DIY Sonar

First lets start with the expected signal level.

Based on the 400RT160 datasheet (http://www.robot-electronics.co.uk/files/t400s16.pdf) and the Pro-Wave Electronics Corporations application note (http://www.prowave.com.tw/pdf/an0508309.pdf) the approximate signal level is in the order of 1 to 10 mV:

Here I have assumed 5v rms transmitter voltage rather than 10v rms as per the standard.

The sensor input impedance is 3.9k ohms.

The signal input (3 mV) suggests a minimum receiver gain of 60dB (x1000) is required.

I have designed two circuits (more options are available for consideration).

The first is a two stage transistor amplifier (74dB gain based on the Tina simulation) followed by a diode detector. For this circuit I assumed the sensor impedance was about 20k as I had not found the right datasheet.

A very important note is that I have not designed the supply noise filtering for this circuit. With 74dB of gain (x5000) and the input in-phase with the output, the risk of feedback is almost certain.

Hers is the frequency and transient analyses:

The second version is less sensitive but should not suffer feedback oscillation:

The choice of detector diode determines the receiver sensitivity. A germanium diode would work best followed by schottky diodes and lastly conventional silicon signal diodes.

Preliminary Transistor Amplifier Design

The preliminary transistor amplifier design is as follows:

The design formula works okay except that Tina predicts a much lower gain.

Still it is a useful starting point.

Pre-selection LC filter

In both these sonar receivers I have used a pre-selection LC filter to reduce out of frequency response and to match the sensor to the transistor input impedance.

I used capacitor tapping to scale up the impedance in the two circuits:

RL~RS*((C1+C2)/C1)^2, where C1= 4n7F and C2=2n2F in circuit 2

In this case I used Tina to determine the exact capacitor values but there are formulas for this if you look on the Internet.

Another design

I also played with this one:

And:

The low limit of detection is in the order of 100uV which is a bit of overkill.

The second version looks about right to me if I use a Ge diode (or a Ge silicon clone).

Probably time to make a prototype using strip-board, here is a layout:

Transmitter

Based on the limited data on the EPS-12E the digital pins can source/sink 12 mA. This is similar to the AVR specification. So an estimate of the "source" resistance is 275 ohm (=3.3v/.012A). The effective source resistance is probably lower. The ultrasonic transmitter has a load impedance of about 500 ohm (based on the datasheet). So I matched the source impedance ad load impedance using a Pi network and updated the strip-board design. I calculate the output power to be 1.6 mW (2.55v pp based on Tina). Here is the Tina schematic:

Here is the strip-board design:

Checking the range:

A return range of 53 cm (26.5 cm one way) is practically unusable!

Okay, I either increase the transmitter power or increase the receiver gain.

Increasing Receiver Gain

Looking at the receiver I had a play with reducing the supply voltage down to 3.3v to eliminate the need for a 5v supply. The sensitivity "reduced" to 20 mv (from 10 mv):

It would seem that I should revisit the two transistor pre-amp schematic.

Increasing Transmitter Power

In order to increase the transmitter power I have to reduce the source resistance by buffering. Many options but here is one that I looked at:

The output is 5.5v pp which equated to 6.4 mW into a 500 ohm load.

In theory you could push this circuit for higher power but the Q increases rapidly making tuning necessary and tricky. Higher Qs also slow the power ramp up.

Revised 3 transistor design

I have revised the design for 3.3v and a simplified "T" high pass filter matching network. I have added a 220pF to roll off the high frequencies faster. Here is the schematic and the response to a 200 uV signal:

(Note:the 3.9k resistor on the input is part of the sensor.)

The receiver can detect a 200uV signal which is more than enough for the low power transmitter:

Now I have to source the inductors and capacitor values I have chosen and then update the strip-board design.

Strange

Well I was looking at what inductors I had and what I had to buy.

A 30 mH would be an ebay order (2-3 weeks) but I had some 15 mH. So no problem, just double them up.

But I though I would check if I can come up with a 15 mH design.

Now, I had noticed that the webpage I was using gave component values that were a bit off (peak at 30 kHz instead of 40 kH etc). I had countered this by optimising the design in Tina (slow but effective).

Up until now I thought it was due to the influence of the transistor.

But i noticed that the components values where not the same as a practically identical but different webpage.

Upon further investigation (and calculating the component values by hand) I found the webpage I was using was wrong. Tina said my calculations were correct.

So I started using the new webpage but even then some of the networks gave wrong answers. I some cases the component value labels were swapped around.

So I built a spreadsheet for the calculations. All good. I worked out a band-pass network using just one 15 mH inductor:

Updated Transmitter

Next was to check the transmitter.

I decided to look at a single transistor design using a collector "choke".

A two stage design is required else the Q would be too high.

Here is the design I can up with:

Because the circuit is driven by a square wave, the "choke" generates inductive voltage spikes.

I tried a capacitor at the base of the transistor (did not work) and a diode across the 'choke".

The diode worked but I was still concerned about the inductive spikes (~0.7v) superimposing on the power supply.

The final design uses a 6.8v 400 mW zener to clip the inductive spike.

Even Stranger

Following is the Tina simulation with the zener diode:

You will notice the ringing (~5.3 MHz, this should not be a problem).

What you will not know is that the output voltage (21.6v pp) has increased (from 17.3v pp) when the zener was added.

So the inductive power has been put to good use!

The output power has increased from 75 mW to 116 mW.

I have heard of parametric amplifiers but I am still quite astonished.

Updated Signal Maths

So with the new transmitter, the expected signal is about 5 mV (rms) at 800 cm return distance.

So I can use the two transistor receiver as it is sensitive to about 5 mV (pp):

Back to the strip-board design!

Here is the latest design layout:

Parts

I wandered down to the local electronics shop and purchased enough components for 5 sets of sensors.

The only hiccup was that the bag of sensors that I order on ebay last February (which were meant to be pairs) were all transmitters.

Still I have two other sensor receivers that I can use for test-work..

So I am not going to be able to finish the fours sensors I need for the Dalek in the next two weeks. Not good. Time is running out.

AlanX