Transformers + Weak signals

[dknguyen]

I am using a bipolar switching driver to drive my transducer so this driver also shares connections with my receiver (which also use the transducer). TO keep the signal within the right common mode range I need to tie on end of the receiver down to ground, but since the driver is doing bipolar switching this creates shorts since the driver just passes all current to this ground. My solution was to use a 1:1 transformer to "re-reference" the signal before it reaches the receiver.

But the echo is a weak signal and I am wondering if transformers at some point are unable to pass signals that are too weak due to magnetizing currents. I have high impedance inputs on the secondary of the transformer, but I am worried about the transformer.

 

[Nigel Goodwin]

Bear in mind transformers are used for RF inputs in radios, at FAR lower signal levels than you are talking about.

However, when you say 'high impedance', how high are you talking about?, and is it matched to the transducer?.

 

[ericgibbs]

hi,
Can you post a drawing of the 'front end' of the TX/RX section, showing the BPT transistor switching.?

 

[dknguyen]

Bear in mind transformers are used for RF inputs in radios, at FAR lower signal levels than you are talking about.

However, when you say 'high impedance', how high are you talking about?, and is it matched to the transducer?.

I was just told the receive signal is on the order of uV or mV. The transformers I am using are 1/4W 0.0035MHz-100MHz RF transformers.

When I say high impedance, I mean op-amp inputs, because I have no idea what kind of load the transducer will be able to drive. It's not matched. The capacitance on the transducer is 400-500pF, so 3100R at 100kHz or 6400R at 50kHz.

I'll post everything when I get home later tonight.

 

[Nigel Goodwin]

So not matched, and not tuned, and VERY wideband RF transformers - this doesn't sound like you have a prayer?.

 

[RadioRon]

But the echo is a weak signal and I am wondering if transformers at some point are unable to pass signals that are too weak due to magnetizing currents. I have high impedance inputs on the secondary of the transformer, but I am worried about the transformer.

My transformer theory is too weak to give a direct answer, but I've never run across this concern before. The practical evidence out there is that there are many ferrite and powdered iron core transformers used in radio receivers, like for example in the old 450 KHz tuned IF strips that we used to find in old AM radios. And they are all dealing with uV level signals. Not to mention that current and voltage baluns used with antennas are a kind of transformer and they certainely don't cause any kind of weak signal cutoff or limiting. Of course, the type of core may have a lot to do with it. All high frequency cores are ferrite or powdered iron, not solid iron, but this has more to do with linear losses or saturation issues. I haven't heard of iron core transformers causing a cut-off or limiting effect on very weak signals.

 

[dknguyen]

Here are the schematics. THe main of ones of interest for the original question are the transmitter circuit, receiver circuit, and bias circuit.

I should mention the transmitter circuit only has one driver on it right now for one terminal of the transducer. THe circuit is identical for the other terminal of the transducer, it just goes to AC- instead of AC+

 

[ericgibbs]

hi,
This is the front end of the TXR I used on my Hydrographic Digital echo sounders. It works at 200KHz giving a depth range limit of about 250mtrs.

This version was used on the shallow rivers in Bangladesh, it was required to measure as shallow as 0.3mtrs, resolution was 1cm.

It may give you an alternative ideas.

The point to be aware of is, if you are measuring distance is the fall off in echo signal strength as the distance increases. The rate of rise of the echo's front edge of a strong signal, will tend to give a 'shorter' range than a weaker echo.
Thats why this system uses two stages of TGC [timed gain controlled] opa's