Philips Semiconductors

Product specification

NE/SA600

1GHz LNA and mixer

1993 Dec 15

59

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 50

Ω input and

output impedance matching can truly appreciate the elegance and

simplicity of the NE/SA600 input and output impedance matching to

50

Ω. 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 50

Ω

so that 50

Ω measurements can be made on the demo board.

The NE/SA600 applications evaluation board schematic is shown in

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

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 50

Ω

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 50

Ω 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 50

Ω 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

Table NO TAG shows the S11, S21, S22 and S21 for the LNA from

800-1200MHz. Typical measurements at 900MHz for the critical

etc. as measured on an applications evaluation board are as follows

:

LNA gain = 16.5dB

LNA through = –7dB

Mixer gain =–3dB (into a 50

Ω load)

LNA noise figure = 2dB

Mixer noise figure = 14dB

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

50

Ω is about 14dB. In an actual system the output impedance of

the mixer is usually much higher than 50

Ω (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