Bruce Varley
Guest
Wed Aug 31, 2011 1:48 pm
"SFD" <sfd_at_127000.00> wrote in message
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Quote:
"Bruce Varley" <bv_at_NoSpam.com> wrote in message
news:aamdnduT--gxfMHTnZ2dnUVZ8qGdnZ2d_at_westnet.com.au...
"SFD" <sfd_at_127000.00> wrote in message
news:d_17q.1432$7r4.944_at_viwinnwfe02.internal.bigpond.com...
Hello All, I'm looking for circuit ideas/suggestions for the following:
A receiver to receive a very small 8.5kHz signal using a 470uH choke as
the pick-up sensor, or any other off the shelf similar small component.
The transmitted source is a loop 'antenna' of wire ranging between 100m
to 300m total length.
It's not clear just what the geometry of your transmitting antenna is. Is
it a single conductor 100-300m long, ie. about 80 metres diameter, or a
smaller diameter with multiple turns?
It's a receiver that will be used with existing equipment.
The equipment being for domestic animal containment.
A length of single core wire is installed around the property perimeter.
The animal (cat/dog) wears a collar that is activated when brought within
about 300mm of any point of the wire.
The wire 'loop antenna' therefore has no fixed regular shape.
Unfortunately I have no control over the design of the Tx as that already
exists.
All I have access to are the pulses (protocol if you like, transmitted by
the Tx).
The Tx transmits burst of pulses 187 every 15mS.
The Tx output is variable. The output comprises a TDA2003 audio amp.
The signal measured at the output of the Tx is 3v P-P into a 5ohm load,
about 210mA.
I have yet to measure the resistance of a typical 300m 'antenna'.
The avaiable signal level is very low, unfortunately I do not have the
exact level details available.
The receiver should be able to detect the signal from a distance of
about 300mm from its source, in other words bringing the receiver within
300mm from anywhere of the loop antenna.
A lot depends on the current in your transmitting loop, something you
haven't told us.
The signal applied to the loop antenna is coded - that is not a problem
as I have the info to decode.
So what I am looking for is a front-end preamp that will provide me an
approximate 2v p-p signal that I can apply to a microP to be
decoded/processed.
Ideas comments views criticisms of description etc., ...and so on
greatly appreciated.
With a bit more info people here should be able ot provide oyu with
better advice.
Thanks for you input - looking forward to your comments.
There are plenty of suggestions for receiver sensors in the other posts, I
suggest you set up a test rig and try things out. There are many variables
involved. But also be aware that when you move to the field, the level of
interference you find may be unexpectedly high.
Your main challenge is likely to be achieving a bandwidth across your total
signal path that allows you to transmit hte data you require. These
magnetically coupled setups often depend on high Q to achieve sensitivity
and noise reduction, but that can easily wreck the capability to send
digital data.
SFD
Guest
Wed Aug 31, 2011 9:54 pm
"Jasen Betts" <jasen_at_xnet.co.nz> wrote in message
news:j3l0d0$qti$1_at_reversiblemaps.ath.cx...
Quote:
On 2011-08-30, SFD <sfd_at_127000.00> wrote:
The Tx transmits burst of pulses 187 every 15mS.
that's a 12.5kHz repetition rate
Is that 187 bursts every 15 ms ?
187 pulses every 15ms
bursts of 187 pulses every 15ms?
where are you getting 8.5kHz from?
.... quite correct. I should have clarified. The existing Tx is 12.5 kHz.
The Tx will be modified to 8.5kHz (187 x 20mS). These devices will be
distributed in the US and apparantly have to be under 10kHz - so I am told.
The problem has been solved. Using a higher value sensing device (>10mH
choke) into a FET and opamp to stabalize and clean the signal.
Thanks for you input, appreciated.
SFD
Guest
Wed Aug 31, 2011 10:00 pm
"Phil Allison" <phil_a_at_tpg.com.au> wrote in message
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Quote:
"SFD"
"Phil Allison"
Tried the audio approach with limited success. Tried opamps MCP6002.
The other point is power consumption - battery life is important.
On the micro side its OK, I obtain 80uA standby by placing the micro
into sleep mode.
** The design of the pick up coil is gonna be crucial - you need high
inductance and high Q.
I doubt an inductor sold for use in SMPS is any good at all.
Best wind your own on a ferrite rod with *lots* of very fine wire -
then find a cap value that tunes it to 8.5kHz .
Op amps with large GBW products and low input noise are never low
current too, so you need to maximise the performance of that coil.
Thanks Phil, youre probably well aware of "The Trials and Tribulations of
an Electronics Bod!"
Your suggestion for pickup would be the ideal scenario, unfortunately the
customer (bless their bank account!) almosts insists on using a small
antenna.
They have two existing similar devices working using a choke as the
pickup. The choke they use is virtuall identical in shape and size as the
one supplied by Electus/Jaycar type LF-1108 470uH - having said that, I
have yet to remove a choke and measure its inductance.
** A coil something like that MIGHT do, but will need a large number of
turns on it and high inductance.
Hundreds of mHs, if possible.
This one from Element14 is getting closer.
http://au.element14.com/toko/8rb273k/inductor-27mh/dp/1193622?Ntt=1193622
13nF cap to resonate, Q = 18, suit a high impedance input.
.... Phil
Problem solved. Measured the choke used in existing device and found it to
be 10mH.
Made up a test cct, choke into FET, into dual copamp to stabalize and clean
up the signal - perfect.
Sensitivity will no doubt (hopefully) be better with the 27mH device.
I was being over optimistic (stupid comes to mind) trying to get a 470uH
choke to work, a few simple calculations would have told me otherwise. Oh
well, when will we ever learn that shortcuts and assumptions often take
longer!
