How does a heart rate monitor pickup the heart bear and transmit?

I would like to know in details how a Polar or Mia Heart Rate Monitor
(HRM) pickup my heart beat and how it transmit the signal.

1) I want to know why does the HRM need moisture?

2) Why the paralleled grooves are important on the transmitter side of
the HRM?
There are two, one on each side of the transmitter?

3) What is the wave length of the HRM?

4) Is the length of the plastic "wings" are important for the
transmitter or the pickup of the heart beat?

Does the antenna use a Yagi or coplanar antenna for the transmitter
and for the reception?

5) Has every knows the heart is submerged in a saline fluid where only
Ultra Short wave length can reach the heart due this saline solution.
How does the HRM picks up the heart beat?
Is it by induction?
Is it by conduction?
Is it by using OPamps?
etc

6) How does the electronic circuit works on the transmitter and
receiver side?

7) I've checked the length of the strap and I was able to verify that a
regular cleaning improve the transmission and low down the interference
level? why?

8) How is the transmitter ?receiver encrypted? Is it the ASK code? or
other ?

9) Is there a HRM which can download directly its data to a laptop
through the infrared port?
Guy (gnh888)

wrote:
I would like to know in details how a Polar or Mia Heart Rate Monitor
(HRM) pickup my heart beat and how it transmit the signal.

1) I want to know why does the HRM need moisture?

To improve the electrical connection between the surface of the
skin and the pickup.


2) Why the paralleled grooves are important on the transmitter side of
the HRM?
There are two, one on each side of the transmitter?

I haven't got a polar, but the cheap ALDI transmitters I have
are slightly grroved; I suspect it's to trap sweat and help
stop the belt sliding about.


3) What is the wave length of the HRM?


It's not usually radio, it's inductive coupling. The belt generates
short bursts of about 5kHz (each burst corresponds to one beat).
In coded belts, the 5kHz burst contains some modulation to
indicate the belt. There may be some belts using other systems,
but gym machines look only for simple bursts and most belts
will work with them.

To pick up the signal, you need a coil somewhat like a telephone
pickup coil, some filtering tuned to 5kHz and amplification.
But just connecting a coil of wire to a 'scope and holding
it very close to the belt will show something.

The advantage of inductive coupling is that it's very short
range (less problem interfering between users) and doesn't
have any radio licensing problems making it easier to sell
the same product in many countries.


4) Is the length of the plastic "wings" are important for the
transmitter or the pickup of the heart beat?

Probably not for the transmission, but the electrodes on your
chest want a bit of separation. Not critical.


Does the antenna use a Yagi or coplanar antenna for the transmitter
and for the reception?


No, just a coil of wire. Not very directional.


5) Has every knows the heart is submerged in a saline fluid where only
Ultra Short wave length can reach the heart due this saline solution.
How does the HRM picks up the heart beat?
Is it by induction?
Is it by conduction?
Is it by using OPamps?

Same method as ECG : measure small voltages from surface of skin
generated by heart muscle contractions. Amplify with opamps and
couple to display with induction. So yes, yes and yes !


6) How does the electronic circuit works on the transmitter and
receiver side?

Nothing special : just sensitive amplifiers with filtering to
remove interference. The belts seem to synchronise to your heart
and generate pulses only when the rate is constant enough to
indicate a reliable connection.


7) I've checked the length of the strap and I was able to verify that a
regular cleaning improve the transmission and low down the interference
level? why?

Improved connection to skin ?


8) How is the transmitter ?receiver encrypted? Is it the ASK code? or
other ?


PSK, I think. But it probably varies with manufacturer.


9) Is there a HRM which can download directly its data to a laptop
through the infrared port?
Guy (gnh888)


I think Polar make one, but the IR connnection is from the watch
to the PC, not direct from the belt to the PC.


-adrian