**Category:**Software

Hello I'm trying to transmit and receive (via loopback with a BNC cable) QPSK symbols on channel 0 of the FSCOMMS5 . Unfortunately the received QPSK symbols seem to have 4 levels of amplitude as opposed to 2 (-1,1) for in-phase and quadrature. In the attached image, figures 0 & 1 plot the samples of transmitted in-phase and quadrature QPSK samples respectively. Whereas figures 2 & 3 plot the received samples of in-phase and quadrature QPSK samples respectively. I understand that a phase mismatch is expected, what I don't understand is why the received stream has 4 different amplitude levels when only two were expected.

I'm including the code below. It's basically a slightly modified version of the PlutoSDR transmit and receive example on PySDR.org

import numpy as np import adi import matplotlib.pyplot as plt sample_rate = 1e6 # Hz center_freq = 915e6 # Hz num_samps = 500000 # number of samples per call to rx() sdr = adi.FMComms5() sdr.tx_enabled_channels = [0] sdr.rx_enabled_channels = [0] sdr.sample_rate = int(sample_rate) # Config Tx sdr.tx_rf_bandwidth = int(sample_rate) # filter cutoff, just set it to the same as sample rate sdr.tx_lo = int(center_freq) sdr.tx_hardwaregain_chan0 = 0 # Increase to increase tx power, valid range is -90 to 0 dB # Config Rx sdr.rx_lo = int(center_freq) sdr.rx_rf_bandwidth = int(sample_rate) sdr.rx_buffer_size = num_samps sdr.gain_control_mode_chan0 = 'manual' sdr.rx_hardwaregain_chan0 = 10.0 # dB, increase to increase the receive gain, but be careful not to saturate the ADC # Create transmit waveform (QPSK, 16 samples per symbol) num_symbols = 1000 x_int = np.random.randint(0, 4, num_symbols) # 0 to 3 x_degrees = x_int*360/4.0 + 45 # 45, 135, 225, 315 degrees x_radians = x_degrees*np.pi/180.0 # sin() and cos() takes in radians x_symbols = np.cos(x_radians) + 1j*np.sin(x_radians) # this produces our QPSK complex symbols samples = np.repeat(x_symbols, 16) # 16 samples per symbol (rectangular pulses) samples *= 2**14 # The PlutoSDR expects samples to be between -2^14 and +2^14, not -1 and +1 like some SDRs plt.figure(0) plt.plot(np.real(samples[0:1000])) plt.figure(1) plt.plot(np.imag(samples[0:1000])) # Start the transmitter sdr.tx_cyclic_buffer = True # Enable cyclic buffers sdr.tx(samples) # start transmitting # Clear buffer just to be safe for i in range (0, 10): raw_data = sdr.rx() # Receive samples rx_samples = sdr.rx() print(rx_samples) # Stop transmitting sdr.tx_destroy_buffer() # Calculate power spectral density (frequency domain version of signal) psd = np.abs(np.fft.fftshift(np.fft.fft(rx_samples)))**2 psd_dB = 10*np.log10(psd) f = np.linspace(sample_rate/-2, sample_rate/2, len(psd)) # Plot time domain plt.figure(2) plt.plot(np.real(rx_samples[:1000])) plt.figure(3) plt.plot(np.imag(rx_samples[:1000])) plt.xlabel("Time") # Plot freq domain plt.figure(4) plt.plot(f/1e6, psd_dB) plt.xlabel("Frequency [MHz]") plt.ylabel("PSD") plt.show()

The figures are also attached.

I'd appreciate any pointers as to why the received and transmitted data has a different number of amplitude levels.