Using PLLs as FM Receivers: Why It Isn't Possible

[THRILLHO]

I don't understand why a PLL alone can't be used as an FM receiver. I've got a project showing how PLLs are used and I'd like to use an FM transmitter (which I have a schematic for) transmitting wirelessly a signal that is demodulated by the PLL a few feet away. Seems pretty simple, where I set the lock range of the PLL to the desired level, an antenna to the input, and the PLL outputting the original signal. Why can't I just have a PLL followed by an amp to drive a speaker?

 

[berkeman]

I don't understand why a PLL alone can't be used as an FM receiver. I've got a project showing how PLLs are used and I'd like to use an FM transmitter (which I have a schematic for) transmitting wirelessly a signal that is demodulated by the PLL a few feet away. Seems pretty simple, where I set the lock range of the PLL to the desired level, an antenna to the input, and the PLL outputting the original signal. Why can't I just have a PLL followed by an amp to drive a speaker?

Seems like it would work, but probably with some distortion. Why are you thinking that it will not work?

Welcome to the PF, BTW.

Oh, and be careful to keep your power level very low in your transmission. What frequency range are you going to use? What size antennas? Wht powre level? You need to be sure not to transmit any interference in frequency bands that are licensed.

 

[THRILLHO]

Seems like it would work, but probably with some distortion. Why are you thinking that it will not work?

Because I see FM radio kits all over the internet, and none consist of solely a PLL and antenna.

Welcome to the PF, BTW.

Thanks.

Oh, and be careful to keep your power level very low in your transmission.

Why?

What frequency range are you going to use? What size antennas? Wht powre level?

This is the schematic for the transmitter I'm making:
http://www.simplecircuitdiagram.com...p-fm-transmitter-for-short-range-application/

Says 88 to 108, I guess since it was designed to transmit to common FM radios. I've got to assume the 2 POTS adjust the carrier frequency, but not sure as to why two are needed or which one actually changes the frequency. Not sure what you mean by power level. P at the output of the transmitter? Ptotal of the transmitter?

You need to be sure not to transmit any interference in frequency bands that are licensed.

Even if it's only functional within a very limited distance?

 

[THRILLHO]

Seems like it would work, but probably with some distortion. Why are you thinking that it will not work?

Because I see FM radio kits all over the internet, and none consist of solely a PLL and antenna.

Welcome to the PF, BTW.

Thanks.

Oh, and be careful to keep your power level very low in your transmission.

Why?

What frequency range are you going to use? What size antennas? Wht powre level?

This is the schematic for the transmitter I'm making:
http://www.simplecircuitdiagram.com...p-fm-transmitter-for-short-range-application/

Says 88 to 108, I guess since it was designed to transmit to common FM radios. I've got to assume the 2 POTS adjust the carrier frequency, but not sure as to why two are needed or which one actually changes the frequency. Not sure what you mean by power level. P at the output of the transmitter? Ptotal of the transmitter?

You need to be sure not to transmit any interference in frequency bands that are licensed.

Even if it's only functional within a very limited distance?

 

[THRILLHO]

I'm having a lot of trouble understanding the demodulation aspect of the PLL. I understand FM modulation pretty well. I understand the three components of the PLL and how the react to each other. What I don't understand is how the supposed demodulated output of the PLL in any way resembles a demodulated signal. It's simply a DC level. Using a function generator, I send a carrier signal of 15kHz with an FM modulating signal that I sweep anywhere from 2kHz to 30kHz. The tracking aspect of the PLL is definitely working, and that DC level (which, again, is at the same pin that all schematics of a PLL I've seen have listed as "demodulated output") does change as I move adjust the carrier frequency around within the lock range (about +/- 60% of the natural frequency of the PLL, which I have set around 15kHz) so I think it's working properly. I just don't see how that DC level is in any way useful.

One more thing, that DC level doesn't change at all when I play with the modulating frequency, only the carrier.

 

[Bob S]

The NE565 is a well-known (since~1975) complete PLL plus demodulator circuit in a single chip. See FM receiver PLL plus demodulator circuit in

http://www.bucek.name/pdf/ne565.pdf

The NE565 is good only up to ~500 kHz, but by now there should be something available in the 100 MHz range.

Bob S

 

[THRILLHO]

Yeah, I'm using a 565 (Motorola's). It's arranged pretty much identically to the schematic on the top left of page 87 of that PDF.

 

[vk6kro]

You would take the output of the PLL from the DC control voltage. This will not be constant if the input signal is changing frequency, as an FM signal does. It will be attempting to follow the frequency of the input signal.

However, you need to operate these chips at a much lower frequency than 90-100 MHz. This would be done by building a converter and mixing the signal down to a much lower frequency.

Another suitable chip is the 74HC4046 which operates to about 30 MHz.

I have seen complete receiver chips which just take a 90 MHz FM signal and turn it into audio.

However it is probably better and cheaper to pick up a $5 radio somewhere and use that.

 

[THRILLHO]

This will not be constant if the input signal is changing frequency, as an FM signal does. It will be attempting to follow the frequency of the input signal.

I know it's not constant, but it's pretty steady on the O-scope. The DC level at pin 7 doesn't appear to be affected at all by the modulated frequency. It changes from 3.5 V near the upper lock frequency (about 24khz) and 5.6V around the lower lock frequency (about 9khz). But that is only done changing the carrier frequency directly (ignoring the modulating frequency). I assumed that the DC level @7 would be oscillating at a frequency identical to that of the modulator. As it turns out, as I change the modulating frequency, it has NO effect whatsoever on that voltage. Only when I change the carrier fairly significantly (like, say, going from 15khz to the upper lock of 24khz in increments of 1kHz) does that voltage change, and in no way is it oscillating.

Here's the childishly drawn schematic with values, which I am taking out of a industrial electronics book:

[PLAIN]http://img22.imageshack.us/img22/5302/pllq.jpg

 

[vk6kro]

Are you FREQUENCY modulating the carrier?

Just from your figures, if you varied the carrier frequency from 9 KHz to 24 KHz the output would vary from 5.6 Volts to 3.5 volts. This is 2.1 volts of output from an FM signal.

Your circuit looks pretty much like one in my book, except that pin 6 is not connected to the demodulated output on pin 7.
[PLAIN]http://dl.dropbox.com/u/4222062/565.jpg

 

[THRILLHO]

Yeah, I'm frequency modulating the carrier.

"Just from your figures, if you varied the carrier frequency from 9 KHz to 24 KHz the output would vary from 5.6 Volts to 3.5 volts. This is 2.1 volts of output from an FM signal."

Which I am. But, again, that DC level is pretty steady, and in no way looks like a demodulated signal.

 

[THRILLHO]

http://www.most.gov.mm/techuni/media/EcE_02012_7.pdf

Page 175 shows exactly what the DC level should look like. I can NOT get this kind of response. Again, on the o-scope and DMM, all I'm getting is a fairly steady DC level that only changes when I make major changes to the carrier. Any ideas?

 

[vk6kro]

How much deviation are you using with your modulation?
ie what is the centre frequency and the lowest and highest frequency of your input carrier?

What is the frequency of the modulation? ie how many times a second does it do a complete sweep of frequencies, up then down?

What are you using to produce this modulation? Audio generators that can produce frequency modulated output would be fairly expensive devices.


You should be able to produce slow frequency modulation by swinging the dial of an audio signal generator across the range of frequencies.

 

[THRILLHO]

"How much deviation are you using with your modulation?
ie what is the centre frequency and the lowest and highest frequency of your input carrier?"

Saw "deviation" on the function generator and wasn't sure what it was. I know I played around with it and swung it slowly from around 2khz to 15khz I think. I thought the lowest and highest levels of the carrier were based on the amplitude of the modulating frequency.

"What is the frequency of the modulation? ie how many times a second does it do a complete sweep of frequencies, up then down?"

I played around with it from 1khz to 50khz. The voltage level slightly changed when I moved it from 1khz to around 7khz or so, then above that the voltage level at pin 7 was unaffected by the frequency. Again, the voltage was not oscillating at all, just rising a bit.

"
What are you using to produce this modulation? Audio generators that can produce frequency modulated output would be fairly expensive devices."

A pretty expensive looking function generator. I don't recall the model number.

"You should be able to produce slow frequency modulation by swinging the dial of an audio signal generator across the range of frequencies. "

Again, I only get gradual rises and drops in the frequency range. The PLL oscillated at about 10khz with no input signal (forgot what the frequency is called, central frequency?). So at the upper lock range (about 16khz), the V @ pin 7 is about 3.5V. At lower lock range (4khz) it is about 5.6V. So basically, swinging the carrier frequency from 4khz to 16khz just raises the V at pin 7 by 2V.

I'm getting really desperate. My teacher is insisting that the PLL is doing exactly what it is supposed to be doing and that it is simply supposed to output a fairly steady DC level. All that I've read, however, implies that the level at pin 7 should be varying at a frequency identical to the modulating frequency.

 

[vk6kro]

The oscillator works from 9 KHz to 24 KHz so you could try a centre frequency of 16.5 Khz (that is half way between the upper and lower limits.)

Then the deviation is how far the signal moves from the centre frequency. No point in trying to drive it where it won't go. You could go to 7.5 KHz either side of centre, but play safe and go for 5 KHz.

Now, how fast can you do this? Inside the chip is a 3600 ohm resistor and this works with the 0.1 uF capacitor to filter the control line. It also limits how fast the modulation can be tracked by the PLL.
In this case, the upper limit is about 277 Hz.
To try this out, you would need to set the modulating frequency to about 20 Hz and gradually move the modulating frequency upwards.

If you do all this, you should start to see output at pin 7.

 

[THRILLHO]

Will try tomorrow, thanks for the advice.

 

[THRILLHO]

One last question, I promise. I can't for the life of me find the typical cutoff frequency of the LPF in the PLLs. Any idea? Thanks a lot for the help.

 

[mheslep]

I don't understand why a PLL alone can't be used as an FM receiver. I've got a project showing how PLLs are used and I'd like to use an FM transmitter (which I have a schematic for) transmitting wirelessly a signal that is demodulated by the PLL a few feet away. Seems pretty simple, where I set the lock range of the PLL to the desired level, an antenna to the input, and the PLL outputting the original signal. Why can't I just have a PLL followed by an amp to drive a speaker?

As your reference in post 3 suggests, the PLL can act as the demodulator, but there is more to a receiver than just a demodulator. In particular the receiver also needs some kind of filter to reject unwanted and out of band signals (a tuner) and also to reduce the bandwidth presented to the demodulator. That filter aids in the receiver's overall sensitivity (by boosting S/N) and selectivity. Your antenna acts as filter to a degree, but you likely need more than that. In this case you probably want something like an 88 to 108MHz RF band filter fed from the antenna. For a super-heterodyne receiver, the filtered RF is then down converted to an intermediate frequency, and that signal is fed to another very narrow band filter (~20kHz) which can achieve a reasonable Q-factor (f0 / BW) with practical components at the 2nd stage lower frequency. The result is a still frequency modulated 20kHz BW signal which is appropriate to feed to the PLL for demod.

In the odd cases that the transmitter is close enough (feet) and at high enough power, the S/N at the receiving antenna may be already so high that many of these elements can be eliminated, so that the PLL can lock without help, but then one might as well just run a wire.