[App_rpt] USB Radio Interface

David Powell-Williams dpowellwilliams at shaw.ca
Fri Feb 15 13:44:35 EST 2008

A general FYI to those contemplating use of the USB FOB interface with COS
generated by hardware, as opposed to dsp:

I found that my receiver did not drop its COS line all the way to ground
potential in the no signal condition.  I presume the Schottky diode (CM108
pin 48) was specified due to its inherently low junction voltage.  However,
in my case, the receiver COS still did not drop low enough to generate

I replaced the Schottky diode with an NPN transistor (collector to CM108 pin
48, emitter to ground, base to the receiver COS source through 10K).  Of
course, this introduces a COS polarity inversion with respect to the CM108.
The inversion is corrected by changing the 'carrierfrom=<usb | usbinvert>'
polarity selection in usbradio.conf.

One further point:
The pin spacing on the CM108 is 0.5 mm. The wire diameter of resistors and
diodes is about 1 mm.  I found it easier to attach 28 AWG wire-wrap wires to
the chip, nail down the wires with hot-melt glue and attach the specified
components to the far ends of the wire-wrap.

Finally, soldering to the CM108 is best done before coffee.


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