TAS5705
www.ti.com ...................................................................................................................................................................................................... SLOS549–JUNE 2008
AGC
RF
Mixer
IF
Detector
AUDIO
VFO
B0310-01
Figure 45. AM Receiver Architecture
Radio-frequency interference at either the tuned frequency or the 455-kHz IF frequency adversely affects the AM
radio reception.
However, there is another frequency that this receiver architecture also receives. This is a signal at the VFO +
tuned frequency (RF + 2 × IF).
For example, to tune a receiver at 540 kHz, an oscillator of 995 kHz is used to produce a difference frequency of
455 kHz. However, the receiver can also receive a frequency at the oscillator frequency 995 kHz plus the IF
frequency of 455 kHz (= 1450 kHz). This frequency is called the IF image frequency. This is because the 1450 –
955 produces a difference frequency of 455 kHz. It is the role of the RF section to provide sufficient selectivity to
attenuate frequencies that are several hundred kHz from the tuned frequency. However, this is often a weakness
of low-cost receivers. Therefore, the AM interference avoidance approach includes this frequency in its
interference avoidance algorithms.
AM Interference Avoidance
As a result, during AM interference avoidance, a the system select a switching rate such that the radiated
emissions avoid
•
•
•
The IF (455 kHz)
The tuned frequency
IF image frequency (tuned frequency + 2 × 455 kHz)
During AM interference avoidance, the TAS5705 receives the sample rate (38, 44.1, or 48 kHz) and the tuned
frequency (typically 540 kHz to 1840 kHz) from the system controller. The TAS5705 uses these two values to
determine what switching rate to use. The TAS5705 has a sample-rate converter (SRC) that permits the PWM
switching rate to be increased or decreased by a fractional amount. The SRC produces a PWM switching rate
that is 6, 7, or 8 times the data sample rate.
This is done by specifying the order in which the switching rates are tested. The TAS5705 provides four
selectable sequences. The evaluation selects the first rate from the sequence and applies the three tests. If the
switching rate is found to be GOOD in all three tests, then that rate is used. If that switching rate is found to be
BAD in any of the tests, then the next rate in the sequence is taken for evaluation.
The sequences are as follows: (see Register 0x22, bits 19–18)
1. 8 × sample rate, 7 × sample rate, 6 × sample rate
2. 8 × sample rate, 6 × sample rate, 7 × sample rate
3. 7 × sample rate, 8 × sample rate, 6 × sample rate
4. 7 × sample rate, 6 × sample rate, 8 × sample rate
Copyright © 2008, Texas Instruments Incorporated
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