niranjan_23

modified 9 years ago

Phase Shift Keying with demodulator

466

05:09:37

⭐The Modulator :-

Simple PSK modulation implemented using relay. Still, works like IC 4051 MUX/DeMUX

⭐ The Demodulator :-

The opamp in the center is an adder that cancels out the 180° phase shifted carrier... The output is that of an ASK modulator. The last opamp is now just an ASK demodulator!

⭐ Working :-

Here, depending on the data bit (either 1 or 0), the carrier is transmitted or the 180° phase-shifted version of the same carrier is transmitted.

⭐ Legend :-

Yellow : Adder Output
Purple : Bit input, 1 or 0
Red : Input signal, either c(t) or -c(t).
Green : Final Output

published 9 years ago

hurz

9 years ago

Actually this is after the opamp ASK as you said. The secret is the third sine wave which is somehow is sync 180° shifted to one of the left sine waves (and this generator must be part of the receiver or this is totally just an ASK demo) . The trick for this kind of modem is how to make even with a PLL this canceling sine generator (in phase AND amplitude) from what input frequency. This is e.g. with a different high low ratio were one phase information is much longer present and a frequency regeneration can sync on it. Here we have a 1:1 ratio, on which one to sync? And how?

niranjan_23

9 years ago

@hurz , sorry i really fail to understand your question... Everything from the adder onwards is the Demodulator.. What i did was, generated an ASK out of PSK then demodulated the ASK wave.. Assuming this assembly is being used, then the ppl will obviously know what carrier freq they are using for modulation so they will use the same at receiving end. The adder has 2 inputs... One is c(t) or -c(t) AND c(t). So whenever -c(t) comes in it is cancelled using adder. That was my logic... Moreover, this is the same setup that i did in my lab experiment last week 😃

hurz

9 years ago

But an ASK transmitter can be made much easier. Actually you already demonstrated. Keep it up.

niranjan_23

9 years ago

@hurz So there's an alternate way to generate ASK?

hurz

9 years ago

Were do you think is the airinterface in this circuit? Orange?

20Mhz

9 years ago

I really didn't get the explanation, of course carrier freq is known, but in real life...how do you sync up in phase and amplitude so you can perfectly cancel? The OPAMP in the center is not a demodulator, is just a non inverting amplifier...your ASK Demodulating magic is just shorting the two sines in quadrature...

niranjan_23

9 years ago

@20Mhz, yes i know they need to be phase-locked in real life as noise might introduce phase errors. I just made it for demonstration purposes.

niranjan_23

9 years ago

@hurz, by air interface, do you mean an antenna?

hurz

9 years ago

@niranjan, yes, to see what kind of modulation is going on air! Would be nice if you can make it clear. Thanx

niranjan_23

9 years ago

@hurz, the output of the relay is the one that is transmitted through air. Assume that an antenna is connected to output of the relay

hurz

9 years ago

Ok, red signal is the end of modem and begin from demodulator. Then back again to my and @20MHz comments. How is the synchronisation done inside demodulator? Especial with this on off ratio (equal 0 and 180degree time) of psk timing? How to detect what is digital High or Low. With upper layer error detection and inverting? Im not sure if you see the problem. Keep it up!

niranjan_23

9 years ago

Its just demonstration of PSK Modulation and demodulation... Not a realistic setup where the receiver is 100s of miles away and PLLs could be used... Anyway, the adder cancels the 180° shifted wave using a non shifted wave... Signal ≥ 10 volts here is considered as bit 1 else bit 0.

20Mhz

9 years ago

Dude, I really thing you're missing the real issue... Let's say you're half mile away, receiver and transmitter are isolated...now assume your local canceling signal has the same amplitude that the recovered signal, even then...how do you start your local oscillator so it's synced up so to cancel the received phase and then once you solve that, what symbol is it 0 or 1...Btw, I'm not trying to bug you, just sharing some critical thinking, cheers!

hurz

9 years ago

So amplitude can easily normalised by an limiter amplifier. But what is logic 1 or 0 is still not known by the receiver.. Anyway this must be done on a higher heuristic level in controller for example by error rate detection. This will loose under bad receiving conditions quite a low data throw put. However a longer sync packet does also waste some bandwidth. Nice to meet you guys! Please keep it up!!!

hurz

9 years ago

Some time ago, i published a PSK Modem - check this, hope you like it http://everycircuit.com/circuit/5277691255193600

niranjan_23

9 years ago

@20Mhz, haha dude, i understood.. Both tx and rx oscillator should be in perfect sync... But this is just a demo as i said... 😀

niranjan_23

9 years ago

@hurz, u blow me away😁🖖👍👍👍

hurz

9 years ago

You are welcome!

tankwondo

7 years ago

Nice! I like it.

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