AM STEREO TUNER/FM STEREO IF PCB with MC13028, MC145151 and LA1832


Tanınmış Üye
This application combines a Sanyo LA1832M with the Motorola MC13028A AM Stereo decoder IC. The LA1832 provides an FM IF, FM multiplex detection, AM tuning, and the AM IF functions. The MC13028A provides the AM Stereo detection as well as Left and Right audio outputs. An MC145151 synthesizer provides the frequency control of the local oscillator contained within the LA1832. Frequency selection is by means of a switch array attached to the synthesizer.

The MC13028A is designed as a low voltage, low cost decoder for the C–QUAM AM Stereo technology and is completely compatible with existing monaural AM transmissions. The IC requires relatively few, inexpensive external parts to produce a full featured C–QUAM AM Stereo implementation. The layout is straightforward and should produce excellent stereo performance. This device performs the function of IF amplification, AGC, modulation detection, pilot tone detection, signal quality inspection, and left and right audio output matrix operation. The IC is targeted for use in portable and home AM Stereo radio applications.
The amplified C–QUAM IF signal is fed simultaneously to the envelope detector circuit, and to a C–QUAM converter circuit. The envelope detector provides the L+R (mono) signal output which is fed to the stereo matrix. In the converter circuit, the C–QUAM signal is restored to a Quam signal. This is accomplished by dividing the C–QUAM IF signal by the demodulated cos Φ term. The cosΦ term is derived from the phase modulated IF signal in an active feedback loop. Cosine Φ is detected by comparing the envelope detector and the in–phase detector outputs in the high speed comparator/feedback loop. Cosine Φ is extracted from the I detector output and is actively transferred through feedback to the output of the comparator. The output of the comparator is in turn fed to the control input of the divider, thus closing the feedback loop of the converter circuit. In this process, the cos Φ term is removed from the divider IF output, thus allowing direct detection of the L–R by the quadrature detector. The audio outputs from both the envelope and the L–R detectors are first filtered to minimize the second harmonic of the IF signal. Then they are fed into a matrix circuit where the Left channel and the Right channel outputs can be extracted at Pins 15 and 16. (The outputs from the I and Q detectors are also filtered similarly.) At this time, a stereo indicator driver circuit, which can sink up to 10 mA, is also enabled. The stereo output will occur if the input IF signal is: larger than the stereo threshold level, not too noisy, and if a proper pilot tone is present. If these three conditions are not met, the blend circuit will begin to force monaural operation at that time.
A blend circuit is included in this design because conditions occur during field use that can cause input signal strength fluctuation, strong unwanted co–channel or power line interference, and/or multi–path or re–radiation. When these aberrant conditions occur, rapid switching between stereo and mono might occur, or the stereo quality might be degraded enough to sound displeasing. Since these conditions could be annoying to the normal listener, the stereo information is blended towards a monaural output. This circuit action creates a condition for listening where these aberrant effects are better tolerated by the consumer.
Intentional mono operation is a feature sometimes required in receiver designs. There are several ways in which to accomplish this feat. First, a resistor from Pin 10 to ground can be switched into the circuit. A value of 1.0 k is adequate as is shown in the schematic in Figure 18. A second method to force the decoder into mono is simply to shunt Pin 10 to ground through an NPN transistor (collector to Pin 10, emitter to ground), where the base lead is held electrically “high” to initiate the action.
A third method to force a mono condition upon the decoder is to shunt Pin 8 of the decoder to ground through an NPN transistor as described above. Effectively, this operation discharges the blend capacitor (10 μF), and the blend function takes over internally forcing the decoder into mono. This third method does not necessarily require extra specific parts for the forced mono function as the first two examples do. The reason for this is that most electronically tuned receiver designs require an audio muting function during turn on/turn off, tuning/scanning, or band switching (FM to AM). When the muting function is designed into an AM Stereo receiver, it also should include a blend capacitor reset (discharge) function which is accomplished in this case by the use of an NPN transistor shunting Pin 8 to ground, (thus making the addition of a forced mono function almost “free”). The purpose of the blend reset during muting is to re–initialize the decoder back into the “fast lock” mode from which stereo operation can be attained much quicker after any of the interruptive activities mentioned earlier, (i.e. turn on, tuning, etc.).
The VCO in this IC is a phase shift oscillator type design that operates with a ceramic resonator at eight times the IF frequency, or 3.60 MHz. With IF input levels below the stereo threshold level, the oscillator is not operational. This feature helps to eliminate audio tweets under low level, noisy input conditions.
The following information provides circuit function, part number, and the manufacturer’s name for special parts identified by their schematic symbol. Where the part is not limited to a single source, a description sufficient to select a part is given.
U1 IC – AM Stereo Decoder
MC13028AD by Motorola
U2 IC – AM/FM IF and Multiplex Tuner
LA1832M by Sanyo
U3 IC – Frequency Synthesizer
MC145151DW2 by Motorola
T1 AM IF Coil
A7NRES–11148N by TOKO
F1 AM IF Ceramic Filter
SFG450F by Murata
F2 FM IF Detector Resonator
CDA10.7MG46A by Murata
F3 FM Multiplex Decoder Resonator
CSB456F15 by Murata
F4 AM Tuner Block
BL–70 by Korin Giken
X1 10.24 MHz Crystal, Fundamental Mode,
AT Cut, 18 pF Load Cap, 35 Ω maximum series R.
HC–18/U Holder
X2 3.6 MHz AM Stereo Decoder Resonator
CSA3.60MGF108 by Murata
S5 8 SPST DIP Switch

Figure 1. AM STEREO TUNER/FM STEREO IF Circuit with MC13028, MC145151 and LA1832

Figure 2. AM STEREO TUNER/FM STEREO IF PCB with MC13028, MC145151 and LA1832

The LA1832 tuner IC (U2) is set for AM operation by switch S2 connecting Pin 12 to ground. An AM Stereo signal source is applied to Pin 2 of the RF coil contained within the BL–70 tuning block. That coil applies the signal to Pin 21 of U2. The L.O. coil is connected from Pin 23 to VCC. The secondary is tuned by a varactor which is controlled by a dc voltage output from the synthesizer circuit. The reactance of this oscillator tank is coupled back to Pin 23. It is through this reactance that the frequency of the L.O. is determined. A buffered output from the L.O. emerges at Pin 24. This signal is routed to Pin 1 of the synthesizer (U3), thus completing the frequency control loop.
The mixer output at Pin 2 is applied to the IF coil T1. Coil T1 provides the correct impedance to drive the ceramic bandpass filter F1. The IF signal returns to U2 through Pin 4, and also to the input, Pin 4 of the AM Stereo decoder (U1). The ceramic filter F1 is designed to operate into a load resistance of 2.0 kΩ. This load is provided at Pin 4 of U2.
The stereo outputs exit from Pins 15 and 16 of U1. The design amplitudes of the audio outputs will vary according to the values used for the resistors to ground at Pins 15 and 16 of the decoder, While the values chosen for RO are left to the discretion of the designer, the numbers chosen in this data sheet are reflective of those required to set the general industry standard levels of audio outputs in receiver designs.
Pins 15 and 16 are also good locations for the insertion of simple RC filters that are used to comply with the United States NRSC requirement for the shape of the overall receiver audio response. There are many design factors that affect the shape of the receiver response, and they must all be considered when trying to approximate the NRSC de–emphasis response. The mixer output transformer (IF coil, T1), and ceramic filter probably have the greatest contribution to the frequency response. The ceramic filter can be tailored from its rated response by the choice of transformer impedance and bandwidth. When designing an overall audio response shape, the response of the speakers or earphones should also be considered.
The frequency to which the test circuit will tune is set by the eight binary switches contained in the S5 assembly, numbered from 1 to 8. Number 1 connects to Pin 11 of U3 and number 8 connects to Pin 18. The other switches connect to the pins in between and in order. Each individual switch is a SPST type.
To tune to a specific RF frequency, a computation must be made in order to ascertain the divide ratio to input to the synthesizer via the switch array. The divide ratio is simply the eight digit binary equivalent number for the local oscillator frequency divided by 10 kHz. The local oscillator frequency is the desired RF frequency plus 450 kHz, the IF frequency. Any local oscillator value within the AM band can be represented
by a binary number. Each binary bit represents a switch setting where a “1” is an open switch and a “0” is a closed switch. The most significant bit represents switch 8 which is connected to Pin 18.
To illustrate, consider the setting for an input frequency of 1070 kHz. (This frequency was used to test the circuit board as described further on.) The local oscillator frequency is 1070 kHz plus 450 kHz which equals 1520 kHz. Dividing by 10 kHz yields the number 152. The binary number for 152 is 10011000. Thus the switches are set to:

Switch Configuration


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