How to Modify an Existing Design from a TDA5200 or TDA5201 to a MAX1470 Superheterodyne Receiver
- Your assembled receiver board
- A soldering iron
- A MAX1470 IC
- A 6.6128MHZ (433.92MHz operation) or a 4.7547MHz (315MHz operation) crystal
- 2x100pF and 1x3.0pF capacitors for 433.92MHz or 2x100pF and 1x4.7pF capacitors for 315MHz operation
- 1x68nH, and 2x15nH inductors for 433.92MHz or 1x15nH, 1x27nH, and 1x120nH inductors for 315MHz operation
Figure 1. Typical TDA5200 based circuit (433.92MHz) and the four conversion steps.
The four simple steps for this conversion are described here:
- Supply voltage: The MAX1470 requires a 3.3V supply. Care must then be taken not to connect the TDA's 5V supply to the MAX1470 based circuit.
- IC and XTAL oscillator: Replace the TDA5200 with the MAX1470. The MAX1470 PLL uses a 64x clock divider which forces the XTAL to be 6.6128MHz for a 433.92MHz receive frequency. Depending on the crystal used, capacitors C15 and C16 may have to be different, to compensate for the crystal being pulled away from its stated operating frequency due to the difference in load capacitance. This is referred to as crystal pulling. Refer to application note 1017, "How to Choose a Quartz Crystal Oscillator for the MAX1470 Superheterodyne Receiver" for more details.
- Input matching and degeneration inductor: The MAX1470 based design replaces C1, C2, C3, and L1 with a series 100pF cap and a series 68nH inductor. In addition, replace C5 with a 15nH inductor. The two new inductors will need to be positioned in place of the C3 and C5 capacitors. The values given in Table 1 are nominal but may need to be slightly tuned since they can be affected by the PC board traces.
- LC tank and mixer input. L2, C6, and C8 will have to be changed to the values listed in Table 1. Once again, the values given are nominal but may need to be slightly tuned since they are affected by the PC board traces.
Table 1. Component differences between the TDA5200 and MAX1470 based designs
|Infineon Circuit||Maxim Circuit|
|XTAL||13.2256MHz||6.6128MHz (433.92MHz), 4.7547MHz (315MHz)|
|C4||100pF||Unchanged or 10nF|
The resulting circuit is shown in Figure 2.
Figure 2. Typical MAX1470 based circuit (433.92MHz).
If the tank tuning and antenna matching are done correctly, you should expect the sensitivity to exceed