[time-nuts] LTE-Lite module

Bob Camp kb8tq at n1k.org
Sun Oct 19 17:13:24 EDT 2014


> On Oct 19, 2014, at 5:00 PM, Jim Lux <jimlux at earthlink.net> wrote:
> On 10/19/14, 1:08 PM, Bob Camp wrote:
>> Hi
>>> On Oct 19, 2014, at 3:35 PM, Charles Steinmetz
>>> <csteinmetz at yandex.com> wrote:
>>> Bob wrote (alluding also to something Poul-Henning wrote):
>>>> The phase comparison part of the PLL is pretty straightforward if
>>>> you are looking at two RF frequencies. An XOR gate is one
>>>> solution, there are many others. Getting something like 100 to
>>>> 200 ns full scale on the phase comparator makes the rest of the
>>>> gizmo much easier.
>>> All true.  However...
>>>> A 12 bit ADC on a MCU will get you to 100's of ps per bit.  That
>>>> is more resolution (it's < 1 ns) than you need for this.
>>> Getting an ADC to sample fast and accurately enough to provide that
>>> honest resolution is not trivial.  And if you have that, you'll
>>> almost certainly have the resources to do the phase comparator
>>> digitally, too, which brings many advantages -- so I see no reason
>>> to use an analog PC.
>> If you take a look at some of the newer ARM MCU’s they are getting
>> 13+ solid bits out of their ADC’s at a > 10 KHz rate. That’s more
>> than good enough for anything you are trying to do with this design.
>> There’s no need to make it any more complex.
> I'm using the Freescale Kinetix K20 parts, which have 16 bit differential input ADCs, and built in averaging.  The raw ADC can sample at about 400kHz.
> You can easily get 14 bit performance from these at tens of kHz rates.
> I need I/Q, so I sample two inputs at 50 kHz (read one, then the other) without averaging (so they're about 2.5 microseconds apart), and then decimate them through a 2 stage CIC and a 13 tap FIR filter down to 200 Hz.  This takes about 60% of the processor running at 48MHz.

I’m using parts from the same family, but not doing the whole DDS thing. Single input and control loop - the part sleeps about 98% of the time. The demo boards (Freedom boards) are all below $15 and free if you go to one of their (often free) classes. 


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