Analog Devices AD8342 specifications If Port Impedance

AD8342

IF PORT

The IF port comprises open-collector differential outputs. The NPN open collectors can be modeled as current sources that are shunted with resistances of ~10 kΩ in parallel with capacitances of ~1 pF.

The specified performance numbers for the AD8342 were measured with 100 Ω differential terminations. However, dif- ferent load impedances may be used where circumstances dic- tate. In general, lower load impedances result in lower conver- sion gain and lower output P1dB. Higher load impedances result in higher conversion gain for small signals, but lower IP3 values for both input and output.

If the IF signal is to be delivered to a remote load, more than a few millimeters away at high output frequencies, avoid unin- tended parasitic effects due to the intervening PCB traces. One approach is to use an impedance transforming network or transformer located close to the AD8342. If very wideband out- put is desired, a nearby buffer amplifier may be a better choice, especially if IF response to dc is required. An example of such a circuit is presented in Figure 45, in which the AD8351 differen- tial amplifier is used to drive a pair of 75 Ω transmission lines. The gain of the buffer can be independently set by appropriate choice of the value for the gain resistor, RG.

50		0.5
45		0.4
40
35		0.3	CAPACITANCE(pF)
30		0.2
25
20		0.1
RESISTANCE(kΩ)		0
15
10		–0.1
5
0	100M 200M 300M 400M 500M 600M 700M 800M 900M	–0.2	05352-045
0		1G
	FREQUENCY (Hz)

Figure 44. IF Port Impedance

The high input impedance of the AD8351 allows for a shunt differential termination to provide the desired 100 Ω load to the AD8342 IF output port.

It is necessary to bias the open-collector outputs using one of the schemes presented in Figure 47 and Figure 48. Figure 47 illustrates the application of a center tapped impedance trans- former. The turns ratio of the transformer should be selected to provide the desired impedance transformation. In the case of a 50 Ω load impedance, a 2-to-1 impedance ratio transformer should be used to transform the 50 Ω load into a 100 Ω differ- ential load at the IF output pins. Figure 48 illustrates a differen- tial IF interface where pull-up choke inductors are used to bias the open-collector outputs. The shunting impedance of the choke inductors used to couple dc current into the mixer core should be large enough at the IF operating frequency so it does not load down the output current before reaching the intended load. Additionally, the dc current handling capability of the selected choke inductors needs to be at least 45 mA. The self- resonant frequency of the selected choke should be higher than the intended IF frequency. A variety of suitable choke inductors are commercially available from manufacturers such as Murata and Coilcraft. Figure 46 shows the loading effects when using nonideal inductors. An impedance transforming network may be required to transform the final load impedance to 100 Ω at the IF outputs. There are several good reference books that explain general impedance matching procedures, including:

•Chris Bowick, RF Circuit Design, Newnes, Reprint Edition, 1997.

•David M. Pozar, Microwave Engineering, Wiley Text Books, Second Edition, 1997.

•Guillermo Gonzalez, Microwave Transistor Amplifiers: Analysis and Design, Prentice Hall, Second Edition, 1996.

	90
120	60
150	30
	50MHz

+VS

AD8342						+V
COMM	8		RFC			S
IFOP	7					+
			Ω		RG	AD8351
IFOM	6		100			–
COMM	5		RFC
		+V
		S	Z	L	= 100Ω

Tx LINE ZO = 75Ω

ZL

Tx LINE ZO = 75Ω

05352-046

		REAL
		CHOKES
180		0
		50MHz
	500MHz	IDEAL
		CHOKES
210		330
	500MHz
240	300
		-049

Figure 45. AD8351 Used as Transmission Line Driver and Impedance Buffer

270 05352

Figure 46. IF Port Loading Effects Due to Finite Q Pull-Up Inductors

(Murata BLM18HD601SN1D Chokes)

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