Asking for a friend...

who's having trouble sending a digital signal from the digital output of microcontroller to a transceiver. The signal looks good at both the uC and the end of the connector cable (a ~1.5m coax cable), but when it's connected to the transceiver it disappears. The input on the transceiver is a transistor with a resistor in series.

Any observations as to what might be happening? Wish I could provide a schematic but apparently it's "confidential", I can probably make up a simplified version once I actually see the circuit. Thanks.

 

Did you try twisted pair? Coax has a characteristic impedance of usually 50 or 75 ohm. Ethernet cable goes longer than 1.5m and they sure don't use coax...CAT5 is twisted pair.

 

Without more information I would guess the output of the MCU is loaded down by the input to the transceiver. The input may be designed to match a higher level signal and the MCU just can't deal with the load.

 

Did you try twisted pair? Coax has a characteristic impedance of usually 50 or 75 ohm. Ethernet cable goes longer than 1.5m and they sure don't use coax...CAT5 is twisted pair.

I'm not sure if he's tried one but I'll give it a shot (likely going to look at the circuit tomorrow).

Without more information I would guess the output of the MCU is loaded down by the input to the transceiver. The input may be designed to match a higher level signal and the MCU just can't deal with the load.

This was along the lines of my own guesswork. If this were the case, I take it the solution would be an Op-Amp input buffer? Does that sound about right?

 

This was along the lines of my own guesswork. If this were the case, I take it the solution would be an Op-Amp input buffer? Does that sound about right?

That is certainly a valid approach.

 

That is certainly a valid approach.

Appreciate the reassurance. We'll see how it goes.

 

A uC serial connection
https://datasheets.maximintegrated.com/en/ds/MAX3483-MAX3491.pdf

 

OK, so just a couple of questions. First, could the receiver be bad and have a shorted input? Maybe it had an ESD event and popped the protection diodes if it has them, or the junction is shorted? Second, if it's soldered to a PCB, make sure there are no shorts in the board or solder bridge on the pins. (You didn't say if this was PCB mounted or something else.) Third, you mentioned what the input looked like so I'm wondering if, as was mentioned, the input impedance is too low for the uC and it can't drive, (voltage level or reflection switching) the transceiver input? If the input is a digital signal, I'm guessing the impedance is not critical, (I'm only saying that in general terms, I know about controlled impedance. ) but if it is driving a cable, it's seems to me that the input to the transceiver is looking for a specific impedance of either 50 or 75 ohms as was also mentioned. I would think that the uC might have a higher output impedance and might require a termination or matching network of some type to really process the output signal correctly or as I said before, the voltage level isn't enough? Just some random thoughts, kind of like random noise, it's there like it or not.

 

My first take is that it is a soldering problem or something similar.

 

So I checked out the circuit yesterday, whipped up a little voltage follower with an op-amp I had laying around - low and behold it works. Well, sort of...

The uC functions normally but the op-amp (an LM741CN, something I had leftover from school) is too slow to produce accurate data. I observed this with a function generator at the same clock speed as the uC (about 100 kHz) and there's a clear ramp up/down where there's supposed to be a leading/falling edge:


(channel 1 is function generator, channel 2 is 741 output)

So I tried a quicker op-amp - an LM318 (also something I had leftover from school). This one looked good on the scope as far as timing, but I had quite a bit of trouble cleaning the signal up (I've just been using a solderless breadboard to test these buffers). Unfortunately it didn't work in circuit - the uC crashed like it does without any buffer. Here's what the signal looked like with the function generator:



I'm wondering if maybe there's some difference between the 741 and the 318 that I should be looking at that allows the uC to work for one and not the other? Something to do with impedance, perhaps, as others have mentioned in this thread (I see that the 741's input resistance is 2MΩ while the 318 is 3MΩ - could that be it)? Perhaps my filters on 318's power line caused the rift?

 

Slew rate makes of the LM318 an outstanding opamp vis a vis the old 741.

Edit/ 70 against 0,5 /Edit

 

OK, so just a couple of questions. First, could the receiver be bad and have a shorted input? Maybe it had an ESD event and popped the protection diodes if it has them, or the junction is shorted? Second, if it's soldered to a PCB, make sure there are no shorts in the board or solder bridge on the pins. (You didn't say if this was PCB mounted or something else.) Third, you mentioned what the input looked like so I'm wondering if, as was mentioned, the input impedance is too low for the uC and it can't drive, (voltage level or reflection switching) the transceiver input? If the input is a digital signal, I'm guessing the impedance is not critical, (I'm only saying that in general terms, I know about controlled impedance. ) but if it is driving a cable, it's seems to me that the input to the transceiver is looking for a specific impedance of either 50 or 75 ohms as was also mentioned. I would think that the uC might have a higher output impedance and might require a termination or matching network of some type to really process the output signal correctly or as I said before, the voltage level isn't enough? Just some random thoughts, kind of like random noise, it's there like it or not.

Appreciate these thoughts. At this point I have no reason to believe the transreceiver is bad, given that there's more than a few of them around and the same problem persists with each of them. They're PCB mounted, for the record.

As for your comments on impedance, I might need some clarification based on my previous post. In the meantime, I'll be skimming the pages of the Art of Electronics...

 

Could it be that the pins are connected wrong on the board ?
and your shorting the signal out

When you do a quick mod circuit you put fixed the pin problem ?

share your circuit please

 

Was just about to say: my circuit is just a voltage follower without any external resistors. Basically this:



I did add a 50 nF cap from V+ to GND on the 318 circuit to clean up the line a bit.

 

Could it be that the pins are connected wrong on the board ?
and your shorting the signal out

When you do a quick mod circuit you put fixed the pin problem ?

share your circuit please

Given that the input/output worked in the function generator test I have no reason to believe that the buffer is wired incorrectly. Same goes for the transceiver itself - it works for slower op-amp. However, I'll be sure to maintain vigilance over this aspect at all times moving forward...

 

It sure seems like the input is loading down the signal, if an OP-Amp is having a hard time driving it? Maybe an Open collector type using an external pull-up could help or a transistor buffer? I'm not an engineer but I've been doing PCB Design for 30+ years so I've seen a lot of different circuits and this just seems odd that there is this much trouble running that transceiver? I wouldn't think building the test circuit on the breadboard would make any difference, the runs are pretty short so I wouldn't think stray capacitance or an impedance mismatch would make that much difference. That said, the edge rate of the uC could be pretty fast and for a signal as "slow" as this one is, I would think there is plenty of time for things to settle out? Be sure and put de-coupling caps on the OP-Amps power pin(s) too, you need to make sure they have the power available to them to drive the outputs just like any other IC. Also, make sure the output of your OP-Amp can pull the level low enough for the Transceiver too. Good Luck and let us know what ends up working.

 

Useful thoughts all around. I guess at this point my question boils down to why does it "work" (as in the uC doesn't get loaded down) with the 741 and not with the 318?

 

That is interesting, but a couple more questions for you and that may help others, but what voltage are you running the OP-Amps on and are you using a single supply? Looking at some of the specs, (and as I said, I'm not an EE) you may not be reaching the correct voltage swings needed when using the uC to drive the OP-Amp(s). The signal/function generator would need to be the same as the uC, which I'm guessing you have it set that way, and typically a signal generator will have a 50 or maybe a 75 ohm output impedance, (50 most likely) and can have a pretty good drive strength. Do you know if the uC has the ability of adjusting the output drive and or speed, etc. of it's output pins? If so, perhaps a slower and stronger drive would help? I don't know all that much about uC, but FPGA's typically have that type of adjustments available, so I'm just guessing at this point on the uC. Just tossing out ideas here... I don't believe your power caps are the issue, but I you put 2 on the power leads, a .1 uF and .01 uF next to the chip to be sure. The .01 should be closest if you have that option. Just more thoughts..

 

Asking for a friend...

who's having trouble sending a digital signal from the digital output of microcontroller to a transceiver. The signal looks good at both the uC and the end of the connector cable (a ~1.5m coax cable), but when it's connected to the transceiver it disappears. The input on the transceiver is a transistor with a resistor in series.

Any observations as to what might be happening? Wish I could provide a schematic but apparently it's "confidential", I can probably make up a simplified version once I actually see the circuit. Thanks.

What do you see with an oscilloscope? Any ringing? If the signal just dies, you're not providing enough current.

 

That is interesting, but a couple more questions for you and that may help others, but what voltage are you running the OP-Amps on and are you using a single supply? Looking at some of the specs, (and as I said, I'm not an EE) you may not be reaching the correct voltage swings needed when using the uC to drive the OP-Amp(s). The signal/function generator would need to be the same as the uC, which I'm guessing you have it set that way, and typically a signal generator will have a 50 or maybe a 75 ohm output impedance, (50 most likely) and can have a pretty good drive strength. Do you know if the uC has the ability of adjusting the output drive and or speed, etc. of it's output pins? If so, perhaps a slower and stronger drive would help? I don't know all that much about uC, but FPGA's typically have that type of adjustments available, so I'm just guessing at this point on the uC. Just tossing out ideas here... I don't believe your power caps are the issue, but I you put 2 on the power leads, a .1 uF and .01 uF next to the chip to be sure. The .01 should be closest if you have that option. Just more thoughts..

I'm running the op-amps at +/- 6V, dual supply. I hadn't really considered the supply level would impact the performance of the buffer, but now you've got my interested in experimenting with different supply levels. For the record, I would love to eliminate the need for a dual power supply. Everything else runs on +5 and 12 V respectively.

As for your remark about adjusting the output of the uC, I'm not certain but I suspect the answer is no. It's just a digital signal from a GPIO on a Red Pitaya, although I've never used one of these so I'll have to have a look at the datasheet and see.