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dc1774acf
DEMO MANUAL DC1774A-C
OPERATION
ADDITIONAL INFORMATION
The demo circuit 1774A-C is a high linearity fixed gain
amplifier. It is designed for ease of use. Both the input
and output are internally matched to 50Ω single-ended
source and load impedance which is compatible with most
test equipment input and output. Figure 2 is shown demo
board’s S-parameters.
Figure 3 shows the demo circuit’s schematic. It requires
a minimum of passive supporting components. The input
and output DC blocking capacitors (C1 and C3) are re-
quired because this device is internally biased for optimal
operation. The frequency appropriate choke (L1) and the
decoupling capacitors (C21 and C22) provide bias to the
RF output node. Only a single 5V supply is necessary for
the V
CC
pins on the device.
The particular element values shown in the demo board
schematic are chosen for wide bandwidth operation.
Depending on the desired frequency, performance may
be improved by properly selecting one of the supporting
components.
As with any RF device, minimizing ground inductance is
critical. Care should be taken with the board layout because
of these exposed pad packages. The maximum number of
minimum diameter vias holes should be placed underneath
the exposed pad. This will ensure good RF ground and low
thermal impedance. Maximizing the copper ground plane
will also improve heat spreading and low inductance. It is
a good idea to cover the via holes with solder mask on the
back side of the PCB to prevent solder from wicking away
from the critical PCB to exposed pad interface.
The DC1774A-C is a wide bandwidth demo board but
it is not intended for operation down to DC. The lower
frequency cutoff is limited by on-chip matching elements.
The Schematic Diagram section shows the PCB’s schematic
of the amplifier family LTC643X-YY. Hence, the board
can be modified for multiple demo board versions. For
example, both DC1774A-A and DC1774A-B demo boards
have a differential amplifier at U1; therefore, the board
An optional parallel 62pF (C8) and 348Ω (R2) input network
has been added to insure low frequency stability.
Table 2 shows the function of each input and output on
the board.
Table 2. DC1774A-C Board I/O Descriptions
CONNECTOR
FUNCTION
J7 (IN+)
Single-Ended Input. Impedance Matched to 50Ω. Drive
from a 50Ω Network Analyzer or Signal Source.
J10 (OUT+)
Single-Ended Output. Impedance Matched to 50Ω. Drive
from a 50Ω Network Analyzer or Spectrum Analyzer.
E3 or J11 (V
CC
)
Positive Supply Voltage Source.
E6 or J18 (GND) Negative Supply Ground.
is using transformers to transform from differential to
single-ended input and output. Likewise, the DC1774A-C
is a single-ended demo board; consequently, it uses the
LTC6431-15 for single-ended input and output.
Setup and Testing Signal Sources
The LTC6431-15 is an amplifier with high linearity per-
formance; therefore, output intermodulation products are
very low. For this reason, it drives most test equipment
and test setups to their limits. Consequently, accurate
measurement of the third-order intercept point for a low
distortion IC such as the LTC6431-15 requires certain
precautions to be observed in the test setup and testing
procedure.
Setup Signal Source
Figure 8 shows a proposed IP3 test setup. This setup has
low phase noise, good reverse isolation, high dynamic
range, sufficient harmonic filtering and wideband imped-
ance matching. The setup is outlined below:
a. High performance signal generator 1 and 2, the
HP8644A, were used to characterize the LTC6431-15.
These generators have low harmonic distortion and
very low phase noise.
