Transmitter Reference Design for a 900MHz Full-Duplex Radio

Figure 1. Reference design board features the MAX2902 single-chip transmitter, which is designed for use in 868MHz to 915MHz frequency band. The device complies with FCC CFR47 part 15.247 902MHz to 928MHz ISM band specifications

Figure 1. Reference design board features the MAX2902 single-chip transmitter, which is designed for use in 868MHz to 915MHz frequency band. The device complies with F_CC CFR47 part 15.247 902MHz to 928MHz ISM band specifications

Figure 2. Reference design block diagram for the MAX2902 900MHz BPSK transmitter. Shows characterization report (v1b), block diagram, and interface connects. Dedicated L_DO for Tx IC VCO and TCXO is to mitigate coupling from PA. The second L_DO output supplies the synthesizer and the Tx IC (all but the PA). The transmitter's PA is supplied directly by the system (V_{CC_MAIN}). An additional PA is required to get > 18dBm output power while meeting 20% EVM requirement at 8:1 VSWR. The 40MHz WLAN TCXO was used as a temporary replacement for the intended 16.36MHz GPS reference originally specified. The P_LL can accommodate any common reference frequency from 1MHz to 50MHz. The customer should select their TCXO based on cost and tolerance for phase noise of the unmodulated carrier.

Lab Measurements

System Performance Summary

Measured at ANT RF SMA; V_CC = 3.3V; TA = +25°C; REF-OSC Freq = 40MHz; P_LL locked at 900MHz; L_DOEN = high; CSB = high; SCLK = D_IN = Low; 2.5V_P-P 1.0Mbps PN bit stream (unfiltered) applied to BPSK input, unless otherwise noted.

Parameter	Conditions and Comments			Target	Measured	Units
>Supply Current	Shutdown mode (L_DOEN = L_OW), V_CC = 4.5V (worst case)				1.7	µA
>Supply Current	For +17.5dBm at LPF output (V_ASK = 2.1) Includes LMX2353 synthesizer and TCXO supply current.			141	104	mA
Tuning Range	Range of V_CO, freq determined by P_LL			902 to 928	902 to 928	MHz
LO Integrated Phase Error	In 2MHz BW of demodulator, no modulation				1.6	°RMS
Phase EVM	EVM contribution from the above RMS phase error (for small angles, approximate EVM by Φ_ERR/(360/2pi)				2.8	%
Magnitude EVM	No modulation				0.2	%
Magnitude EVM	1.0Mbps PN sequence BPSK modulation				13 to 14	%
Full-Power EVM	V_ASK = 2.1V, 8:1 VSWR (worst-case angle) Dominated by distortion due to VCO pulling	V_{CC_MAIN} = 2.7V, I_CC = 106mA (50Ω pwr = +16.9dBm)		20	40.2	%
		V_{CC_MAIN} = 3.3V, I_CC = 114mA (50Ω pwr = +19.2dBm)		20	43.6
		V_{CC_MAIN} = 4.0V, I_CC = 138mA (50Ω pwr = +20.6dBm)		20	53.3
		V_{CC_MAIN} = 4.5V, I_CC = 299mA (50Ω pwr = +21.3dBm)		20	30.1
Output Power	50Ω load, 1.0Mbps BPSK modulation (Power at balun output is approx 1dB higher, due to BPF, T-line, SMA) EVM for specified VSWR (worst-case phase angle) is less than 20%	V_ASK = 2.1V (for 1:1 VSWR tolerance)	V_{CC_MAIN} = 2.7V, I_CC = 103mA		16.9	dBm
			V_{CC_MAIN} = 3.3V, I_CC = 133mA	17.5	19.2
			V_{CC_MAIN} = 4.0V, I_CC = 141mA		20.6
			V_{CC_MAIN} = 4.5V, I_CC = 145mA		21.3
		V_ASK = 1.7V (for 4:1 VSWR tolerance)	V_{CC_MAIN} = 2.7V, I_CC = 103mA		16.7
			V_{CC_MAIN} = 3.3V, I_CC = 108mA		18.2
			V_{CC_MAIN} = 4.0V, I_CC = 112mA		19.0
			V_{CC_MAIN} = 4.5V, I_CC = 114mA		19.4
		V_ASK = 1.6V (for 8:1 VSWR tolerance)	V_{CC_MAIN} = 2.7V, I_CC = 95mA		16.2
			V_{CC_MAIN} = 3.3V, I_CC = 99mA		17.2
			V_{CC_MAIN} = 4.0V, I_CC = 102mA		17.8
			V_{CC_MAIN} = 4.5V, I_CC = 104mA		18.1
Reference Spurs	f_COMP = 2MHz, worst-case condition: V_CC = 4.5V, +21.3dBm output power Actual measurement was < -63dBc (below noise floor of VSA)			< -41	< -42	dBm/MHz
Output Harmonics	2^nd harmonic levels, at LPF output (limit is -27dBm/MHz in nonrestricted bands)			< -30	-42	dBm/MHz
Output Harmonics	3^rd harmonic levels, at LPF output (limit is -41dBm/MHz in restricted bands)			< -49	-54	dBm/MHz

Measurement Data

Figure 3. Output power and supply current at various supply voltages as a function of PA gain (V_ASK). Note that the reference design incorporates a 900MHz BPF (approx. 0.6dB IL) at Tx output. Reference design and EV kit provide almost identical output power at the PA. Supply current for the reference design is less than 10mA higher than the EV kit at full power. This is due to the increased bias to the PA that decreases the VCO pulling.

Figure 4. Output power and EVM at various VSWRs, as a function of PA gain (V_ASK). Recommended operating points reflect V_ASK levels to keep EVM below 20% at a desired max VSWR (worst-case phase). Reference design allows a higher nominal output power for same EVM.

Figure 5. Output power and EVM at various VSWRs as a function of V_CC. V_ASK is optimized in the design according to the worst-case VSWR the system can tolerate. Lowering V_ASK drops P_OUT, but keeps EVM in check for increasing VSWR. Use the following V_ASK settings to limit EVM to 20%:

Design for 1:1 VSWR tolerance ≥ 2.1V

Design for 4:1 VSWR tolerance ≥ 1.7V

Design for 8:1 VSWR tolerance ≥ 1.6V

Detailed Description

The MAX2902 is single-chip transmitter designed for use in 868MHz to 915MHz frequency bands. It complies with FCC CFR47 part 15.247 902MHz to 928MHz ISM band specifications.

The transmitter IC offers a high level of integration while minimizing external components. The transmit modulator, power amplifier, RF VCO, 8-channel frequency synthesizer, and baseband PN sequence lowpass filter are all integrated. By filtering BPSK modulation, the spurious emissions are reduced, enabling up to 8 independent transmit channels in the U.S. ISM band. Inputs are provided for spread-spectrum BPSK, ASK, and OOK. FM can be achieved by directly modulating the VCO. The device is intended primarily for use with external differential antenna.

In this reference design the MAX2902 is applied as the transmit IC in the 900MHz band. It is controlled by a National Semiconductor LMX2353 fractional-N P_LL.