Forums

BPSK Demodulation with LPC214X

Started by radim100 April 27, 2007
Hi,
Did anybody have some experience with demodulating BPSK signal with
LPC213X or LPC214X MCU. Our telemetry is receiving BPSK coded data
from sensor with approx 2400 baud ( bit/sec ) rates . There is 100ms
burst of data and 2.9 sec gap etc. Sensor is DC powered ( 12-20VDc )
with BPSK sinewaves signal ( 0.5V ) modulated on DC .
Thanks Radim.

An Engineer's Guide to the LPC2100 Series

--- In l..., "radim100" wrote:
>
> Hi,
> Did anybody have some experience with demodulating BPSK signal with
> LPC213X or LPC214X MCU. Our telemetry is receiving BPSK coded data
> from sensor with approx 2400 baud ( bit/sec ) rates . There is 100ms
> burst of data and 2.9 sec gap etc. Sensor is DC powered ( 12-20VDc )
> with BPSK sinewaves signal ( 0.5V ) modulated on DC .
> Thanks Radim.
>

I've implemented a QAM demodulator (for V.22bis modem) on a LPC213x,
which is similar but more complex. If you implement AGC or can rely on
the signal strength being steady, you can also use the built-in 10-bit
ADCs. From memory the receiver (analog signal to byte stream) took less
than 20 MIPS (with the code all in 'C').

8 Khz sampling is generally used (if it's 2400 baud, I'm assuming a 4
KHz or less carrier?).

Bottom line: I would have thought it ideal for what you're looking at,
as you can do everything on the one chip. All you need is to find/write
the software: there's probably some out there available as a starting
point.

Brendan
On 27/04/2007 at 20:43:01 -0000 Brendan Murphy Wrote:
> --- In l..., "radim100" wrote:
>
> [...SNIPPED...]
>
> I've implemented a QAM demodulator (for V.22bis modem) on a LPC213x,
> which is similar but more complex.

Hi Brendan,

Are you using the modem for internet access ?.

I want to know if is possible use the V.22bis modem standard
for dial-up internet access (Internet Service Provider) ?.

Regards,
Boris Estudiez.-

--

Contacteme en:
* Mail(1): stk freeshell.org
* Mail(2): 43824 electronica.frc.utn.edu.ar
* Mail(3): slicetex hotpop.com
* Website: http://stk.freeshell.org
EOF
Brendan Murphy wrote:
>
> --- In lpc2000@yahoogroups .com ,
> "radim100" wrote:
> >
> > Hi,
> > Did anybody have some experience with demodulating BPSK signal with
> > LPC213X or LPC214X MCU. Our telemetry is receiving BPSK coded data
> > from sensor with approx 2400 baud ( bit/sec ) rates . There is 100ms
> > burst of data and 2.9 sec gap etc. Sensor is DC powered ( 12-20VDc )
> > with BPSK sinewaves signal ( 0.5V ) modulated on DC .
> > Thanks Radim.
> > I've implemented a QAM demodulator (for V.22bis modem) on a LPC213x,
> which is similar but more complex. If you implement AGC or can rely on
> the signal strength being steady, you can also use the built-in 10-bit
> ADCs. From memory the receiver (analog signal to byte stream) took less
> than 20 MIPS (with the code all in 'C').
>
> 8 Khz sampling is generally used (if it's 2400 baud, I'm assuming a 4
> KHz or less carrier?).
>

I've also been looking at using the ADC for 1200bps demodulation... How
are you doing with noise? I've got to use this via an RF circuit and
you know what they can be like, even FM is somewhat noisy.

I was thinking about doing modulation via PWM output from a port pin,
then shaping the signal via a series of low pass active filters.
TomW

--
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net http://cyberiansoftware.com http://openzipit.org
"Windows? No thanks, I have work to do..."
----------------
--- In l..., Tom Walsh wrote:
> I've also been looking at using the ADC for 1200bps demodulation...
How
> are you doing with noise? I've got to use this via an RF circuit and
> you know what they can be like, even FM is somewhat noisy.

We have fairly minimal analog filtering - it's a high volume
application and every cent counts, and hardware must be minimised. If
you have a reasonably powerful processor available, you may as well use
it: the ARM is well capable of the DSP required to do the majority of
the filtering.

>From memory, we have a 3rd order LPF on the i/p, with programmable
gain: the micro adjusts the gain, controlled by software. If you do
this, you can get away with the on-board 10-bit ADC. The i/p system is
then the micro, an op-amp and a few resistors.

>
> I was thinking about doing modulation via PWM output from a port pin,
> then shaping the signal via a series of low pass active filters.

Again, on filtering we'd tend to use the processor. For example, we use
8-bits of DAC with 8 kHz sampling. To reduce the analog/hardware
filtering requirement, we run this through a small oversampling filter
and o/p the samples at 40 kHz.

Having said that, the biggest problem with the LPC2xxx (at least the
versions we've used), is the lack of a hardware trigger on DAC o/p.
Trying to do it in s/w leads to jitter, which causes too much in-band
noise (at least for 2400 bps). It is possible to do "jitter-free" s/w
to get round this, but it's messy to say the least.

We looked at using the PWM for o/p: the problem is that it's so noisy,
even at carefully chosen oversampled rates, that you always get in-band
harmonics that are too high (at least for our purposes) and cannot be
removed by filtering. Obviously, it'll depend on your specific
requirements, but I'd be surprised if you can make it clean enough.

The great thing about doing most of the filtering in software rather
than hardware (apart from less hardware to go wrong) is the ease with
which it can be tweaked. It can also all be developed on a PC using the
tools it has available, and then ported once it all works.

As you might guess, my background is software: no doubt you'll have an
alternative view on this....

Brendan
--- In l..., Boris Estudiez
wrote:
> Are you using the modem for internet access ?.
>

No - but see below.

> I want to know if is possible use the V.22bis modem standard
> for dial-up internet access (Internet Service Provider) ?.
>

We produced a TCP/IP stack, optimised for low-memory systems, and at
one stage ran it on the V.22bis (2400bps) modem. It was a very neat
package: tiny micro, a very few external components (and a lot of
software). We demo'd an application where a Web browser controlled
various devices managed by the same micro.

The problem was that it's a bit like the talking dog: the interest
wasn't so much in what it could do but rather the fact it could do it
at all.

TCP in particular behaves very badly at such low speeds. You have to do
a lot of optimising and application tuning to try and reduce the data
being sent and received.

In the end, it remained just that: an interesting demo. The modem and
IP stack ended up in different (separate) applications/products.

There's plenty of applications out there using low-speed dial-up comms:
in fact for applications where you have a small amount of data to send
it's actually faster than faster line-speed standards, due to the very
fast training time. I'm not aware of any large scale use of TCP/IP at
such speeds, though.

In summary: it can be done, but it's unlikely to be that useful in a
real-world application.

Brendan
--- In l..., "Brendan Murphy"
wrote:
>
> --- In l..., Tom Walsh wrote:
> > I've also been looking at using the ADC for 1200bps
demodulation...
> How
> > are you doing with noise? I've got to use this via an RF circuit
and
> > you know what they can be like, even FM is somewhat noisy.
>
> We have fairly minimal analog filtering - it's a high volume
> application and every cent counts, and hardware must be minimised.
> As you might guess, my background is software: no doubt you'll have
an
> alternative view on this....
>
> Brendan
>
Here is an interesting question with regard to the thoroughness of
preparation for analogue work in the real world.

If you had a choice between two individuals, one with a first class
honours degree in electronic engineering from a British or Irish
university that says ranks them in the top 10% and the other with a
US GRE score that ranks them in the top 10% which would you choose?

Having looked at the GRE tests I would rate the GRE standard as
second year university (British or Irish 1970s standards).

Having seen who came out with first class honours and seen that
examinations take problems verbatim from exercises handed out
throughout the year, I would have no confidence in the first class
honours results without conducting my own tests. Also given the
insanity of many final year projects and the demands they impose, I
would question whether there is much time to do normal examination
preparation.

If I wanted someone for general electronic engineering work who could
get to grip quickly with just common design tasks I would not need
someone with above second year university education, provided they
knew their subjects well.

If I was in a big company and wanted someone for specific leading
edge work then I would take a different attitude.

John Heenan
John Heenan wrote:
>
> --- In lpc2000@yahoogroups .com ,
> "Brendan Murphy"
> wrote:
> >
> > --- In lpc2000@yahoogroups .com ,
> Tom Walsh wrote:
> > > I've also been looking at using the ADC for 1200bps
> demodulation. ..
> > How
> > > are you doing with noise? I've got to use this via an RF circuit
> and
> > > you know what they can be like, even FM is somewhat noisy.
> >
> > We have fairly minimal analog filtering - it's a high volume
> > application and every cent counts, and hardware must be minimised.
> > As you might guess, my background is software: no doubt you'll have
> an
> > alternative view on this....
> >
> > Brendan
> > Here is an interesting question with regard to the thoroughness of
> preparation for analogue work in the real world.
>
> If you had a choice between two individuals, one with a first class
> honours degree in electronic engineering from a British or Irish
> university that says ranks them in the top 10% and the other with a
> US GRE score that ranks them in the top 10% which would you choose?
>
> Having looked at the GRE tests I would rate the GRE standard as
> second year university (British or Irish 1970s standards).
>
> Having seen who came out with first class honours and seen that
> examinations take problems verbatim from exercises handed out
> throughout the year, I would have no confidence in the first class
> honours results without conducting my own tests. Also given the
> insanity of many final year projects and the demands they impose, I
> would question whether there is much time to do normal examination
> preparation.
>
> If I wanted someone for general electronic engineering work who could
> get to grip quickly with just common design tasks I would not need
> someone with above second year university education, provided they
> knew their subjects well.
>
> If I was in a big company and wanted someone for specific leading
> edge work then I would take a different attitude.
>

From my experience, it is not the credentials that are as important as
the individual themselves. I was doing interviews for the company I was
working at and went through a bunch of people. All were well qualified,
but the individual who caught my attention was the guy who was very
quiet, it was a difficult interview, until... Until I asked him about
any hobby interests he had in electronics. He then lit up and went on
to describe in detail this computer to remote control car thing that he
was building, it was a setup where he would put the car into a maze and
have the computer learn the maze from the car's sensors.

That is the guy I recommended that they hire. The one who was driven to
"play" with this stuff. Unfortunately, they went with a guy who had a
very nice grade point average and school. That one lasted five months
before he left.

It appears that a lot of people miss the who point of higher learning.
They seem to think that college will teach them all they need to know to
do engineering work. Unfortunately, they miss the point that college
teaches you how to think, not what to think...

My point is that anything / anybody can look good on paper.

Regards,

TomW
--
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net http://cyberiansoftware.com http://openzipit.org
"Windows? No thanks, I have work to do..."
----------------
--- In l..., Tom Walsh wrote:
>
> John Heenan wrote:
> >
> > --- In lpc2000@yahoogroups .com
40yahoogroups.com>,
> > "Brendan Murphy"
> > wrote:
> > >
> > > --- In lpc2000@yahoogroups .com
40yahoogroups.com>,
> > Tom Walsh wrote:
> > > > I've also been looking at using the ADC for 1200bps
> > demodulation. ..
> > > How
> > > > are you doing with noise? I've got to use this via an RF
circuit
> > and
> > > > you know what they can be like, even FM is somewhat noisy.
> > >
> > > We have fairly minimal analog filtering - it's a high volume
> > > application and every cent counts, and hardware must be
minimised.
> > > As you might guess, my background is software: no doubt you'll
have
> > an
> > > alternative view on this....
> > >
> > > Brendan
> > >
> >
> > Here is an interesting question with regard to the thoroughness of
> > preparation for analogue work in the real world.
> >
> > If you had a choice between two individuals, one with a first
class
> > honours degree in electronic engineering from a British or Irish
> > university that says ranks them in the top 10% and the other with
a
> > US GRE score that ranks them in the top 10% which would you
choose?
> >
> > Having looked at the GRE tests I would rate the GRE standard as
> > second year university (British or Irish 1970s standards).
> >
> > Having seen who came out with first class honours and seen that
> > examinations take problems verbatim from exercises handed out
> > throughout the year, I would have no confidence in the first class
> > honours results without conducting my own tests. Also given the
> > insanity of many final year projects and the demands they impose,
I
> > would question whether there is much time to do normal examination
> > preparation.
> >
> > If I wanted someone for general electronic engineering work who
could
> > get to grip quickly with just common design tasks I would not need
> > someone with above second year university education, provided they
> > knew their subjects well.
> >
> > If I was in a big company and wanted someone for specific leading
> > edge work then I would take a different attitude.
> > From my experience, it is not the credentials that are as
important as
> the individual themselves. I was doing interviews for the company
I was
> working at and went through a bunch of people. All were well
qualified,
> but the individual who caught my attention was the guy who was very
> quiet, it was a difficult interview, until... Until I asked him
about
> any hobby interests he had in electronics. He then lit up and went
on
> to describe in detail this computer to remote control car thing
that he
> was building, it was a setup where he would put the car into a maze
and
> have the computer learn the maze from the car's sensors.
>
> That is the guy I recommended that they hire. The one who was
driven to
> "play" with this stuff. Unfortunately, they went with a guy who
had a
> very nice grade point average and school. That one lasted five
months
> before he left.
>
> It appears that a lot of people miss the who point of higher
learning.
> They seem to think that college will teach them all they need to
know to
> do engineering work. Unfortunately, they miss the point that
college
> teaches you how to think, not what to think...
>
> My point is that anything / anybody can look good on paper.
>
> Regards,
>
> TomW

What you say is common in many disciplines. But knowing that most of
those you are interviewing are hopelessly unrealistic, untried, have
no respect for those they see as standing in the way of their bright
ideas fantasies, have dubious honesty and have been clever at playing
the game does not alter that they are the stock you must choose from

However some engineering students at college think that engineering
will give them a leg up into a management position where all the hard
engineering work is done by companies wanting to sell them their turn
key solutions. Isn't the biggest job of the FAE to determine what
potential customers actually want and ensure no one stuffs up and
gets embarrassed?

Anyway I take back what I said about the US GRE (Graduate Record
Examination). There are numerous dodgy questions and bizarre
mathematical questions where you have to guess what a stupid examiner
or sets of examiners who are incapable of clear expression are
looking for. In the second edition of 'Practicing to take the GRE
Engineering Test, 2nd Edition 1989' there is a single transistor
question. It is on page 87 and was apparently used in a real test. An
NPN transistor looks like it is in a common collector configuration
to provide current gain but a second inspection suggests a common
emitter configuration (providing voltage gain) rather confusedly
across both a resistor and the power source! The bias consists of a
voltage source! How many decent engineering graduates in a hurry did
not get past option B and marked the 'Voltage gain may be greater
than one' as false whereas the probable false answer is D 'The
voltage gain is positive in the midfrequency region' which just about
anyone who is clueless about circuits should get since single
transistors always invert.

John Heenan
--- In l..., "John Heenan" wrote:
>
> --- In l..., Tom Walsh wrote:
> >
> > My point is that anything / anybody can look good on paper.
> >
> > Regards,
> >
> > TomW
>
> What you say is common in many disciplines. But knowing that most of
> those you are interviewing are hopelessly unrealistic, untried, have
> no respect for those they see as standing in the way of their bright
> ideas fantasies, have dubious honesty and have been clever at playing
> the game does not alter that they are the stock you must choose from
[[and plenty more of the same....]]

Guys, what possible relevance has any of this to a Group devoted to the
NXP LPC2xxx?