J
John Popelish
- Jan 1, 1970
- 0
Rubicon said:Hello,
I have been trying to make a 40kHz ultrasonic transmitter and receiver
without much success until the recent "Opamp problem" post and the
subsequent replies. It answered many questions for me.
My desire is to have the receiver battery powered and be reliable.
I have put a schematic on the net at:
http://www.geocities.com/talionis.geo/Temp/temp.html
Eliminate C14 and tie pins 3 and 5 together.
Unable to locate a suggested LM6132 low power opamp locally I am
trying a LM833 in its place.
The LM6132 has a wider input common mode range and output voltage
swing but this may not be a problem if you expect the final amplitude
to be well below the supply rails.
With the LM833 (GBW of 10MHz min - 15MHz
max, slew rate of 7V/uS) I set both halves of the opamp with a 1K and
33K for a gain of 33*33=1089.
The square root of a desired 1000 stage gain = 31.6 so a standard
value 33K negative feedback resistor was chosen for each stage. The
GBP would then be 10*33*40000=13.2MHz which is within the upper and
lower limits of the LM833. Correct so far?
Sounds reasonable.
There is a LM393 comparator in place of an LM358 opamp as with a
transistor based receiver circuit I built with a CA3140 opamp as a
comparator (1.5m range) I found that a drop in battery voltage caused
it to stop working. Re-adjusting the threshold trimmer fixed this but
I don't want to have to keep doing that as the battery depletes.
Then you will need to replace the supply with a stable reference as
the source of the decision voltage at TR1. You may be able ot build a
single voltage reference that is shared by all functions that need
one.
I like the 15 mA dual Schottky, BAS70-04.I've made the D1, D2 diodes schottky types, not quite the right ones I
know. 1SS106 barrier diodes would be better but are unavailable.
http://rocky.digikey.com/WebLib/Zetex/Web Data/BAS70.pdf
The yellow dots on the schematic are some things I'm not very sure
about. What are the functions of R4, C2 and C14 and are their values
correct?
I doubt R4 is necessary. C2 has to have a much lower impedance around
40 kHz than the parallel combination of R1,5 to clean the amplifiers
of supply feedback. Of course R 1 and 5 could be higher values before
the two amplifier bias currents cause much of a bias point shift.
With the LM6132 they could easily each be a meg ohm (reduces battery
current a bit). The LM833 has a worst case bias current of 1 uA, so a
pair of 1 meg resistors supplying that to two stages would shift the
bias point a volt. I would probably use 100k resistors with that
amp. This gives a 50k divider impedance. At 40k, a capacitor with
500 ohms impedance (100 times reduction of any 40 kHz getting in
through the positive supply) would be .008 uf. So 10 uf is pretty
generous. I would probably use a .01 or .1 uf ceramic (not Y5V or Z5U
because of their microphonics) or film.
As I said earlier, eliminate C14.
Do I need to add anything to have a long shielded cable to TX1?
Put the 100 ohm resistor below the transducer, and tie the shield to
the positive rail.
I have a basic 555 transmitter I made for the transistor based
receiver circuit and using it on the above LM833 circuit it didn't
work at all. Changing the opamps resistor values to Part-A 10k/1M and
Part-B 10k/100K from a printed off rangefinder circuit it did but at
only 2-3cm.
You may want ot start thinking about how to transform these two gain
stages into band pass filters with gain to reduce interference.
I am not sure why the transmitter worked on the second setup and not
the first. I read where it'd be better to stay away from large value
resistors to decrease noise with such high gain.
Depending on your construction it may have more to do with stray
capacitance across the feedback resistors, which reduces gain. Lower
values of resistance reduce this effect.
It's not the best
transmitter but it must have been roughly tuned to 40kHz to work at
all. I do not have a scope to measure it but I have seen a frequency
meter kitset that I think might just do to tune it or its replacement.
Connect a signal diode (1N4148, etc.) in series with your digital volt
meter and adjust the frequency for maximum positive voltage at the
drain of MOS1.