SA600 Datasheet, PDF (13/17 Pages) NXP Semiconductors – 1GHz LNA and mixer

Philips Semiconductors
1GHz LNA and mixer
Product specification
NE/SA600
And last but not least, is the impedance matching at LNA inputs and
outputs and mixer RF and LO input ports. Only those who have
toiled through discrete transistor implementations for 50Ω input and
output impedance matching can truly appreciate the elegance and
simplicity of the NE/SA600 input and output impedance matching to
50Ω. Also, the mixer output impedance is high, so matching to a
crystal or SAW IF filter becomes extremely easy without the need for
additional IF impedance transformers (tapped-C networks with
inductors or baluns).
The NE/SA600 applications and demo board features standard low
cost 62mil FR-4 board. A top-side ground plane is used and 50Ω
coplanar transmission lines are used. LO and RF_INA traces are
perpendicular. Provisions for the image reject filter between RF_OUTA
and RF_INMX are provided. A simple LC match for 80MHz IF is used
so that 50Ω measurements can be made on the demo board.
The NE/SA600 applications evaluation board schematic is shown in
Figure 1. The V_CC (Pin 1) and V_CCMX (Pin 14) are tied together and
the power supply is bypassed with capacitors C5 and C6. These
capacitors should be placed as close to the device as practically
possible.
C1 is the DC blocking capacitor to the input of the LNA. L1 provides
additional input matching to the LNA for an improved return loss
(S11). This inductor can be a surface-mount component or can be
easily drawn on the printed circuit board (small spiral or serpentine).
This additional match improves the gain of the LNA by 0.4dB and
lowers the noise figure to 2dB or less. If the typical gain of the LNA
of 16dB is acceptable with 2.2dB of noise figure, then L1 can be
eliminated. If the LNA input is fed from a duplexer or selectivity
filter after the antenna, C1 can also be eliminated since the filter will
also provide DC blocking. The LNA bypass capacitor C3 should be
at least 100 times C1 or C9 for low frequency stability. Switch S1
toggles the LNA gain/through function. R1 is used only to limit the
maximum current into the enable pin and only necessary if enable
may power up before the V_CC.
C4 is a DC blocking capacitor for the LO input pin and may not be
needed in actual applications if the VCO output is isolated and will
not upset the internal DC biasing of the mixer. The image reject
filter goes between the output of the LNA and the RF input to the
mixer. Since the LO input, RF output and mixer input are all 50Ω
matched impedances internally, there is no need for any external
components. C8 and C9 are DC blocking capacitors to the
connectors and will not be needed in an actual application.
R2 and L2 are the load to the mixer output which is typical of the IF
crystal or SAW filters. C2 and L3 provide a match from the high
impedance mixer output to a 50Ω test set-up (spectrum analyzer,
etc.) and C7 is a DC blocking capacitor for the mixer output.
The printed circuit board layout for the schematic of Figure 1 is
shown in Figure 14. It is a very simple printed circuit board layout
with all the components on a single side. The layout also
accomodates a two pole image reject filter between the LNA outupt
and mixer input. All the input and output traces to the LNA and
mixer should be 50Ω tracks with the exception of mixer output,
which can be very narrow due to the higher impedances of the filter.
The NE/SA600 internal supply is very well regulated. This is seen
from Figure 15 which shows the I_CC vs. V_CC for the NE/SA600.
Table NO TAG shows the S11, S21, S22 and S21 for the LNA from
800-1200MHz. Typical measurements at 900MHz for the critical
parameters such as gain, noise figure, IP₃, 1dB compression point,
etc. as measured on an applications evaluation board are as follows
:
LNA gain = 16.5dB
LNA through = –7dB
Mixer gain =–3dB (into a 50Ω load)
LNA noise figure = 2dB
Mixer noise figure = 14dB
LNA IP₃ = –10dBm (in gain mode)
LNA IP₃ = +26dBm (in through mode)
LNA 1dB compression point = –20dBm
Mixer 1dB compression point = –4dBm
The shunt inductor L1 for input match is optional. Figure 16 shows
the effect of the inductor value from 8.2nH to 15nH on gain, noise
figure and input match.
The total power gain for the LNA and mixer (excluding the image
reject filter) in a system where the output of the mixer is loaded with
50Ω is about 14dB. In an actual system the output impedance of
the mixer is usually much higher than 50Ω (more like 1kΩ or higher)
and so it is more important to consider the voltage gain from the
input at the LNA to the mixer output. The voltage gain in this case
will be about 29.85V/V. The total noise figure for the LNA and mixer
combination is be about 3.27dB. The input third order intercept
point for the LNA and mixer is about -11dBm. In the LNA through
mode, the intercept point for the combination is higher than
+19dBm. This LNA through feature provides an additional boost to
the total dynamic range of the system.
The NE/SA600 finds applications in many areas of RF
communications. It is an ideal down converter block for high
performance, low cost, low power RF communications transceivers.
The front-end of a typical AMPS/TACS/NMT/TDMA/CDMA cellular
phone is shown in Figure 13. This could also be the front-end of a
VHF/UHF handheld transceiver, UHF cordless telephone or a
spread spectrum system.
The antenna is connected to the duplexer input. The receiver output
of the duplexer is connected to the RF input of the LNA. If the
additional improvement in noise figure and gain are not needed to
meet the system specifications then L1 and C1 can be eliminated.
In TDMA systems, the NE/SA600 can be totally powered down by
Q1 and the two resistors. In this mode the current consumption will
be zero mA. Care should be taken in the software of the system to
insure that the enable pin on NE/SA600 tied to the LNA gain control
port is held low while the device is in total power down mode. L2
and C2 can be tuned to the IF frequency and to match to the IF filter
impedance.
A complete analysis of the front-end shows that the total voltage
gain from the antenna input to the mixer output is about 9.5V/V. This
value includes a 3.2dB loss for the duplexer and a 1.8dB loss for the
bandpass filter. The noise figure as referred to the antenna is 7dB
and the input third order intercept point is about -7.5dBm. In LNA
through mode the input third order intercept point increases to about
+24dBm.
During normal operation of a handheld RF receiver the received
signal strength (RSSI) is nominally greater than -100dBm. The
signal only drops below this level due to severe multipath fading,
shadow effect or when the receiver is at extreme fringes of cell
coverage. The LNA through mode can be used here as a two step
gain control such that when RSSI is below a certain threshold level
(e.g. -90dBm), the LNA has a -7dB loss and the total current
consumption of the NE/SA600 is only 4.3mA. The sensitivity of the
system will not suffer because the received RF signal is much higher
than the noise floor of the system. When the RSSI falls below a
certain threshold (e.g. -95dBm) the LNA is enabled to give the full
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