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.

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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 2. 5th-order low pass Butterworth filter.


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.
• V_CC: 3.3V
• V_BIAS = 1.809V
• VGC = 2.003V
• LO = 200MHz, -13dBm