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FM demodulator using a 4046 PLL

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schmitt trigger

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I'm designing -as a hobby project- some IR headphones.

When I design anything, I divide the circuit into functional blocks, it allows me to test and debug each independently, before integrating the system.

The block I was testing is the classic demodulator circuit using a PLL, and a plain vainilla IC amp, as shown in the image. This schematic is essentially identical to many examples that appear on the web and app notes.

Since there is a significant source of IR noise at around 40 to 70 Khz, I decided to go for a higher center frequency of 120 Khz.

I assembled the circuit on a protoboard, and to test it I used an Agilent signal generator that can generate FM. The circuit works very well, and I can get a demodulated audio output of 100 mV RMS with 1% distortion to drive the LM386 audio amp, which then drives some 32 ohm headphones.

So far so good......the problem is that the circuit has a significant amount of white noise, enough to be very irritating.
Even if I feed an unmodulated carrier, the noise persists. I also figured out that the noise is being generated by the PLL, because if I inhibit it, the output from the LM386 is completely quiet. And before you ask, yes, the PLL is locked to the input signal.

I'm using a well regulated +6v supply, and the ceramic caps are all NPOs and the resistor are film types. As you can see from the schematic, I'm using proper decoupling too.

My question is: has anyone built a similar circuit? Have you experienced noise issues too?
View attachment IR receiver.bmp
 

betwixt

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I haven't built one quite like it but your 'significant' amount of noise in reality is probably very small, in many other applications it might be considered normal. I would guess it is less than 1mV.

Things to try:
1. it is possible that the hiss is originating in the LM386 which is a notoriously noisy device. Check that when you inhibit the PLL you aren't producing some other signal at the SFOUT pin that is quietening the LM386. Perhaps temporarily disconnecting R5 will confirm this.

2. reduce C4 to say 100pF and note whether the noise level changes. If it drops, leave it at 100pF and add a series RC across it with C being much larger than 1nF. Essentially you would be using the large capacitance and it's series resistor to stabilize the VCO frequency while still allowing the error voltage to change rapidly. If you try this be careful that you do not compromise the audo frequency resonse by letting the VCO track the modulation rather than the carrier.

Brian.
 

schmitt trigger

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" I would guess it is less than 1mV." 5 mV peak to peak actually.

"....hiss is originating in the LM386 which is a notoriously noisy device..." I' m also suspecting that the residual carrier is intermodulating or beating with the LM386. I've ordered a different audio amp to prove this point.

".... leave it at 100pF and add a series RC across it with C being much larger than 1nF..." Interesting idea. When I've used PLLs in the past, I've always added a little resistance in series with the loop filter capacitor, which improves stability. I may try this next. I hadn't done it because all the app notes which show a FM demodulator do not include it, most likely because the carrier is not suppressed properly.
I will try it.

Thanks.
 

FvM

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If the problem is VCO and phase detector phase noise, it may be helpful to use faster HC4046 variants (if you don't already).
 

schmitt trigger

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If the problem is VCO and phase detector phase noise, it may be helpful to use faster HC4046 variants (if you don't already).

Yes I actually have replaced it with a 74HC4046 which is significantly faster. As a matter of fact, the VCO's timing values in the schematic already reflect that, although I forgot to update the CD4046 label to a HC4046 one.

At this point of time, while waiting for some components, my inclination is that it could be an interaction with the LM386.

Another scenario is that I have to increase the output of the demodulator. Right now, the maximum output is about 300 mV pk-pk, whereas the noise is about 10 mV pk-pk. That is giving me a S/N ratio of only 30 dB.
 

schmitt trigger

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Thanks for everyone who replied...... I've solved the problem.

Turns out that the ground where I was returning the audio filter cap was noisy. Moving the cap position a pair of mm away diminished the noise level to a tolerable level.

I've been doing analog design for many years, and are very cognizant of proper grounding, but this still showed me a lesson. I had originally returned the cap's ground to the LM386 ground pin. When I returned it closer to the 4046's ground pin, the noise abated.
Upon a little reflection, I understand why: all the noise current should be returned to the source, which in this case is the 4046.

I hope this experience is helpful to other forum readers.
 
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