The effect of hand around the antenna of a digital transceiver

On Feb 21, 10:38�am, cincy...@gmail.com wrote:
> On Feb 20, 7:31&#4294967295;pm, "ytach" <ytachw...@ou.edu> wrote:
>
>
>
>
>
> > Hello Guys,
>
> > I have built a basic DBPSK transceiver and I was verifying the
> > implementation when I found out that when I put my hand on the antenna, the
> > signal error rate goes up big time. My question to you guys, is it possible
> > to implement a basic DBPSK transceiver with no error correction or
> > equalization or AGC and yet can sustain the multipath and attenuation
> > effect of placing hand around the antenna? When do I need really these
> > components (Please guys let me know if my assumptions below are wrong):
>
> > 1- AGC : The TX and RX are so close (15 cm apart) and the TX power can go
> > from -6dBm to 6 dBm and I am sending at -3 dBm. since the TX and RX are
> > fixed in place I did not implement AGC.
>
> > 2- Synchronizer : I did not implement carrier equalization because I am
> > using differential coding so I can live without carrier synch with loss 3
> > dB. and I did not implement the symbol synch because I am clocking both TX
> > and RX from the same source so I do not need timing extraction.
>
> > 3- Equalizer
> > The bandwidth of my signal is around 400kHz (using BPSK and 0.5 sqrt
> > cosine pulse shaping) which I am assuming less than the channel bandwidth
> > coherency in indoor environment so an equalizer can be ignored.
>
> > Is it is really possible to implement DBPSK in an indoor environment and
> > receive good BER without the components mentioned above?
>
> > I am ready to provide any further technical details as needed and would
> > appreciate sharing your experience with me
>
> A dose of reality: if you are clocking the TX and RX off of the same
> source, that implies some sort of physical connection between the two
> devices. If that's available, why not use that connection to carry
> your DBPSK signal instead?
>
> What I'm getting at is that you will (almost) never have perfect time
> synchronization. Your other questions, as to what you can "get away
> without having," will have differing answers depending upon the exact
> topology of your system; there's no single answer. You seem to have a
> basic communications system in place, which will work in the presence
> of AWGN and no channel dispersion or attenuation. Those additional
> challenges require solutions to combat them, so the answers to your
> questions are "yes, you need AGC/equalization/time sychronization" if
> the structure of your system demands it.
>
> Jason- Hide quoted text -
>
> - Show quoted text -

I totally agree with you Jason, I am going to build all these blocks
after I verify that by transceiver is built correctly. My attempts to
test that were two:

1- looping back DAC of TX to ADC of RX (so this becomes a regular
modem and not a wireless one) and the transceiver is working fine.
2- go wireless! so I tried to control the environment as much as I
can. They transceivers are not moving, yet they are probably too
close. I eliminate the timing problem by clocking both sides from the
same source and here you go, a signal is received but a burst of
errors show up from time to time and some time lengthy busts. When I
see errors coming and going this way I feel it is clocking problem but
I could be wrong. I do not know for sure how wireless channel behave
in that short range. Do you have any debugging techniques to suggest.
Thank you for your reply Jason

On Feb 21, 12:34&#4294967295;pm, Eric Jacobsen <eric.jacob...@ieee.org> wrote:
> On Fri, 20 Feb 2009 18:31:38 -0600, "ytach" <ytachw...@ou.edu> wrote:
> >Hello Guys,
>
> >I have built a basic DBPSK transceiver and I was verifying the
> >implementation when I found out that when I put my hand on the antenna, the
> >signal error rate goes up big time. My question to you guys, is it possible
> >to implement a basic DBPSK transceiver with no error correction or
> >equalization or AGC and yet can sustain the multipath and attenuation
> >effect of placing hand around the antenna? When do I need really these
> >components (Please guys let me know if my assumptions below are wrong):
>
> Your physically touching the antenna? &#4294967295;Depending on how your receiver
> is built you're probably screwing up the input impedance or something.
> As Sebastian mentioned, your antennas may already be in near-field,
> and your hand certainly is if you're touching the antenna. &#4294967295; &#4294967295;At that
> point, all bets are off.
>
> >1- AGC : The TX and RX are so close (15 cm apart) and the TX power can go
> >from -6dBm to 6 dBm and I am sending at -3 dBm. since the TX and RX are
> >fixed in place I did not implement AGC.
>
> Even wired systems implement AGC. &#4294967295; AGC is pretty fundamental to
> getting deterministic receiver response. &#4294967295; Deleting it implies that
> the operating conditions are unusually stable, and for a wireless
> system that means nothing changes in the environment, no people moving
> around, nobody ever opens or closes a door, etc., etc.
>
> >2- Synchronizer : I did not implement carrier equalization because I am
> >using differential coding so I can live without carrier synch with loss 3
> >dB. and I did not implement the symbol synch because I am clocking both TX
> >and RX from the same source so I do not need timing extraction.
>
> As previously mentioned, use the timing connection to carry the data.
>
> >3- Equalizer
> >The bandwidth of my signal is around 400kHz (using BPSK and 0.5 sqrt
> >cosine pulse shaping) which I am assuming less than the channel bandwidth
> >coherency in indoor environment so an equalizer can be ignored.
>
> Maybe. &#4294967295; Maybe not.
>
> >Is it is really possible to implement DBPSK in an indoor environment and
> >receive good BER without the components mentioned above?
>
> IMHO not for any practical application. &#4294967295; If the connection exists for
> the synchronization, that's probably a good connection to use for the
> data, too. &#4294967295;AGC is fundamental. &#4294967295; Only in very unusual circumstances
> can you get away without an AGC if you want a practical system. Study
> "dynamic range" in receivers.
>
> Eric Jacobsen
> Minister of Algorithms
> Abineau Communicationshttp://www.ericjacobsen.org
>
> Blog:http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php

Hello Eric,

I have first to thank you for your post on pulse shaping, it is really
a great post and helped me to tune up my understanding of pulse
shaping. From the answers so far, it might be really scaling issues in
the receiving chain so problably I have to build the AGC.

Also, changing the impedence of the antenna is a good hint. does AGC
combat these problems (impedence change which I would guess cause a
drop in the wireless signal strength and hence AGC comes to play fix
this) ?

As a conclusion from your post, I should study the dynamic range of my
transceiver. However, as I said earlier, the errors are in burst
nature and sometimes lengthy ones. So do you think that AGC is the
only problem? Can I trust my modem design completely if I got it
working by looping the DAC of TX to the ADC of RX?

Also regarding the equalizer, I am also sharing your opinion that I
may probably need it and probably not, when I read the book of
Rappaport "Wireless Communications"  He has mentioned in an example
that an outdoor wireless channel has a coherent bandwidth of 146kHz
and therefore, systems like GSM with 200 kHZ bandwidth do require
equalizers. But I am not sure the coherent bandwidth for indoor
environment and do not have any idea what could be the range of them.
Do you have any clue on this.

Sorry for too many questions. But again thank you for sharing your
experience and would be greatful for any hint that can help me going
in the right direction.

Thank you,

On Feb 21, 11:38�am, cincy...@gmail.com wrote:
>
> A dose of reality: if you are clocking the TX and RX off of the same
> source, that implies some sort of physical connection between the two
> devices. If that's available, why not use that connection to carry
> your DBPSK signal instead?
[snip]

To add to Jason's comment, also check to make sure that your
wired clock connection does not cause the overall system
response to change (i.e., the wire does not conduct RF signals).

This has happened to me before ....  and when I unplugged the
clock cable to check the PHY, the antenna response changed
dramatically and I had to start from scratch again!!

Julius

On Feb 22, 3:36&#4294967295;pm, julius <juli...@gmail.com> wrote:
> On Feb 21, 11:38&#4294967295;am, cincy...@gmail.com wrote:
>
> > A dose of reality: if you are clocking the TX and RX off of the same
> > source, that implies some sort of physical connection between the two
> > devices. If that's available, why not use that connection to carry
> > your DBPSK signal instead?
>
> [snip]
>
> To add to Jason's comment, also check to make sure that your
> wired clock connection does not cause the overall system
> response to change (i.e., the wire does not conduct RF signals).
>
> This has happened to me before .... &#4294967295;and when I unplugged the
> clock cable to check the PHY, the antenna response changed
> dramatically and I had to start from scratch again!!
>
> Julius

Wooow Julius, very interesting hint. I am wondering if this is what is
happening in my case, never thought of this! I will probe the clocks
and see how they look with/without RF board.

On Feb 21, 5:27&#4294967295;pm, YTach <y.tachw...@gmail.com> wrote:
[snip]
> Also regarding the equalizer, I am also sharing your opinion that I
> may probably need it and probably not, when I read the book of
> Rappaport "Wireless Communications" &#4294967295;He has mentioned in an example
> that an outdoor wireless channel has a coherent bandwidth of 146kHz
> and therefore, systems like GSM with 200 kHZ bandwidth do require
> equalizers. But I am not sure the coherent bandwidth for indoor
> environment and do not have any idea what could be the range of them.
> Do you have any clue on this.

Rather than argue qualitatively about this, how about actually
measuring
the effective channel in your system?  If your DAC and ADC are fed off
the same clock, this is as simple as sending a proper pseudo-random
sequence and cross-correlating the receiver symbols.

Then you can try the same exercise with your hand in the way, and you
can then "see" the difference.

Julius