I have been playing around with the cool RTL dongles (more on rtl-sdr dongles on superkuh’s web page or rtl-sdr.com) that you can buy on e-bay for about US$8 (including shipping). These are very capable 8-bit digital receivers that have up to 2.4 MHz bandwidth and can tune anywhere between 24 MHz and 1850 MHz
I recently came up with a trivial hack to build a receiver with multiple coherent channels using the RTL dongles. I do this basically by unsoldering the quartz clock on the slave units and cable the clock from the master RTL dongle to the input of the buffer amplifier (Xtal_in) in the slave units (I’ve attached some pictures).
I originally drove the master crystal with both dongles, which also worked. However, Ian Buckley pointed out to me that a more typical way of doing this is feeding the signal into Xtal_In (in the pictures below). So I tried that too, and it also worked. I’m still not sure what the optimal setup is, as there is no schema for the dongle, but both methods I’ve tried so far have worked in practice.
|This is how you make a dual coherent channel digital receiver with $16. The clock drive probably won’t be enough for many of these, but this can be fixed with a buffer or some other active splitter.|
|The oscillator is wired using a piece of 75 Ohm antenna coax that came with the dongle. It’s like they designed the dongle for multi-channel coherent applications.|
This has some implications for low cost geophysical instruments. It will be possible to use this receiver for the 150/400 MHz beacon satellite receiver, as this only requires that the receivers have clocks that are locked with each other. Interferometry and passive radar are other application examples. With more than two locked channels, applications such as imaging start to become possible.
I’ve made some relative phase noise measurements, and the systems don’t have detectable sample drift over two hours, and their relative phase is also pretty stable.
|Spectrum at 1 Hz sample rate of the relative z_1/z_2 phase signal going into two receivers.|
|IQ plot of the z_1/z_2 relative phase signal over ~6000 seconds at 1 Hz sample rate.|
And oh, by the way, I found this nice usb hub, which I’m going to use to hopefully get a 7 channel coherent rtl system.
|Hub with the right usb port orientation for rtl dongles.|
Stay tuned for more results. I already have some pretty nice passive radar results using the system, which I’ll be posting in a few days.
Update: Apparently three dongles will also run fine from one master clock. I know the clock isn’t split correctly, but adding any components would increase the total cost and the whole point of this exercise is to determine what the lower bound is for software defined radios.
|Three channel coherent RTLSDR receiver.|