2-11
RF2938
Rev A8 010418
11
TRANSMITTER
TX LPF and Mixers
The transmit section starts with a pair of 5-pole Bessel
filters identical to the filters in the receive section and
with the same 3 dB frequency. These filters pre-shape
and band-limit the digital or analog input signals prior
to the first upconversion to IF. These filters have a high
input impedance and expect an input signal of
200mVPP typical. Following these low pass filters are
the I/Q quadrature upconverter mixers. Each of these
mixers is half the size and half the current of the RF to
IF downconverter on the RF2444. Recall that this
upconverted signal may drive the same SAW filter (in
half duplex mode) as the RF2444 and therefore share
thesameload.Havingthe sum of the twoBBtoIF mix-
ers equal in size and DC current to the RF to IF mixer,
will minimize the time required to switch between RX
and TX, and will facilitate the best impedance match to
the filter.
TX VGA
The AGC after the SAW filter starts with a switch and a
constant gain amplifier of 15dB, which is identical to
the circuitry on the receive IF AGC. This was, done, as
on the RX signal path, so that the input impedance will
remain constant for different TX gain control voltages.
Following this 15dB gain amplifier is a single stage of
gain control offering 15dB gain range. The main pur-
pose of adding this variable gain is to give the system
the flexibility to use different SAW filters and image fil-
ters with different insertion loss values. This gain could
also be adjusted real time, if desired.
TX Upconverter
The IF to RF upconverter is a double-balanced differ-
ential mixer with a differential to single-ended con-
verter on the output to supply 0dBm peak linear power
to the image filter. The upconverted SSB signal should
have -6 dBm power at this point, and the image will
have the same power, but due to the correlated nature
of the signal and image, the output must support 0 dBm
of linear power to maintain linearly.
+6dBm PA Driver
The SSB output of the upconverter is -6dBm of linear
power. The image filter should have at most 4dB of
insertion loss while removing the image, LO, 2LO and
any other spurs. The filter output should supply the PA
driver input -10dBm of power.
The PA driver is a two-stage class A amplifier with
10dB gain per stage and capable of delivering 6 dBm
of linear power to a 50
Ω load, and has a 1dB compres-
sion point of 12 dBm. For lower power applications, this
PA driver can beusedto drive a 50
Ω antenna directly.
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