TDC-GP30 in gas meters
4 Circuit Design in Detail
The circuit design and the layout of the Gasmeter V1.1 can be seen in section 5.
Basically, the Gasmeter V1.1 is designed to send ultrasonic fire pulses that have relatively high
peak values and amplify the received signal to a value which is meaningful for the GP30. The circuit
was designed for development and test purposes, therefore there are many things added to
increase the flexibility, observability and immunity against other disturbances.
May be the number of devices used is also not quite as less as it could be. For instance the quantity
of some analog switches, e.g. U2 and U17, could be decreased to one, or instead of separate
housing 2 switches at one device could be used… However to decrease crosstalks and increase
the robustness such savings haven’t been done.
In the circuit the jumpers can be seen almost everywhere (J19, J20, J21, etc.). This was done for
the sake of flexibility. Because it makes it quite easy to separate certain parts from the whole circuit
and see its advantages/disadvantages to the system.
Moreover, the circuit has been equipped with some test points. So that it is easily observed what is
actually going on in that certain point of the circuit. On J5 and J6 the sent signal or the received
signal before the amplification can be seen. The amplified received signal can also be seen on J31.
The circuit is coming with a default assembly of 560 Ohm resistors in the driver/Receiver patch. For
transducers with different impedance resistors R1, R5 and R6 need to be matched to the
transducer.
Figure 8: Resistors to adopt to transducer’s impedance
The figure below was taken from the oscilloscope while the GP30 was performing zero flow
measurement. The oscilloscope was triggered by the signal on J5 and the amplified received signal
was seen on J31. The fire pulses (yellow signal) have 13Vpp values while the received signal on
the transducer is only around 20mVpp. Without amplification this value is just senseless and
impossible to be detect by GP30. Therefore an external amplification unit has been used on the
circuit which also be controlled by the GP30. After opamp, the received packet reaches to
500mVpp.
