TQ5121 View Datasheet(PDF) - TriQuint Semiconductor
Part Name
Description
Manufacturer
TQ5121 Datasheet PDF : 10 Pages
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TQ5121
Data Sheet
Table 1. TQ5121 Noise Parameters
Freq |Gopt| <Gopt Fmin
Rn
(MHz)
(dB)
(W)
835
0.678
33
1.34
61.6
850
0.655
34
1.38
61.1
865
0.652
36
1.36
61.2
880
0.652
38
1.35
60.9
895
0.649
38
1.36
61.3
910
0.659
40
1.35
61.2
925
0.687
41
1.35
65.6
LNA Output Match
The output impedance of the LNA was designed to interface
directly with 50Ω terminations. This internal match serves to
reduce the number of external components required at this port.
An additional benefit accrues as an improvement in IP3
performance, return loss and power gain.
The output of the LNA will most often be connected to an image
stripping filter. Depending on the filter type, additional
components might be needed to present a better match to the
LNA output. The TQ5121 general applications circuit (page 4)
shows a TOYOCOM (637-881A) saw filter. A series inductor
“Lx”of 10nH is added to the filter input to improve the match.
This series inductor also smoothes out excessive ripple in the
filter passband improving the overall performance of the circuit.
Mixer
The mixer of the TQ5121 is implemented by a common source
depletion FET. The mixer is designed to operate on supply
voltages from 3V to 5V. An on-chip buffer amplifier simplifies
direct connection of the LO input to a commercial VCO at drive
levels down to -7dBm. The common-gate LO buffer provides a
good input match, and supplies the voltage gain necessary to
drive the mixer FET gate. The "open-drain " IF output allows for
Fig 3. Mixer Section
LO Bias and
Mixer LO Tuning
Input
Mixer RF
Input
Mixer IF
Output
flexibility in matching to various IF frequencies and filter
impedance’s. See Figure 3.
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.
Mixer Input
Although the 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.
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 4, 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 3 indicate a good suppression
of the interfering signals.
6
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