CN-0248 Datasheet, PDF (3/6 Pages) Analog Devices – An IQ Demodulator-Based IF-to-Baseband Receiver with IF and Baseband Variable Gain and Programmable Baseband Filtering

Circuit Note
CN-0248
Measurement Results
0
A 4-QAM, 5 MSPS modulated signal was applied to the input of
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the ADL5336. For more information on the test setup, see the
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Circuit Evaluation and Test section.
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EVM is a measure of the quality of the performance of a digital
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transmitter or receiver and is a measure of the deviation of the
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actual constellation points from their ideal locations, due to
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both magnitude and phase errors. This is shown in Figure 2.
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Q
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MAGNITUDE ERROR
(I/Q ERROR PHASE)
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VGA1 = 88mV rms, VGA2 = 88mV rms
VGA1 = 88mV rms, VGA2 = 707mV rms
VGA1 = 250mV rms, VGA2 = 250mV rms
VGA1 = 707mV rms, VGA2 = 88mV rms
VGA1 = 707mV rms, VGA2 = 707mV rms
VGA1 = 88mV rms, VGA2 = 250mV rms
VGA1 = 250mV rms, VGA2 = 88mV rms
VGA1 = 250mV rms, VGA2 = 707mV rms
VGA1 = 707mV rms, VGA2 = 250mV rms
MEASURED
SIGNAL
ERROR
VECTOR
PHASE ERROR
(I/Q ERROR PHASE)
0
IDEAL SIGNAL
(REFERENCE)
I
Figure 2. EVM Plot
Figure 3 shows the system EVM vs. the input power to the
ADL5336 while the maximum gains on the VGAs are set to
15.2 dB and 19.5 dB for VGA1 and VGA2, respectively. Several
AGC setpoint combinations were tested. Figure 4 is also system
EVM vs. the input power to the ADL5336; however, the gain of
the VGA’s was set 9.7 dB and 13.4 dB, respectively. The same
AGC setpoint combinations were tested.
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VGA1 = 88mV rms, VGA2 = 88mV rms
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VGA1 = 88mV rms, VGA2 = 707mV rms
VGA1 = 250mV rms, VGA2 = 250mV rms
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VGA1 = 707mV rms, VGA2 = 88mV rms
VGA1 = 707mV rms, VGA2 = 707mV rms
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VGA1 = 88mV rms, VGA2 = 250mV rms
VGA1 = 250mV rms, VGA2 = 88mV rms
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VGA1 = 250mV rms, VGA2 = 707mV rms
VGA1 = 707mV rms, VGA2 = 250mV rms
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PIN (dBm)
Figure 4. System EVM, Digital VGA Gains = 00
Figure 3 and Figure 4 illustrate the importance of keeping the
signal levels applied to the ADRF6510 low enough not to
compress the input stage and/or filter. At the highest of AGC
setpoints (500 mV rms and 707 mV rms), the input of the
ADL5387 IQ demodulator is starting to compress and adds
additional degradation to the EVM. The best EVM is achieved
when the AGC setpoints are at their lowest (88 mV rms). EVM is
already beginning to degrade when the setpoints are 250 mV rms.
Figure 5 compares the EVM between the minimum and the
maximum digital gain settings (both VGAs were set to either a
gain code of 11 or a gain code of 00) on the ADL5336 VGAs when
VGA1 and VGA2 setpoints are 250 mV rms and 88 mV rms,
respectively.
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GAIN = 11
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GAIN = 00
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–95 –85 –75 –65 –55 –45 –35 –25 –15 –5 5 15 25 35
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PIN (dBm)
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–95 –85 –75 –65 –55 –45 –35 –25 –15 –5 5 15 25 35
PIN (dBm)
Figure 3. System EVM, Digital VGA Gains = 11
Figure 5. System EVM with VGA1 Setpoint = 250 mV rms and
VGA2 Setpoint = 88 mV rms
For the given AGC setpoints, when the maximum gain code
was 11, the handoff from VGA2 to VGA1 occurs after VGA2
runs out of the gain range; therefore, the signal level being applied
to the ADRF6510 continues to increase (and degrade EVM) until
VGA1 reaches its setpoint. Once VGA1 reaches its own setpoint,
the EVM levels off again; therefore, the signal level being applied to
the ADRF6510 does not change until VGA1 runs out of the gain
range at about 5 dBm of input power. When the maximum gain
code was set to 00, both VGAs have more attenuation available,
thus allowing VGA2 to shift its dynamic range such that it does not
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