CN-0134 Datasheet, PDF (4/5 Pages) Analog Devices – Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter

CN-0134
Circuit Note
Table 3. Single-Carrier W-CDMA Composite EVM Results Comparing Filter vs. No Filter on ADF4350 RF Outputs
(Measured As Per 3GPP Specification Test Model 4)
Frequency (MHz)
2140
1800
900
Composite EVM No LO
Filtering
3.50%
3.40%
3.30%
Composite EVM with LO Filtering,
Filter C
1.80%
1.50%
0.90%
Modulator Output Power
(dBm)
−7
−7
−7
Error vector magnitude (EVM) is a measure of the quality of
the performance of a digital transmitter or receiver and is a
measure of the deviation of the actual constellation points from
their ideal locations, due to both magnitude and phase errors.
This is shown in Figure 5.
EVM measurements are given in Table 3 comparing results with
and without the filter. In this case the baseband I/Q signals were
generated using 3GPP test model 4 using a Rhode and Schwarz
AMIQ I/Q Modulation Generator with differential I and Q
analog outputs. Filter b was also used. A block diagram of the
test setup for EVM is given in Figure 6.
Adjacent channel leakage ratio (ACLR) is a measure of the
power in adjacent channels relative to the main channel power
and is specified in dBc.
The LO phase noise and the linearity of the modulator are the
main contributors to ACLR. The ACLR test setup is the same
as for EVM with the exception that coaxial filters were placed
on the I/Q outputs of the signal generator to reduce aliasing
products.
R&S AMIQ GEN.
I+ I– Q+ Q–
CN-0134
EVALUATION
BOARD
5.5V
SPECTRUM ANALYZER
[R&S FSQ 8]
RF OUT
POWER SUPPLY
Figure 6. EVM Measurement Setup (Simplified Diagram)
In addition to the improvement in sideband suppression and
EVM, there is also a performance benefit to driving the
ADL5375 LO inputs differentially. This improves modulator
OIP2 performance by 2 dB to 5 dB, compared with single-
ended LO drive. Note that most external VCOs only come with
a single-ended output, so using the differential outputs on the
ADF4350 provides a benefit over an external VCO in this case.
Figure 7 shows sideband suppression results using an 850 MHz
to 2450 MHz filter (filter b).
10
SSB #10 +5dBm
SSB #10 +2dBm
0
SSB #10 –1dBm
SSB #10 –4dBm
–10
–20
–30
–40
–50
–60
0
500 1000 1500 2000 2500 3000 3500
FREQUENCY (MHz)
Figure 7. Sideband Suppression Results for 850 MHz to 2450 MHz Filter b
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0134-DesignSupport.
COMMON VARIATIONS
It is possible to use the auxiliary outputs on the ADF4350 to
switch between two filter types where wideband operation
beyond that possible with one single filter is required. This
is shown in Figure 8. An RF double-pole, 4-throw switch
(DP4T) is used to select the differential outputs of either
Filter 1 or Filter 2.
Rev. B | Page 4 of 5