The use of multiple RTL-SDR dongles as coherent receiver has been proposed since 2013.
In http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html Juha Vierinen posted a two units hack.
Many others can be found Googling “rtl-sdr coherent receiver”

This pages describe my own project of a SDR coherent receiver named 3radio

The first goal is a setup for coherence measure without sophisticate instruments.

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                                      Block diagram rev 1.0

It is a compact device that uses a single USB cable to acquire 3 SDR sample streams.

Thermal cooling is important to reach reliable performance.

The unit is housed into an aluminum box with the rtl-sdr screwed to the box wall.

 

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                           Temperature range from 25°C to 85°C

The original MCX connector, the IR receiver and the led have been removed. Please keep the solder dots as flat as possible on the solder side.

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The rtl-sdr PCB has a 2.0 mm hole that can be used to fix the unit near the led position. A second 2.0 mm diameter hole is drilled in the position indicated in the picture.
Two 2.0 mm screws will rest the board against the aluminum wall. An electrical insulated thermal pad is placed in between the board and the aluminum.
90° SMA Female connectors has been used in the prototype.

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          Preliminary test wiring with a xtal oscillator feeding the other units.

As the TCXO 28.8MHz is not jet arrived in the mail, one of the original xtal oscillators has been used to drive the other two ones using a 14 pF capacitors (2*6.8pF in parallel).

The USB connectors has been replaced by wires to place the boards against the aluminum near the HUB PCB.

The red wire that goes to the antenna diode is carrying the pseudo random noise PRNG signal to the antenna inputs via a BAV99 diode gate.

The HUB  is a FE1.1s chip while the PRNG is a AT90USB162 wired to the 4th port of the HUB.

The plan is to use the PRNG sequence to synchronize the received streams during an initial calibration phase or every time the USB samples streams are stopped.

At the beginning the generator stream is used to measure some coherence figure...

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                                                                   early prototype

 3radio - Part2