应用笔记 1835

REP039: The MAX2510 IF Transceiver with the MAX5183 DAC


摘要 : RF systems, which require direct coupling between the baseband output digital-to-analog converter (DAC) and the input to the I/Q modulator, present a design challenge. This unique requirement of direct coupling does not exist in many RF systems because the modulation and encoding are contrived to avoid the need for DC response at this point in the system. When the need for very low frequency response exists, a way must be found to route the DAC output signal into the I/Q modulator while providing acceptable RF performance. This application note describes tests carried out on the MAX2510 IF transceiver to measure the impact of DC coupling into the modulator, presents the test results, and proposes a simple circuit to provide the DC interface to the MAX5183 DAC.


Rapid engineering prototypes are real circuits that Maxim application engineers have built and measured in our labs. They can provide a starting point for new RF designs. They are not available as evaluation kits.

Additional Information: The MAX2510 is a highly integrated IF transceiver featuring four operating modes for advanced system power management. Typically, the receiver downconverts a high IF/RF (up to 600MHz) to a low IF (up to 30MHz) using a double-balance mixer. Also included in the receiver function are an IF buffer that can drive an off-chip filter, an on-chip limiting amplifier offering 90dB of receive-signal-strength indication (RSSI), and a robust differential limiter output driver designed to directly drive a CMOS input. The transmitter section upconverts I and Q baseband signals to an IF in the 100MHz to 600MHz range using a quadrature modulator.

The MAX5183 is a dual, 10-bit, 40MHz current/voltage output DAC with simultaneous update. It is designed to provide a high level of signal integrity for the least amount of power dissipation. The device has three modes of operation: normal, low-power standby, and complete shutdown.

Often, customer systems require DC response between the MAX5183 DAC, and the I/Q modulator in the MAX2510. When space is limited, a simple interface between the DAC and the modulator is possible, if the impact of DC offset voltages on the I/Q inputs is managed. This application note presents data showing the impact of modulator I/Q offset voltages on the modulator's RF performance and proposes an interface solution.

Conclusion

The offset voltage at the input of the I/Q modulator impacts the LO suppression. Other modulator performance is only weakly impacted. ~10mV of offset on one modulator input will degrade the LO suppression from -43dBc to -20dBc. Offset voltages of ±20mV on both I and Q inputs degrades the LO suppression to -15dBc.

Bench Test Equipment List
Description
Maker
Model
Serial
Spectrum Analyzer/Mainframe
HP
HP141T
1615A12622
Spectrum Analyzer/RF
HP
HP8555A
1416A03976
Spectrum Analyzer/IF
HP
HP8552B
1437A07067
Signal Generator
HP
HP8640B/01/02/03
1851A09868
Signal Generator
HP
E4432B
US38441362
Oscilloscope
HP
1740A
2026A13971
Power Supply
PowerOne
C5–6 (modified)
N/A
DVM
Fluke
12
51620479

Figure 1. Circuit Modification and Test Set Up.
Figure 1. Circuit Modification and Test Set Up.

Figure 2. 5th-order low pass Butterworth filter.
Figure 2. 5th-order low pass Butterworth filter.

Figure 3. Proposed interface solution.
Figure 3. Proposed interface solution.

Measured Modulator Performance: Vary Just the Q Offset
Q Offset Voltage
(mV)
LO Level
(dBc)
I Offset Set for Minimum
(mv)
SA Ref. Level
LO
VQ pp
VBIAS
VGC
0
-43
2
-13dBm
200MHz, -13dBm
.4V
1.890V
2.003V
-4
-28
2
-
-
-
-
-
-9
-22
2
-
-
-
-
-
-19
-15
2
-
-
-
-
-
6
-22
2
-
-
-
-
-
11
-18
2
-
-
-
-
-
21
-13
2
-
-
-
-
-

Measured Modulator Performance: Vary the I Offset
I Offset Voltage
(mV)
LO Level
(dBc)
Q Offset Set for Minimum
(mv)
0
-44
-1
3
-33
-1
8
-23
-1
13
-18
-1
18
-15
-1
-7
-21
-1
-12
-18
-1
-22
-13
-1

Measured Modulator Performance: Vary Both the I and Q Offset
Q Offset Voltage
(mV)
I Offset Voltage
(mV)
LO Level
(dBc)
Lower Sideband
(dBc)
2nd Harmonic
-3
2
-44
-40
-34dBc
-20
2
-20
-40
-
-20
20
-17
-40
-
20
20
-16
-40
-
20
-20
-15
-40
-32dBc

Test Condition:
  • All tests at room temperature
  • Type of modulation: none, CW used.
  • VCC: 3.3V
  • VBIAS = 1.809V
  • VGC = 2.003V
  • LO = 200MHz, -13dBm