To: rsgb_lf_group@blacksheep.org, SpectrumLabUsers@yahoogroups.com
From: =?UTF-8?Q?Wolfgang_B=c3=bcscher?= <dl4yhf@freenet.de>
Message-ID: <56BDED97.5060308@freenet.de>
Date: Fri, 12 Feb 2016 15:35:03 +0100
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101
 Thunderbird/38.5.1
MIME-Version: 1.0
Subject: LF: PIC-based GPSDO with A/D converter / supported serial baudrates ?
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Status: RO

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 .