Circuit Ideas?

[SFD]

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.
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.

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.

 

[Jasen Betts]

On 2011-08-30, SFD <sfd@127000.00> wrote:

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.

Small antennas never work well.

ideally you want several hundered turns of fine enameled wire on a
form with the largest cross-section practical, (fit the largest dimensions
of the case you plan to use.)

Ideas comments views criticisms of description etc., ...and so on greatly
appreciated.

build an 8.5Khz linear oscilator (eg: twin tee) and turn the loop gain down
until it stops, then couple the antenna to the input

otoh you could probably connect the antenna to a DAC and do the
detection in software.

The signal applied to the loop antenna is coded - that is not a problem as I
have the info to decode.

what sort of coding, and what symbol rate it makes a difference
to the max Q you can use if you use a tuned circuit.

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.

--
⚂⚃ 100% natural




--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---

 

[Bruce Varley]

"SFD" <sfd@127000.00> wrote in message
news:d_17q.1432$7r4.944@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?

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.

 

[John Tserkezis]

SFD wrote:

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.
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.

The signal applied to the loop antenna is coded - that is not a problem as I
have the info to decode.

This smells something like the wireless heart-rate and/or bicycle
sensors and associated receiver watch.

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.

Make sure the "antenna" inductor is structured like a tuned circuit
around the transmitted carrier to at least attempt to attenuate other
unwanted signals.

From there, treat it like audio, though exactly how it would depend on
how the signal is carried before you can know how to condition it from
there.

I've only observed these signals in the past, I hadn't had to do
anything with them, so I can't offer too much past there.
--
Sumo Wrestling: survival of the fattest.

 

[SFD]

"John Tserkezis" <jt@techniciansyndrome.org.invalid> wrote in message
news:j3iqe9$grj$1@dont-email.me...

SFD wrote:
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.
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.

The signal applied to the loop antenna is coded - that is not a problem
as I
have the info to decode.

This smells something like the wireless heart-rate and/or bicycle sensors
and associated receiver watch.

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'.

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.

Make sure the "antenna" inductor is structured like a tuned circuit
around the transmitted carrier to at least attempt to attenuate other
unwanted signals.

From there, treat it like audio, though exactly how it would depend on
how the signal is carried before you can know how to condition it from
there.

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.

I've only observed these signals in the past, I hadn't had to do anything
with them, so I can't offer too much past there.
--
Sumo Wrestling: survival of the fattest.
Le Big Mac!

Thanks for your input.

 

[SFD]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:j3ilbg$cio$1@reversiblemaps.ath.cx...

On 2011-08-30, SFD <sfd@127000.00> wrote:
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.

Small antennas never work well.

ideally you want several hundered turns of fine enameled wire on a
form with the largest cross-section practical, (fit the largest dimensions
of the case you plan to use.)
Agreed, unfortunately these are the parameters in which I have to work, I

have no choice.
The phsical size of the choke is important to the construction of the
design.

Ideas comments views criticisms of description etc., ...and so on
greatly
appreciated.

build an 8.5Khz linear oscilator (eg: twin tee) and turn the loop gain
down
until it stops, then couple the antenna to the input

I like this osc idea of yours, I think I will give that a try....

otoh you could probably connect the antenna to a DAC and do the
detection in software.

Will give this one some thought - not enough signal directly from the
antenna, thats why I need to front end to amplify the received signal and
use it to provide a clean signal to the micro.

The signal applied to the loop antenna is coded - that is not a problem
as I
have the info to decode.

what sort of coding, and what symbol rate it makes a difference
to the max Q you can use if you use a tuned circuit.

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'.

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.

Thank you for you thought and help.

 

[SFD]

"Bruce Varley" <bv@NoSpam.com> wrote in message
news:aamdnduT--gxfMHTnZ2dnUVZ8qGdnZ2d@westnet.com.au...

"SFD" <sfd@127000.00> wrote in message
news:d_17q.1432$7r4.944@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.

 

[no one]

http://www.ebay.com.au/itm/Cat-Door-Staywell-400-Series-Magnetic-4-Way-Locking-/180659516845?pt=AU_Pet_Supplies&hash=item2a102575ad

http://www.ebay.com.au/itm/Staywell-Petsafe-Petporte-MICROCHIP-SMART-FLAP-CAT-DOOR-/180716435280?pt=AU_Pet_Supplies&hash=item2a1389f750


"SFD" <sfd@127000.00> wrote in message
news:Iwd7q.1459$7r4.823@viwinnwfe02.internal.bigpond.com...

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:j3ilbg$cio$1@reversiblemaps.ath.cx...
On 2011-08-30, SFD <sfd@127000.00> wrote:
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.

Small antennas never work well.

ideally you want several hundered turns of fine enameled wire on a
form with the largest cross-section practical, (fit the largest
dimensions
of the case you plan to use.)
Agreed, unfortunately these are the parameters in which I have to work, I
have no choice.
The phsical size of the choke is important to the construction of the
design.

Ideas comments views criticisms of description etc., ...and so on
greatly
appreciated.

build an 8.5Khz linear oscilator (eg: twin tee) and turn the loop gain
down
until it stops, then couple the antenna to the input


I like this osc idea of yours, I think I will give that a try....

otoh you could probably connect the antenna to a DAC and do the
detection in software.

Will give this one some thought - not enough signal directly from the
antenna, thats why I need to front end to amplify the received signal and
use it to provide a clean signal to the micro.


The signal applied to the loop antenna is coded - that is not a problem
as I
have the info to decode.

what sort of coding, and what symbol rate it makes a difference
to the max Q you can use if you use a tuned circuit.


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'.


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.


Thank you for you thought and help.

 

[SFD]

"no one" <krawczuk@adam.com.au> wrote in message
news:gM2dnRo0AJMe9sDTnZ2dnUVZ_gydnZ2d@adnap.net.au...

http://www.ebay.com.au/itm/Cat-Door-Staywell-400-Series-Magnetic-4-Way-Locking-/180659516845?pt=AU_Pet_Supplies&hash=item2a102575ad

http://www.ebay.com.au/itm/Staywell-Petsafe-Petporte-MICROCHIP-SMART-FLAP-CAT-DOOR-/180716435280?pt=AU_Pet_Supplies&hash=item2a1389f750


"SFD" <sfd@127000.00> wrote in message
news:Iwd7q.1459$7r4.823@viwinnwfe02.internal.bigpond.com...

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:j3ilbg$cio$1@reversiblemaps.ath.cx...
On 2011-08-30, SFD <sfd@127000.00> wrote:

Thanks for then eBay info - wish it was as simple as that ;-)
This is part of a design and prototype for a customer for a product that is
to tie in with existing equipment already in the market place.
Thanks anyway.

 

[Phil Allison]

"SFD"

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.



...... Phil

 

[SFD]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9c5h7pFuf0U1@mid.individual.net...

"SFD"

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.



..... Phil

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.
Trying to reverse engineer whats already been done is a no go due to epoxy
encapsulation.
Thanks for your input.

 

[Phil Allison]

"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

 

[SFD]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9c5liiFr5rU1@mid.individual.net...

"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

Thanks for that info Phil, I have ordered a few pieces. Other similar chokes

I'd seen in the mH range appeared physically too big - this is a good find!
The spec indicates its inductance is measured at 79.6kHz, will be
interesting to see its final in-circuit performance at 8.5kHz.
Will keep you posted once they arrive and put a test cct put together.
Cheers.

 

[Jasen Betts]

On 2011-08-30, SFD <sfd@127000.00> wrote:

build an 8.5Khz linear oscilator (eg: twin tee) and turn the loop gain
down
until it stops, then couple the antenna to the input

I like this osc idea of yours, I think I will give that a try....

If you need to save power Phil's tuned circuit idea is better
especially if the frequency is stable.

otoh you could probably connect the antenna to a DAC and do the
detection in software.

Will give this one some thought - not enough signal directly from the
antenna, thats why I need to front end to amplify the received signal and
use it to provide a clean signal to the micro.

do a tuned curcuit and use the B-E junstion if a transistor as the
detector C-E pulling a microcontroller pin down and a 100K pulling it
up

The signal applied to the loop antenna is coded - that is not a problem
as I have the info to decode.

what sort of coding, and what symbol rate it makes a difference
to the max Q you can use if you use a tuned circuit.


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?

and what's the coding you wrote about?

basically I want to know the bandwidth of the signal


--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---

 

[Phil Allison]

"Jasen Betts"

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?

and what's the coding you wrote about?

basically I want to know the bandwidth of the signal

** Knock me over with a feather ...

Jase has actually asked an insightful question.

Will wonders never cease...............




..... Phil

 

[Bruce Varley]

"SFD" <sfd@127000.00> wrote in message
newsxd7q.1461$7r4.131@viwinnwfe02.internal.bigpond.com...

"Bruce Varley" <bv@NoSpam.com> wrote in message
news:aamdnduT--gxfMHTnZ2dnUVZ8qGdnZ2d@westnet.com.au...

"SFD" <sfd@127000.00> wrote in message
news:d_17q.1432$7r4.944@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]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:j3l0d0$qti$1@reversiblemaps.ath.cx...

On 2011-08-30, SFD <sfd@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]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9c5liiFr5rU1@mid.individual.net...

"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!