The function of the circuit shown in Figure 1 is to route the RF signal through an IF bandpass filter. This circuit uses 3 140 MHz IF SAW filters with different bandwidths and 2 ADG904-R SP4T CMOS RF switches. The ADG904-R switch controls which bandpass filter the RF signal passes through. Only one filter can be selected at a time. Such switches are generally referred to as "filter switches". This application is very common in wireless infrastructure such as cell phone base stations and point-to-point radio RF front-ends. Selecting IF filters with different bandwidths can increase the flexibility of these systems to control different data rates.

For optimal system performance, the insertion loss must remain flat with respect to frequency in the IF band. The ADG904-R switch has a very flat insertion loss versus frequency curve, making it an ideal choice for this circuit. Insertion loss is also flat over the entire supply voltage and temperature range. Off isolation is another very important parameter that determines the performance of this circuit. The ADG904-R has excellent off-isolation characteristics, with values ​​greater than −50 dB below 200 MHz.

This circuit consists of 2 ADG904-R CMOS RF switches and 3 IF SAW filters. The circuit connection is shown in Figure 1. The supply voltage range of the switch is 1.65 V to 2.75 V, and the nominal voltage of 2.5 V was used for evaluation. The RF input is connected to pin 10 (RFC) of ADG904-R switch U1. The ADG904-R switch is a single pole four throw switch and the RFC pin is the common connection for all four switches (RF1, RF2, RF3 and RF4). Pin 4 of U1 (RF1) is connected to the input of the Triquint 856592 SAW filter. This filter is a single-ended filter with a characteristic impedance of 50 Ω, a center frequency of 140 MHz, and a typical 1 dB bandwidth of 20.7 MHz. Note that achieving optimal termination impedance of 50 Ω requires lumped element matching. The output of the filter is connected to pin 17 (RF2) of ADG904-R U2.

Similarly, a second SAW filter (Triquint 856684) is connected to U1 and U2. At this point, the filter is connected to pin 17 (RF2) of U1 and pin 4 (RF1) of U2. The 856684 filter has a center frequency of 140 MHz and a typical 1 dB bandwidth of 16.18 MHz.

The third SAW filter (Triquint 856656) is connected to pin 7 (RF3) of U1 and pin 14 (RF4) of U2. The 856656 filter has a center frequency of 140 MHz and a typical 1 dB bandwidth of 11.82 MHz.

The PCB pad layout recommended by the SAW filter manufacturer must be used. To achieve maximum isolation from the input port to the output port, there is a plating bath below the filter. Increasing this isolation helps achieve adequate out-of-band attenuation and minimal in-band ripple. The input and output ports of each filter are matched to 50 Ω using 0603 size inductive and capacitive lumped components, which is an important aspect to ensure good performance on the circuit evaluation board. To further improve isolation, the 50 Ω transmission lines connecting the filter to the switch and RF edge connector are designed as coplanar waveguides.

The independent insertion loss versus frequency for the 11.82 MHz bandwidth filter is shown in Figure 3. This data was measured with the filter on a separate circuit evaluation PCB, using the same transmission line design and matching components, but without switches in the RF path. This response is used as reference data. Insertion loss at 140 MHz is −9.17 dB, consistent with the data sheet typical value of −9.2 dB.

 

 

 

Figure 4 shows the test results of the circuit evaluation PCB when the ADG904R switch selects the 856656 SAW filter. This response corresponds well to the independent response, which is also included in the figure for reference. The insertion loss of the filter increases by 1 dB due to the losses of the two ADG904-R switches in series. Ripple within the passband is important for wireless infrastructure applications. In the passband of 135.5 MHz to 144.5 MHz, from minimum peak to maximum peak, the in-band ripple is 0.47 dB, a level that is well within the data sheet ripple specification (0.8 dB maximum). In this application, the switch must have high off-isolation to minimize leakage from one filter channel to the next. If the switch has poor off-isolation, the in-band insertion loss and filter ripple will increase. This phenomenon was not found in the test results.

Figures 5 and 6 show the test results for the 856592 and 856684 SAW filter channels respectively. The results shown include the loss of two ADG904-R switches in series, which is approximately 1.0 dB. Both filter channels show good and flat insertion loss, as well as high levels of out-of-band attenuation. The results shown are within the tolerance range of the manufacturer's data sheet and clearly show that the ADG904-R switches did not introduce any ripple, mismatch, or affect the flatness of the filter in any way.