benbiles
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Hi, I am trying to understand how to use an OP AMP to compare two changing DC voltages of a RSSI ( receive strength signal indicator ).
I would like the audio outputs of two RF modules to flipflop to the higher RSSI DC voltage using a quiet switch IC as in the schematic..
I would ultimately like a delay to be set between the flipflop and then eventually I would like to change the flipflop switch to a more subtle fast pan
between the audio outputs of the modules.
But !! I feel I am running way ahead of myself on the OPAMP area of this design in progress !!
Do I need to use some resistors here to set some input parameters for the two DC inputs ? do I have this completely wrong !! should I be using two or even for OPAMPS ?
should I be using two schmitt triggers somehow to smooth things out if the RSSI voltage is erratic?
The Voltage will rise and fall from 1-3 vdc on either input depending on how strong the receive signal will be. However if the two voltages become too similar then it would be useful for the flipflop to latch for a given time.. ( wait for a stronger signal difference between the two voltages )
any ideas would be welcome.. I would like to avoid using a micro controller if possible since the dual receiver will be run on battery.
The LED lights are supposed to show which module is currently being output..
Thanks,
Ben
Hi, in general you have the right idea. you can use an opamp, or you can use a comparator. The comparator is likely to have some hysteresis built-in which will reduce a little bit the amount of switching given similar noisy RSSI values. Also please don't use the term "flipflop" unless you actually mean a real flip flop!In either case, you can use resistors to add additional hysteresis to the comparator, so you don't need a schmitt trigger.
You may also want to confirm whether your analog switch is a break-before-make, so you don't accidentally short the outputs of your audio thing together. This probably isn't a big deal given what's probably a few ohms of switch resistance, but it's good to have a clean design.
Something like ADG884 may make more sense for doing an audio mux. https://www.analog.com/en/switchesmultiplexers/analog-switches/adg884/products/product.html
In either case, your audio will probably make pops and other noises when switching. Is your audio output from Rx module always between ground and 3.0v? If not, you will need to add some opamp circuit to bias it up from under ground.
Your LED2 will never light up. Currently the only way to light it up is if the pointy end (cathode) goes below ground, which it won't do. If you want LED2 to light up, then you need to change TR1 to NPN (with a ~100 ohm resistor before the base), and put LED2 in series with TR1 and R3, pointy end down. Even easier to get one LED for each state is just to let the comparator drive them directly. In this case, you delete TR1 and just put each LED (and its 10k resistor) in series from VCC to ground, then put the comparator output in between. So when the comparator outputs high, only the bottom LED+R lights up, and when the comparator outputs low, only the upper LED+R lights up.
Also 10K for an LED is too high. It'll barely make any light. Use something like 330 ohm, 100 ohm, etc. to get some brightness.
For fast panning, you may want to make your own analog switch. In this case you can get a complementary CMOS pair (nfet and pfet) for each channel, then use some RC circuit to control the gate so it's slow. Put a resistor in series with each channel so you don't short them out.
Also, as for the microcontroller, you are likely to burn more power in your comparators, audio RX, opamps, and LEDs than in the micro, if it's programmed properly to use power-down states, slow clock, etc. Anyway you're right, a micro is overkill for this application.
Hope this helps
It's very unlikely that the FET circuit does what you want. At least transistor DC bias and R and C values seems to be far off.
My first comment is a general one. I'm missing a requirement specification. Without it, nobody can rate the discussed design ideas.
Assuming typical audio signal quality requirements, VGA810 won't be ever used.
Secondly, the standard solution for the addressed problem is a diversity receiver which switches the IF signal.
To avoid permanent switching, the comparator circuit should have at least sufficient hysteresis, or preferably evaluate the switching criterion based on both RSSI values and time, e.g. switch immediately in case of signal dropout, delay switching to the stronger signal if the other signal has still sufficient quality. Under this conditions, hard switching may be an acceptable method.
My first comment is a general one. I'm missing a requirement specification. Without it, nobody can rate the discussed design ideas.
Assuming typical audio signal quality requirements, VGA810 won't be ever used.
Secondly, the standard solution for the addressed problem is a diversity receiver which switches the IF signal.
To avoid permanent switching, the comparator circuit should have at least sufficient hysteresis, or preferably evaluate the switching criterion based on both RSSI values and time, e.g. switch immediately in case of signal dropout, delay switching to the stronger signal if the other signal has still sufficient quality. Under this conditions, hard switching may be an acceptable method.
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