Keywords: rf, rfic, wireless, cell phone, if, saw filter, gps, amps, cdma, switch, IF filters, wireless handsets, IF receiver IC
Maxim's MAX2308 IC is a highly integrated building block for low cost super heterodyne mobile telephone receivers. This IC includes a wide dynamic range dual-input IF variable gain amplifier (VGA), low phase noise IF VCO (voltage controlled oscillator) with PLL (phase locked loop) and an IQ demodulator. Efficiently applying the MAX2308 with three IF filters seems difficult to handset design engineers, since the MAX2308 inherently supports two IF filter paths. The MAX2308 was designed when two IF paths were thought to be sufficient for most wireless phone system standards.
The most commonly used IF frequency for all three filters is now 183.6 MHz. This frequency is too high for any common analog, low-frequency switch. High frequency switches have no problem operating at this frequency, at the cost of increased insertion loss (IL). Another issue the compounds the IL is that the RF switches are designed and specified for 50Ω systems. Most IF SAW filters have output impedances of approximately 150Ω and inputs of the IF VGA are on order of 1k ohm. Under these conditions a typical RF switch presents an insertion loss of roughly 5dB. The test schematic shown in Figure 1 is used to measure the insertion loss of any RF switch with high impedance input and output loading. A typical IF SAW filter has 5 dB of insertion loss, which when added to the 5dB loss of the rf switch makes 10 dB of gain that the receiver must now supply. This may not be possible if the VGA has run out of dynamic range.
This situation is improved if the IF SAW filter is matched to a 50Ω switch and then the 50Ω switch is matched to the high impedance IF VGA (see Figure 2).
This solution has some benefits and well as some disadvantages.
The benefits include:
The disadvantages include:
Another solution is to connect each IF filter through an appropriate matching network to each side of the differential IF VGA. In parallel, each input of the IF VGA is connected through a "large" capacitor to inputs of a SPDT RF switch. The output of the switch is then AC grounded. In operation the desired IF path is selected by employing the RF switch to AC ground the unused path. Because the VGA inputs and the IF SAW output matching networks are high impedance circuits, the switch impedance provides near-perfect short at the IF frequency. Another benefit of this approach is that the switch introduces little or no loss into the path of the active IF filter. An example of this solution is shown in Figure 3.