By Richard Wilson 16th December 2015

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.

·         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 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.

Figure 1. Typical 450MHz EVM measurement of 0.34% RMS.

Figure 1. Typical 450MHz EVM measurement of 0.34% RMS.

A typical EVM measurement at LO = 450MHz is shown in Figure 1, demonstrating an LTC5598 EVM of 0.34% rms, and 0.9% peak.

After the harmonic filter, output power measures +0.4dBm for the same signal. By comparison, a lab-grade signal generator with the same amplitude, frequency, and digital modulation measures 0.28% rms and 0.8% peak, on the same VSA setup. This indicates that the LTC5598 modulation accuracy is nearly as good as the test equipment used to measure it.

EVM vs IQ drive level

·         16-QAM, 1Msample/s, RRC, raised cosine, α = 0.35 (peak-to-average ratio 5.4dB).

·         VBIAS = 0.5V DC. LO = 0dBm.

Figure 4 shows EVM increasing rapidly when the baseband inputs drive the modulator output signal peaks into compression. Even without a VSA to measure EVM, this level of maximum rms output power can be estimated by:

+8.4dBm         LTC5598 Output P1dB (Typ. at f = 450MHz)

− 5.4dB           Crest Factor of 16-QAM Test Waveform

= +3.0dBm     Average Output Power (Peaks Will Be in 1dB Compression)

Figure 4. EVM and RMS Output Power vs IQ Drive Level

Figure 4. EVM and RMS Output Power vs IQ Drive Level

This is a rough estimate. For more complex modulation schemes, even 1dB compression may be excessive, and at the same time, the crest factor will be higher, significantly dropping the average output power that becomes available for highly complex waveforms.

The same test conditions are used:

16-QAM, 1Msample/s, RRC, raised cosine, α = 0.35 (peak-to-average ratio 5.4dB). VEMF = 0.8V differential (1.15VP-P differential), VBIAS = 0.5V.

Figure 5 illustrates how the LTC5598 modulation accuracy is affected near the ends of the IQ modulator frequency range specification. EVM is lowest at midband frequencies from 30MHz to 700MHz. At LO frequency below 30MHz, EVM is reduced with stronger LO drive (consult the data sheet).

At both LO frequency extremes, the main contributor to LTC5598 EVM is quadrature phase error. Some IQ gain imbalance is also present, but generally not much of a contributor to overall EVM. Where necessary, either or both of these error terms can be corrected open-loop in baseband, or in some transmit chains as part of an existing closed-loop PA predistortion correction system.

Slightly higher EVM may be perfectly acceptable in some systems, for example when simple, low-order digital modulation schemes are used.

The LTC5598 provides excellent digital modulation accuracy across many popular VHF and UHF communications bands. In some cases, EVM is comparable to that of a lab-grade signal generator. Where desired or necessary, baseband correction of quadrature phase or gain may be implemented for enhanced accuracy.

Figure 2. Basic Principle of the VSA.

Figure 2. Basic Principle of the VSA.

Modulation accuracy is often measured with a vector signal analyser (VSA). See Figure 2. In brief, the VSA functions as follows:

1.      It downconverts and digitises the input signal at a specified centre frequency over a given bandwidth. The modulation scheme, symbol rate, measurement filter, etc. are user selected. This data becomes known as the measured signal.

2.      It digitally demodulates the measured signal to recover the source digital data stream.

3.      Based on the recovered source data, modulation scheme, etc., the VSA mathematically generates an ideal reference signal.

4.      It calculates error vectors by determining the difference between the measured and reference data vectors, and normalizing to the peak signal level. From the set of error vectors, the rms and peak EVM scalar values are extracted.

Tips for Lowest IQ Modulator EVM

Use “clean” IQ baseband source:

·         IQ DAC clock should be low phase noise and jitter.

·         Be sure DAC reconstruction filter does not encroach upon the baseband bandwidth.

·         Be sure the baseband IQ signal paths have sufficiently flat frequency response.

·         Use a “clean” LO signal source:

·         LO phase noise adds random phase error, increasing EVM. This type of error cannot be later removed.

·         LO harmonics will give rise to quadrature phase error. For best results, adhere to the modulator data sheet recommendation regarding LO harmonic content.

Figure 5. LTC5598 EVM vs LO Frequency.

Figure 5. LTC5598 EVM vs LO Frequency.

Tagged with: Linear Technology Corporation RF Rohde & Schwarz

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