TQ9222 データシートの表示（PDF） - TriQuint Semiconductor

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TQ9222

Dual-Band TDMA LNA/Downconverter IC

TriQuint Semiconductor 

High Band LNA Output

The output of the high band LNA can also be connected directly

to the image stripping filter. However, it has been found that

some SAW filters can cause low image rejection in the circuit as

a whole, even as their individual characteristics show rejection

of 30-40dB. It appears that the very low impedance of the SAW

filter at the image frequency can be reflected back to the LNA

output pin, lessening the effectiveness of the ground, and

leaking into the mixer input.

The optimum solution would be to include a 1/4 wavelength

transmission line between the LNA output and SAW input in

order to invert the impedance. However, a more compact

solution is to bridge a small capacitor across a much shorter

transmission line, giving a much longer effective length. (refer to

Fig 9). It may also be possible to entirely use lumped

components to accomplish the same. On the evaluation board,

the transmission line is looped to that the capacitor can be

varied in its positioning, thus facilitating tuning.

Low Band Mixer: LO Port

As mentioned earlier, a common gate buffer amplifier is

positioned between the LO port and the mixer FET gate in order

to provide a good impedance to the VCO and to allow operation

at lower LO drive levels. The buffer amplifier provides the

voltage gain needed to drive the gate of the mixer FET while

consuming very little current (approximately 1.5mA).

Because of the broadband 50Ω input impedance of the buffer

amplifier and the internal DC blocking capacitor, the user’s VCO

can be directly connected to the LO input via a 50Ω line with no

additional components.

Low Band Mixer Input

Although the low band mixer input port has been designed with

a 50Ω impedance, it has been found that LO leakage out

through the pin can in some cases reflect off the SAW filter and

travel back to the mixer input out of phase, causing some

degradation in conversion gain and system noise figure.

Sensitivity to the phenomena depends on the particular filter

model and SAW-mixer transmission line length. On the

TQ9222

Data Sheet

evaluation board, a small inductance “L8”is added on the SAW

output to compensate. With certain longer line lengths it is

possible to omit “L8”.

Low Band LO Buffer Tune

While the broadband input match of the LO buffer amplifier

makes interfacing easy, the broadband gain means that thermal

and induced noise at other frequencies can be amplified and

injected directly into the LO port of the mixer. Noise at the IF

frequency, and at LO +/- IF will be downconverted and emerge

at the IF port, degrading the downconverter noise figure.

As indicated on the diagram of Fig 5, in order to test the LO

response to these spurious signals, a two-tone signal was

injected into the LO port with the RF port terminated in 50Ω.

One signal generator is set to the LO frequency at its normal LO

drive level usually (-7 dBm). The second signal generator

(spurious signal) is set to the LO +/- the IF frequency. The

combined input power at mixer LO port has to be less than -50

dBm. The results shown in Table 2 indicate a good suppression

of the interfering signals.

50 ohm

SIG 1:

FLO

TQ9222 Mixer

RF

IF

Spectrum

Analyzer

LO

Directional

+

Coupler

SIG 2:

FLO +/- IF

Fig. 5 LO Spurious Response Diagram

LO/Spurious Mixer LO Port

C/V

(MHz)

Input Power

(dB)

991/1101

-57

-71.7

991/1101

-58.9

-71.8

Table 2. LO Spurious Response Data

For additional information and latest specifications, see our website: www.triquint.com

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