AD6630
–1dB 15dB –9dB –2dB 15dB –5dB 24dB –5dB
MAIN
9dBm
4dBm
–15dBm
–28dBm –29dBm –14dBm –23dBm–25dBm–10dBm
LOCAL
OSCILLATOR
AD6600
AD6620
DSP
AD6630
SAW
SAW
DIVERSITY
ANTENNA
–104dBm
–43dBm
–28dBm
–16dBm
–15dBm
AD6630 INPUT
–91dBm
–30dBm
–15dBm
–3dBm
AD6630 OUTPUT
–67dBm
–6dBm
AD6600 INPUT
–71dBm
–10dBm
+4dBm
+9dBm
+9dBm
+9dBm
+4dBm
+4dBm
–2dBm
Figure 6. GSM Design Example
This equation is derived from measured data at 170 MHz. Clamp
levels vary with frequency, see Figure 5. Output clamp levels
less than 8.5 dBm will result in damage to the clamp circuitry
unless the absolute maximum input power is derated. Similarly,
the output clamp level cannot be set higher than 12 dBm.
THEORY OF OPERATION
The AD6630 amplifier consists of two stages of gain. The first
stage is differential. This differential amplifier provides good
common-mode rejection to common-mode signals passed by
the SAW filter. The second stage consists of matched current
feedback amplifiers on each side of the differential pair. These
amplifiers provide additional gain as well as output drive capa-
bility. Gain set resistors for these stages are internal to the de-
vice and cannot be changed, allowing fixed compensation for
optimum performance.
R
CLAMP
GENERATOR
V
EE
Figure 7. Clamp Level Resistor
Matching SAW Filters
The AD6630 is designed to easily match to SAW filters. SAW
filters are largely capacitive in nature. Normally a conjugate
match to the load is desired for maximum power transfer.
Clamping levels for the device are normally set by tying CLLO
or CLHI pins to the negative supply. This internally sets bias
points that generate symmetric clamping levels. Clamping is
achieved primarily in the output amplifiers. Additional input
stage clamping is provided for additional protection. Clamping
levels may be adjusted to lower levels as discussed below.
Another way to treat the problem is to make the SAW filter look
purely resistive. If the SAW filter load looks resistive there is no
lead or lag in the current vs. voltage. This may not preserve
maximum power transfer, but maximum voltage swing will
exist. All that is required to make the SAW filter input or output
look real is a single inductor shunted across the input. When the
correct value is used, the impedance of the SAW filter becomes
real.
APPLICATIONS
The AD6630 provides several useful features to meet the needs
of radio designers. The gain and low noise figure of the device
make it perfect for providing interstage gain between differential
SAW filters and/or analog-to-digital converters (ADC). Addi-
tionally, the on-board clamping circuitry provides protection for
sensitive SAW filters or ADCs. The fast recovery of the clamp
circuit permits demodulation of constant envelope modulated
IF signals by preserving the phase response during clamping.
9.7⍀
The following topics provide recommendations for using the
AD6630 in narrowband, single carrier applications.
400⍀
47nH
3pF
15.2pF
Adjusting Output Clamp Levels
Normally, the output clamp level is set by tying either CLLO or
CLHI to ground or VEE. It is possible to set the limit between
8.5 dBm and 12 dBm levels by selecting the appropriate exter-
nal resistor.
Figure 8. Saw Filter Model (170 MHz)
EVALUATION BOARD
To set to a different level, CLLO and CLHI should be tied
together and then through a resistor to ground. The value of the
resistor can be selected using the following equation.
Figures 9, 10 and 12 refer to the schematic and layout of the
AD6630AR as used on Analog Devices’ GSM Diversity Re-
ceiver Reference Design (only the IF section is shown). Figure
14 references the schematic of the stand-alone AD6630 evalua-
tion board and uses a similar layout. The evaluation board uses
center tapped transformers to convert the input to a differential
signal and AD6630 outputs to a single connector to simplify
evaluation. C8, C9 and L2 are optional reactive components to
tune the load for a particular IF frequency if desired.
14.4–OUTPUT
R =
CLAMP (dBm)
0.0014
REV. 0
–5–
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