Analog Devices AD8342 specifications High if Applications, Ac loading impedance

AD8342

HIGH IF APPLICATIONS

In some applications it may be desirable to use the AD8342 as an up-converting mixer. The AD8342 is a broadband mixer capable of both up and down conversion. Unlike other mixers that rely on on-chip reactive circuitry to optimize performance over a specific band, the AD8342 is a versatile general-purpose device that can be used from arbitrarily low frequencies to sev- eral GHz. In general, the following considerations help to en- sure optimum performance:

•Minimize ac loading impedance of IF port bias network.

•Maximize power transfer to the desired ac load.

•For maximum conversion gain and the lowest noise per- formance reactively match the input as described in the IF Port section.

•For maximum input compression point and input intercept points resistively terminate the input as described in the IF Port section.

As an example, Figure 51 shows the AD8342 as an up- converting mixer for a WCDMA single-carrier transmitter de- sign. For this application, it was desirable to achieve −65 dBc adjacent channel power ratio (ACPR) at a −13 dBm output power level. The ACPR is a measure of both distortion and noise carried into an adjacent frequency channel due to the finite intercept points and noise figure of an active device.

production concerns due to the sensitivity of the match. For this application, it is advantageous to shunt down the ~1 kΩ input impedance using an external shunt termination resistor to allow for a lower Q reactive matching network. The input is terminated across the RFIN and RFCM pins using a 499 Ω termination. The termination should be as close to the device as possible to minimize standing wave concerns. The RFCM is bypassed to ground using a 1 nF capacitor. A dc blocking ca- pacitor of 1 nF is used to isolate the dc input voltage present on the RFIN pin from the source. A step-up impedance transfor- mation is realized using a series L shunt C reactive network. The actual values used need to accommodate for the series L and stray C parasitics of the connecting transmission line seg- ments. When using the customer evaluation board with the components specified in Figure 51, the return loss over a 5 MHz band centered at 170 MHz was better than 10 dB.

External pull-up choke inductors are used to feed dc bias into the open-collector outputs. It is desirable to select pull-up choke inductors that present high loading reactance at the output frequency. Coilcraft 0302CS series inductors were selected due to their very high self-resonant frequency and Q. A 1:1 balun was ac-coupled to the output to convert the differential output to a single-ended signal and present the output with a 50 Ω

ac loading impedance.

				100pF
VPOS				1.82kΩ
0.1pF	100pF
		12	11	10	9
	VPDC		PWDN	EXRB	COMM
	13	COMM			COMM	8
	1nF
	14	RFCM			IFOP	7

VPOS

100pF

34nH

ETC1-1-13

The performance of the circuit is shown in Figure 52. The aver- age ACPR of the adjacent and alternate channels is presented vs. output power. The circuit provides a 65 dBc ACPR at

−13 dBm output power. The optimum ACPR power level can be shifted to the right or left by adjusting the output loading and

100nH	1nF	499	Ω		AD8342
170MHz			15	RFIN			IFOM	6
INPUT
4.7pF	VPOS
			16	VPMX		COMM		5
	0.1∝F	100pF			LOCM	LOIN	COMM
			VPLO
				1	2	3	4
					1nF	1nF
			100pF
						1970MHz
						OSC

1nF

1nF

34nH

100pF

VPOS

2140MHz OUT

05352-052

the loss of the input match.

	–60
	–62	ADJACENT
		CHANNELS
(dBc)	–64

Figure 51. WCDMA Tx Up-Conversion Application Circuit

Because a WCDMA channel encompasses a bandwidth of almost 5 MHz, it is necessary to keep the Q of the matching circuit low enough so that phase and magnitude variations are below an acceptable level over the 5 MHz band. It is possible to use purely reactive matching to transform a 50 Ω source to match the raw ~1 kΩ input impedance of the AD8342. However, the L and C component variations could present

ACPR	–66
					ALTERNATE
	–68				CHANNELS
							05352-053
	–70
		–20	–15	–10	–5	0
	–25

OUTPUT POWER (dBm)

Figure 52. Single Carrier WCDMA ACPR Performance of Tx Up-Conversion

Circuit (Test Model 1_64)

Rev. 0 Page 18 of 20