Help required to implement a simple AFSK transmiter via the Python API with ADALM PLUTO.

Can you explain more of what the problem is here? This script just sends 1x 0.3 second burst...

-Travis

The problem is that the transmission is not the correct length. If I increase the message length to say 10, 20 or any length, the up translated RF still remains a fraction of a second.

20 etc characters I should say.

Can you provide a plot? Putting this in cycling mode with alittle gap shows the right data length.

-Travis

Many thanks Travis, for the prompt reply. I'll have a look at getting a demodulated plot, but the baseband RTTY is fine. I was expecting a direct frequency translation from baseband without recourse to the cyclic buffer, or delays, as the data rate and sampling rate are so low. How would I deploy the cyclic buffer without affecting the code timings? The code at the moment uses 2 stop bits, but usual practice is 1.5 stop bits, so I have around 90ms (half a bit) for a gap? I'll give it a try.

George 

sorry 11ms not 90ms

I don't have a high speed trigger based scope to measure a single transmission. So I just put it in cyclic mode and put a silence gap (zeros) so I could see the rough side of the data.

import numpy as np
import adi

sample_rate = 1e6 # Hz
center_freq = 1e9 # Hz
N = int(sample_rate/45.45) #22002 number of samples to transmit at once
t = np.arange(N)/sample_rate

sdr = adi.Pluto("ip:192.168.2.1")
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.rx_lo = int(center_freq)
# sdr.tx_hardwaregain_chan0 = 0 # Increase to increase tx power, valid range is -90 to 0 dB
# sdr.tx_buffer_size = 10e6

spaceI = np.int16(np.sin(2.0*np.pi*2125*t) * 2**14)
spaceQ = np.int16(np.cos(2.0*np.pi*2125*t) * 2**14)
spaceIQ = spaceI + 1j*spaceQ

markI = np.int16(np.sin(2.0*np.pi*2295*t) * 2**14)
markQ = np.int16(np.cos(2.0*np.pi*2295*t) * 2**14)
markIQ = markI + 1j*markQ

bit_sequence = [0,0,1,0,1,0,1,1,0,1,0,1,0,1,1,1]
waveform = np.array([], dtype=spaceIQ.dtype)
for bit in bit_sequence:
wave = markIQ if bit else spaceIQ
waveform = np.concatenate((waveform, wave))

# Add zeros at the end to allow for noticeable gap
waveform = np.concatenate((waveform, np.zeros(10000, dtype=waveform.dtype)))

print(f"len of waveform: {len(waveform)} samples")

sdr.tx_cyclic_buffer = True
sdr.tx(waveform)

import time
time.sleep(1) # Give the SDR some time to start transmitting

capture_size = len(waveform)*2
sdr.rx_buffer_size = capture_size
sdr.rx_enabled_channels = [0]

data = sdr.rx()

# plot time domain
import matplotlib.pyplot as plt
plt.figure()
plt.plot(np.real(data), label='I')
plt.plot(np.imag(data), label='Q')
plt.title('Received FSK Signal - Time Domain')
plt.xlabel('Sample Index')
plt.ylabel('Amplitude')
plt.legend()
plt.grid()
plt.show()

Travis, many thanks again for your efforts, however the introduction of a delay and setting the tx_cyclic_buffer does not solve the problem. Monitoring the Pluto output in real time on a spectum analyser confirms that the transmission is not correct or of the predicted length. If you lengthen the bit sequence to say 48 bits, for example, the duration of this should be (48 x 0.177s) 8.5 sec and this is not observed, with cyclic buffer off. (The requirement is to just transmit the mark - space coding without repetition). 

best regards George

48 characters. 

48 characters.