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 simple, popular digital modulation scheme. The symbol constellations are as far apart as possible, which is desirable for weak signal operation. BPSK is popular for its relatively simple spread spectrum capability. Therefore, BPSK can be applied to weak signal communication, spread spectrum, ranging and radar systems [1].

The LTC5548 is basically a passive double balanced mixer with a DC-coupled IF port. As a BPSK modulator (Figure 1), the mixer does not provide frequency conversion, so the modulator frequency range is limited to the range of frequencies that both the LO and RF ports can handle. Figure 2 shows the test setup for the BPSK modulator. A lab-level vector signal generator with a differential output will generate a baseband waveform.

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

Figure 2. Test setup for the BPSK modulator

Modulator IF input signal

The baseband drive level of the LTC5548 DC-coupled IF input should meet the following criteria:

The drive should always be differential (balanced) with a 0.0V common mode.

The typical drive level for each IF pin can be ±0.1V continuous (0.2V pp).

The drive level of each IF pin should not exceed ±0.2V at the peak signal (0.4V pp).

The drive level of each IF pin must not exceed ±0.3V absolute maximum rating. Moreover, such large input signals typically produce unacceptably high spectral reproduction at the RF output.

For most applications, low LO leakage is required, which means that the dc offset voltage on the IF input pin should be close to zero volts. Typically, the LO leakage is in a phase that cannot be completely eliminated with DC offset adjustment. Therefore, using DC offset adjustment can reduce LO leakage, but it does not eliminate LO leakage.

Figure 3 shows the IF+ and IF-pin voltages that make up the differential IF input signal. The test circuit is shown in Figure 1. Note that the signal is a differential signal centered around approximately zero volts and meets the drive level criteria listed above.

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