How to optimise digital modulation accuracy in an IQ modulator? Bruce Hemp and Peter Stroet from Linear Technology describe how a vector signal analyser helps optimise EVM performance of the LTC5598 direct quadrature modulator.
EVM, or error vector magnitude, is essentially a scalar measurement of digital modulation accuracy, an important figure of merit for any source of digital modulation. Low modulator EVM is desired because the EVM degrades farther down the line—transmit upconverters, filters, power amplifiers, the communications channel, and the receiver all impair the received signal.
Unless otherwise noted, the following test conditions apply (See Figure 3):
- LTC5598 IQ modulator on Linear Technology demonstration circuit DC1455A.
- LO: 0dBm, f = 450MHz.
- Baseband Modulation: PN9, root raised cosine (RRC) filtering, α = 0.35, symbol rate = 1Msample/s, 16-QAM (four bits per symbol, peak-to-average ratio 5.4dB).
- Baseband drive: VEMF1 = 0.8V differential (1.15VP-P differential). VBIAS = 0.5V.
- VSA measurement filter: RRC, α = 0.35.
- VSA reference filter: Root Cosine (RC).
16-QAM is a relatively common type of digital modulation, readily demonstrating the modulation accuracy attainable with the LTC5598. It is utilised in many wireless communication standards such as LTE/LTE-Advanced, HSDPA, EDGE Evo, CDMA2000 EV-DO, Cognitive Radio IEEE 802.22 (TV white space), PHS, and TETRA.
It is worth noting that VEMF is the differential IQ baseband amplitude, as indicated on the Rohde & Schwarz AMIQ software. Actual I and Q voltage (peak-to-peak differential) measures as shown.