Analysis of working state and performance of BPSK modulator under different LO frequencies

Binary Phase Shift Keying (BPSK), also known as biphasic modulation, is a widely used and straightforward digital modulation technique. One of its key advantages is that the symbol constellation points are maximally separated, making it particularly effective for weak signal communications. Additionally, BPSK is favored for its compatibility with spread spectrum systems due to its simplicity and robustness. As a result, BPSK finds applications in various areas such as weak signal communication, spread spectrum transmission, ranging, and radar systems [1]. The LTC5548 is essentially a passive, double-balanced mixer with a DC-coupled IF port. When used as a BPSK modulator (as shown in Figure 1), it does not perform frequency conversion, meaning the modulator's operational frequency range is limited by the capabilities of both the LO and RF ports. Figure 2 illustrates the test setup for the BPSK modulator. A high-quality vector signal generator with a differential output is used to produce the baseband waveform.

Figure 1. RF mixer resistors R1 and R2 are used as BPSK modulators, each with an IF input of 50Ω, suitable for modern laboratory test equipment.

Figure 2. Test setup for the BPSK modulator

Modulator IF Input Signal

The baseband drive level for the LTC5548’s DC-coupled IF input must meet specific requirements:

- The drive must be differential (balanced) with a common-mode voltage of 0.0V.

- The typical drive level for each IF pin is ±0.1V continuous (0.2V peak-to-peak).

- The peak drive level should not exceed ±0.2V (0.4V pp).

- The absolute maximum rating for each IF pin is ±0.3V. However, exceeding this can lead to significant spectral distortion at the RF output.

In most applications, low LO leakage is essential. This means the DC offset on the IF input pins should be close to zero volts. Although DC offset adjustment can help reduce LO leakage, it cannot completely eliminate it due to phase differences in the LO signal.

Figure 3 shows the voltages at the IF+ and IF- pins that form the differential IF input signal. The test circuit is based on the configuration in Figure 1. The signal is a balanced waveform centered around 0V and meets all the specified drive level criteria.

Figure 3. Typical modulator drive waveform measured at the IF+ and IF- input pins. Symbol rate = data rate = 5 Mbps.

5.08MM Pitch Series

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