Hi Tobias,
Thanks for the information. 12 Mbaud would be fine for a 'real' SDR
already, and far more than a PIC with 40 MHz oscillator frequency ( ->
10 MHz 'cycle' clock) needs. To keep the project homebrew-friendly the
plan is to use an off-the-shelf adapter (like the ones FTDI call 'USB
RS232 Cables' even though they are much more than just a cable..).
The 'Prolific' seems unable to read his own transmitted signal at the
higher bitrates. I tried it with my own software (WinPic, which contains
a terminal emulator for debugging), where I can enter *any* numeric
baudrate. But windows (or the driver) never complains, so there is no
easy way to check it.
In the meantime, 460800 bits/second seem to work, which should be
sufficient for approx. 22 kSamples with one channel, and 16 (averaged)
bits per sample. Not a replacement for a HiFi soundcard (and not "I/Q"
at all), but with a similar principle as recently suggested by G4JNT
(using only one channel but a special sampling sequenze) may work just
as well.
I will find out soon what the PIC16F's max ADC sampling rate of 75
kS/sec can do.. and if the XC8 compiler permits a bit of signal
pre-processing (or if I really need to switch to a dsPIC. I hope not..)
A block diagram of the GPSDO (as it looks like at this very moment)
is at http://www.qsl.net/dl4yhf/gpsdo/gpsdo_pic_main.c .
Should anyone here consider building the GPSDO: please be patient - the
circuit (contained in the sourcecode as 'ascii art') may be subject to
change. I may even decide to replace the PIC16F1783 by a dsPIC30F3013,
which unfortunately has an incompatible pinout (despite the same SDIP-28
footprint).
73,
Wolf DL4YHF .
Am 12.02.2016 um 17:08 schrieb Tobias DG3LV:
Hi Wolf !
Please have a look at :
http://www.ftdichip.com/Products/ICs/FT4232H.htm
it says : "RS232/RS422/RS485 UART Transfer Data Rate up to 12Mbaud.
(RS232 Data Rate limited by external level shifter)."
The datasheet
http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT4232H.pdf claims
:
"Data Transfer rate: The FT4232H supports a data transfer rate up to
12 Mbit/s when configured as an RS232/RS422/RS485 UART interface.
Please note the FT4232H does not support the baud rates of 7 Mbaud 9
Mbaud, 10 Mbaud and 11 Mbaud."
and :
"The Baud Rate Generators provides a x16 or a x10 clock input
to the UART’s from a 120MHz reference clock and consists of a 14 bit
pre-scaler and 4 register bits which provide fine tuning of the baud
rate (used to divide by a number plus a fraction). This determines the
Baud Rate of the UART which is programmable from 183 baud to 12
million baud."
If you can omit the (RS232-) level shifter and just use the 5 Volt
TTL-Level in/output directly, then higher baudrates than 115200 should
be possible at not too long cable-distances. (if possible, use
RS422/RS485 differential signalling) At the QRL we did tests with this
chip at up to 4 MBit some time ago with some success using the VCP
driver. (There may be other chips from FTDI having a similar
performance, we just selected this one.)
The FT4232H chip is, too, available as a (not-too-expensive, €28.71)
mini-module at their webshop. (The more prominent chips from FTDI may
be a target to fakes at the "usual market platforms")
The Windows(/Linux)-program on the PC has to be able to configure
these baudrates. Standard Terminal-programs will support up to 115200
Baud. But self-created programs could go well beyond this (using the
WIN32 -API).
73 de dg3lv Tobias
Am 12.02.2016 um 15:35 schrieb Wolfgang Büscher:
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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