smeter) has a programmable threshold.
The programmability of the IF counter is the same as for the FM IF counter, albeit with different center
frequency, frequency window and sampling time programming.
The SD information or the single weak field strength information or IF counter result are available at the
SD pin.
The Smeter is available both in analog format (buffered at the SMETER pin) and in a 6 bit digital format
(tuner I
5.3 TUNING SECTION
5.3.1
One VCO is used for both AM and FM tuning. To implement the world tuning concept (one VCO applica-
tion for all the geographical areas), the VCO runs at approximately 200 MHz. Digital dividers generate the
LO signals suitable for the AM and FM 1st mixers.
The VCO itself is a two-pin base-input collector-output bipolar amplifier.
A first divider by 1, 2 and 3 is present to generate the FM LO frequency and to be fed to the PLL. A second
divider by 4, 6, 8 and 10 is present to generate the AM LO frequency. For FM tuning it is possible to pro-
gram whether the mixer works in high- or in low-side injection mode. A 90â phase shift circuit operated on
the output of the first divider to generate the I and Q portions of the LO for the FM image rejection mixer.
5.3.2
A 10.25 MHz crystal oscillator is used to generate the reference frequency of the tuning PLL and of the
switched capacitor circuits of the TDA7514. The oscillation frequency can be finely adjusted by program-
ming internal capacitors (5 bit).
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IF NOISE BLANKER
IF AMPLIFIER AND DETECTOR
AUDIO OUTPUT
STATION DETECTION
VCO AND DIVIDERS
XTAL OSCILLATOR
C bus read byte).