Greetings all,
The PIC-based GPSDO I am currently building (using a PIC16F1783, but
maybe in the end it will turn out to be a dsPIC30F3013) is happily
synchronizing the surplus 10 MHz VCOCXO for over a week now, without a
single hiccup.
So far, so good... I'm quite happy with the very low phase noise from
the control loop (which uses two integrator to bring back the phase
error to zero, should there ever be a 'hiccup' which has not happened yet).
The next plan is to use the PIC's internal 12-bit ADC as a replacement
for the soundcard, and possibly the PIC's internal PSMC as a simple
downconverter ( programmable switch mode controller, can supply 40 MHz /
N or 20 MHz / (N + M/16) with the fractional frequency adjustment ).
To keep it simple, without an extra DDS, frequencies which the GPSDO can
produce 'directly' (using the PSMC) are for example:
476190.47619048 Hz
475498.33887043 Hz
474806.20155039 Hz
474114.06423034 Hz
473421.92691030 Hz
472729.78959025 Hz
472037.65227021 Hz
(for the curious reader, the above frequenies are taken from the 10
MHz OCXO, multiplied by the PIC's internal PLL to 40 MHz, then divided
down by the PSMC as described in the PIC16F1782/3 datasheet,
DS40001579E, on page 220. One could even use the PSMC as a crude BPSK
modulator, but that's off topic for the moment).
Since I don't want to develop an own USB driver for the digitized input,
the interface shall use the UART (RS232 or RS422, this is up to the
re-builder). The question is which serial baudrates (bits per second)
are supported by the common "RS232" / USB adapters. They all support
115200 bit/sec but that's a bit low (if most of the digital
downconversion shall be performed by the PC, due to the stepwidth of the
PSMC output), so higher serial baudrates for the transmission of analog
samples (from PIC to PC) are desirable. The question is WHICH higher
baudrates are available, and is there a simple way on a PC (Linux or
Windows, shouldn't matter).
Must these all be multiples of 115200 ? So far I could only test it with
the already mentioned 'Prolific' adapter, but I can see no way how to
enumerate the baudrates which the adapter *really* supports.
Trial-and-error experiments with various bitrates only produced garbage
data. The rise and fall times at the RS232-TXD output are about 800 ns
which is surprisingly steep so I guess the level converter itself cannot
be the problem. Any ideas / recommendations ?
Buy a 'good' FTDI adapter (instead of the Prolific), and if so, which
one exactly ?
If you have one of those, can you switch it to 1 MBit/second, hook it up
to a scope, send a few characters from a terminal program and check the
bit timing + rise & fall times ?
73, Wolf DL4YHF .
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