TA0030
TA0030
RF2607, RF2609: Transmit and Receive AGC Amplifiers for CDMA Cellu-
lar/PCS Phones
Cellular and Personal Communication Services (PCS)
phones that are based on Code Division Multiple
Access (CDMA) need careful regulation of signal levels
on both the forward and reverse channels. In the
reverse channel (mobile phone to base station) a
transmit automatic gain control (AGC) amplifier must
carefully adjust the output power of the mobile so that it
does not dominate the input spectrum at the base sta-
tion.
Each mobile's signal should arrive at the base station
at the same power level; this helps ensure that capac-
ity is maximized. In the forward channel (base station
to mobile) a receive AGC amplifier adjusts to accom-
modate widely varying signal levels coming in from the
base station. At a CDMA mobile phone antenna
numerous signals sent from the cell base station are
layered on a single frequency band and within this
group of signals lays the desired data.
All of these signals (desired and undesired) pass
through a low noise amplifier front end and then
through a downconverting mixer. Immediately following
the mixer the waveforms pass through a CDMA inter-
mediate frequency (IF) bandpass filter. All of the afore-
mentioned signals are in-band and are not filtered.
Because of this condition the receive AGC amplifier
cannot simply limit. The strongest signal would be lim-
ited and compress all other signals that were received,
so if the desired data was not the strongest, it would be
lost. It becomes clear that the receive AGC amplifier
must provide linear amplification and attenuation to
prevent limiting of undesired signals. RF Micro Devices
has developed two integrated circuits (ICs) that per-
form both of these important functions. The RF2607
CDMA/FM Receive AGC Amplifier and the RF2609
CDMA/FM Transmit AGC Amplifier are both monolithic
ICs that are fabricated in an advanced bipolar Silicon
process. Both of these low cost, high performance ICs
pack variable gain differential amplifier stages, gain
control operational amplifiers, and temperature com-
pensation circuitry within small QSOP16 plastic pack-
ages.
directing the RF2609 to increment or decrement its
gain in 1dB steps. As the mobile strays further from the
base station, the RF2609 is directed to increase its
gain and hence, its output power, while the reverse
occurs as the mobile approaches the base station. The
gain of the IC is controlled by a single DC voltage
externally supplied by a digital-to-analog (D/A) con-
verter which is swept from 0VDC to 3VDC. Figure 1
illustrates the gain response of the RF2609 as the gain
control voltage is swept.
RF2609 Gain vs. Gain Control Voltage
(Vcc=3.6 V, 130 MHz)
60
50
40
30
20
+25擄C
-30擄C
+80擄C
Gain (dB)
10
0
-10
-20
-30
-40
-50
-60
0.0
0.5
1.0
1.5
2.0
2.5
3.0
GC (volts)
Figure 1. Gain response of the RF2609 as the gain
control voltage is swept
CDMA system specifications dictate that as the gain is
swept over its entire range, the transmit AGC amplifier
must maintain a minimum input third order intercept
point (IIP3). Another way to state this requirement is
that the adjacent channel power rejection of the ampli-
fier must remain constant regardless of the output
level. This specification ensures that IS-95 transmitted
spectrum requirements are met under the entire gain
range of the AGC amplifier. To clarify how this affects
the design, in a CDMA phone the input signal provided
to the RF2609 will remain constant even while the gain
is increased or decreased. Thus, the output signal will
vary proportionally with the gain of the IC. Under these
conditions, the third order (IM3) products must remain
constant and not rise as the gain moves upward. The
RF2609 uses a proprietary variable gain amplifier
scheme that achieves excellent IM3 performance (see
13-151
13
TECHNICAL NOTES
AND ARTICLES
The RF2609 features a 90dB gain range from -48dB
power gain to +42dB power gain and is powered from
a single 3.6V supply. In a cellular system, the cellular
base station sends control signals to the mobile phone
Copyright 1997-2000 RF Micro Devices, Inc.
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