Some tests have been made using the prototype described in 3radio project - Part 1.
The software uses rtl-sdr library. It runs 3 instances using synchronous transfer. Each stream is recorded in a file as raw bytes. A first graphical analysis and comparison is made using Audacity application.
The application programs all the units to the same frequency. The central frequency is 60.0 MHz because the energy of Pseudo Random Noise Generator (PRNG) sequence is higher at low frequency. The 3radio PRNG prototype has not jet a pulse sharpening circuit.
The PRNG has been programmed to generate at pseudo-random time intervals a sequence of 64 bits at 1Mbps with a good auto-correlation peak. Nevertheless the first measure has been made without any correlation processing, just looking at signal in time with Audacity app.
The picture shows the records made with a sampling rate of 2048 ksps and the gain set to 10dB. All the units are feed by the same PRNG signal. The signals are down-converted and recorded as complex base-band signal I&Q, here shown as stereo audio signals. The time scale is in 10 ms units as Audacity does not accept 2048 kHz sampling and 20.48 kHz is selected instead. The time span is about 500 ms.
The 3 streams are correlated and shown different latency time. I think latency is due to the different recording starting time and serialization over the USB link other than the pll phasing of RTL2832 and R820T.
Lucky the sampling synchronization is kept constant during the measure. So an head synchronization could be maintained for a long time.
Manually phasing the signals and zooming into the 64 bit sequence shows the strong correlation in between the 3 I&Q streams.
A phase rotation in the signals is expected as the PRNG signal is generated by a microprocessor running at 16.0 MHz with a xtal not synchronous to the 28.8MHz clock.
Note: an hardware improvement could be made synchronizing the 16.0 MHz clock of PRNG and the 12.0 MHz clock of HUB with the advantage of decreasing the eterodine signals.