Philips Semiconductors
Product specification
Low voltage high performance mixer FM IF system
with high-speed RSSI
SA626
50.00
48.00
46.00
POWER GAIN (dB)
44.00
42.00
40.00
38.00
36.00
34.00
32.00
30.00
10.00
30.00
50.00
70.00
–50.00
–30.00
–10.00
90.00
V
CC
= 2.7V
V
CC
= 5V
V
CC
= 3V
POWER GAIN (dB)
65.00
63.00
61.00
59.00
57.00
55.00
53.00
51.00
49.00
47.00
45.00
10.00
30.00
50.00
70.00
–50.00
–30.00
–10.00
90.00
V
CC
= 2.7V
V
CC
= 3V
V
CC
= 5V
TEMPERATURE (°C)
TEMPERATURE (°C)
SA626 IF Amplifier Gain vs Temperature vs Supply Voltage
SA626 Limiting Amplifier Gain vs Temperature
vs Supply Voltage
SR00474
Figure 5. Performance Characteristics
CIRCUIT DESCRIPTION
The SA626 is an IF signal processing system suitable for second IF
or single conversion systems with input frequency as high as 1GHz.
The bandwidth of the IF amplifier is about 40MHz, with 38dB of
power gain from a 50Ω source. The bandwidth of the limiter is about
28MHz with about 54dB of power gain from a 50Ω source.
However, the gain/bandwidth distribution is optimized for 10.7MHz,
330Ω source applications. The overall system is well-suited to
battery operation as well as high performance and high quality
products of all types, such as cordless and cellular hand-held
phones.
The input stage is a Gilbert cell mixer with oscillator. Typical mixer
characteristics include a noise figure of 14dB, conversion power
gain of 11dB, and input third-order intercept of -16dBm. The
oscillator will operate in excess of 1GHz in L/C tank configurations.
Hartley or Colpitts circuits can be used up to 100MHz for xtal
configurations. Butler oscillators are recommended for xtal
configurations up to 150MHz.
The output of the mixer is internally loaded with a 330Ω resistor
permitting direct connection to a 10.7MHz ceramic filter. The input
resistance of the limiting IF amplifiers is also 330Ω. With most
10.7MHz ceramic filters and many crystal filters, no impedance
matching network is necessary. To achieve optimum linearity of the
log signal strength indicator, there must be a 3dB insertion loss
between the first and second IF stages. If the IF filter or interstage
network does not cause 3dB insertion loss, a fixed or variable
resistor can be added between the first IF output (Pin 16) and the
interstage network.
The signal from the second limiting amplifier goes to a Gilbert cell
quadrature detector. One port of the Gilbert cell is internally driven
by the IF. The other output of the IF is AC-coupled to a tuned
quadrature network. This signal, which now has a 90
°
phase
relationship to the internal signal, drives the other port of the
multiplier cell.
Overall, the IF section has a power gain of 92dB. For operation at
intermediate frequency at 10.7MHz. Special care must be given to
layout, termination, and interstage loss to avoid instability.
The demodulated output of the quadrature drives an internal op
amp. This op amp is configured as a unity gain buffer. It can drive
an AC load as low as 5kΩ with a rail-to-rail output.
A log signal strength indicator completes the circuitry. The output
range is greater than 90dB and is temperature compensated. This
log signal strength indicator exceeds the criteria for AMPs or TACs
cellular telephone, and RCR-28 cordless telephone. This signal
drives an internal op amp. The op amp is capable of rail-to-rail
output. It can be used for gain, filtering, or 2nd-order temperature
compensation of the RSSI, if needed.
1997 Sept 25
7
PHILIPS [ NXP SEMICONDUCTORS ]
