FM radio design resources...

Time has passed, and the circuit has been built:
https://imgur.com/a/g9y31uO
As it stands thought the output is 7.5MHz, 500mVpp centered at around 2V. It has sidelobes 18 dB down though :(, I was assured by the art of electronics that 60dB was the default!.
I tried increasing C13 to reduce the feedback using the variable capacitor in the picture, but it did not seem to change much, only shifting the frequency slightly. What ideas do people have surrounding how to reduce the sidelobes?
 
Harry Dudley-Bestow wrote:
Time has passed, and the circuit has been built:
https://imgur.com/a/g9y31uO As it stands thought the output is
7.5MHz, 500mVpp centered at around 2V. It has sidelobes 18 dB down
though :(, I was assured by the art of electronics that 60dB was the
default!. I tried increasing C13 to reduce the feedback using the
variable capacitor in the picture, but it did not seem to change
much, only shifting the frequency slightly. What ideas do people have
surrounding how to reduce the sidelobes?
Click to expand...

By \'sidelobes\', do you mean harmonics, FM sidebands, or ....?

Thanks

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Monday, January 3, 2022 at 4:49:01 PM UTC-8, Phil Hobbs wrote:
Harry Dudley-Bestow wrote:
Time has passed, and the circuit has been built:
https://imgur.com/a/g9y31uO As it stands thought the output is
7.5MHz, 500mVpp centered at around 2V. It has sidelobes 18 dB down
though :(, I was assured by the art of electronics that 60dB was the
default!. I tried increasing C13 to reduce the feedback using the
variable capacitor in the picture, but it did not seem to change
much, only shifting the frequency slightly. What ideas do people have
surrounding how to reduce the sidelobes?

By \'sidelobes\', do you mean harmonics, FM sidebands, or ....?

Thanks
Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...
My bad. I mean that the output of the circuit with no modulation or any other input applied is not a particularly perfect sine wave. These pictures should clarify what I mean. It\'s the output of the circuit (which has a DIY 10:1 attenuator on it)
https://photos.app.goo.gl/ayw8ERykW7cTUKv88

I assume this means that the sound will come out all wrong.
 
Harry Dudley-Bestow wrote:
On Monday, January 3, 2022 at 4:49:01 PM UTC-8, Phil Hobbs wrote:
Harry Dudley-Bestow wrote:
Time has passed, and the circuit has been built:
https://imgur.com/a/g9y31uO As it stands thought the output is
7.5MHz, 500mVpp centered at around 2V. It has sidelobes 18 dB
down though :(, I was assured by the art of electronics that 60dB
was the default!. I tried increasing C13 to reduce the feedback
using the variable capacitor in the picture, but it did not seem
to change much, only shifting the frequency slightly. What ideas
do people have surrounding how to reduce the sidelobes?

By \'sidelobes\', do you mean harmonics, FM sidebands, or ....?

http://electrooptical.net http://hobbs-eo.com
My bad. I mean that the output of the circuit with no modulation or
any other input applied is not a particularly perfect sine wave.
These pictures should clarify what I mean. It\'s the output of the
circuit (which has a DIY 10:1 attenuator on it)
https://photos.app.goo.gl/ayw8ERykW7cTUKv88

I assume this means that the sound will come out all wrong.
Click to expand...

Not necessarily. You can put an FM signal through a frequency
multiplier, for instance, and get perfectly good sound nonetheless.

Thing is, the waveform is an amplitude thing, whereas the modulation is
a phase thing.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
Not necessarily. You can put an FM signal through a frequency
multiplier, for instance, and get perfectly good sound nonetheless.

Thing is, the waveform is an amplitude thing, whereas the modulation is
a phase thing.

Cheers
Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...

Interesting! In that case, I shall proceed onwards with constructing the receiver, and deal with new problems as they arise.
 
Harry Dudley-Bestow wrote:
Well time has gone by and I have got the PLL working! Input to a VCO
is green, output from the PLL control signal is blue:
https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6
Click to expand...

Nice.

It locks from between about 10-11MHz and can be used to demodulate FM
audio just fine. For the final \"productionised\" version I would like
to remove the negative rail I am using for my loop filter amplifier.
Currently the loop filter is a simple two transistor diff amp:
https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6 However in order for the
input to go to 0V, this requires a negative voltages source which for
a battery powered device is not ideal.

My question is: What is the simplest possible amplifier where the
input can go to the negative rail?

From some reading in the \"Design of Analog CMOS Integrated Circuits\"
I understand that something may be possible with a pmos diff pair,
but I cannot get anything to work in simulation. @Phil Hobbs any
ideas?
Click to expand...

Well, there are lots of RRIO CMOS op amps out there, e.g. . If you want
something discrete, a PNP diff pair with a resisitive pulldown would
work, and be a lot quieter than discrete small-signal PFETs, which are
pretty scarce nowadays anyway.

Something like that obviously won\'t pull down very strongly near ground,
but then all you\'re driving is a varactor, right?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
Harry - there are numerous op amps that allow the input below the negative rail. I can\'t recall a specific number though. The rail to
rail devices Phil mentions might be a good starting point for a search. These have, I believe, both p & n channel devices on the input.
The ones I vaguely rember were input from below negative to within 2 volts of positive.

Hul


Harry Dudley-Bestow <harry.dudleybestow@gmail.com> wrote:
Well time has gone by and I have got the PLL working!
Input to a VCO is green, output from the PLL control signal is blue:
https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6

It locks from between about 10-11MHz and can be used to demodulate FM audio just fine.
For the final \"productionised\" version I would like to remove the negative rail I am using for my loop filter amplifier.
Currently the loop filter is a simple two transistor diff amp:
https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6
However in order for the input to go to 0V, this requires a negative voltages source which for a battery powered device is not ideal.

My question is: What is the simplest possible amplifier where the input can go to the negative rail?

From some reading in the \"Design of Analog CMOS Integrated Circuits\" I understand that something may be possible with a pmos diff pair, but I cannot get anything to work in simulation. @Phil Hobbs any ideas?
Click to expand...
 
Thanks for the input.

Hul I am aware of rail to rail input amps :). What I would like to do for my project is make something using discrete components.

I will try out the PNP diff pair and see how it goes. I am not so sure where this \"resistive pulldown is supposed to go but all will become clear, I\'m sure.
 
Harry Dudley-Bestow wrote:
Thanks for the input.

Hul I am aware of rail to rail input amps :). What I would like to do for my project is make something using discrete components.

I will try out the PNP diff pair and see how it goes. I am not so sure where this \"resistive pulldown is supposed to go but all will become clear, I\'m sure.
Click to expand...

Two PNPs connected emitter to emitter, with the bases connected across
whatever phase detector you\'re using. (It\'ll need to be biased above
ground if you want any significant output swing from the amp.)

Resistor from the emitters to the supply to provide bias current.

One collector grounded directly, the other connected to ground via a
resistor that provides the output voltage to the varactor.

Some appropriate bypass capacitor across the resistor to give a generous
PLL bandwidth without letting much of the RF get lost in the resistor.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Sunday, July 24, 2022 at 12:10:41 PM UTC-7, Phil Hobbs wrote:
Harry Dudley-Bestow wrote:
Thanks for the input.

Hul I am aware of rail to rail input amps :). What I would like to do for my project is make something using discrete components.

I will try out the PNP diff pair and see how it goes. I am not so sure where this \"resistive pulldown is supposed to go but all will become clear, I\'m sure.

Two PNPs connected emitter to emitter, with the bases connected across
whatever phase detector you\'re using. (It\'ll need to be biased above
ground if you want any significant output swing from the amp.)

Resistor from the emitters to the supply to provide bias current.

One collector grounded directly, the other connected to ground via a
resistor that provides the output voltage to the varactor.
Click to expand...

Thanks for the clarification Phil. That\'s more or less what I had going in LT spice. The tip about biasing the input higher though got the circuit to work in the sense that it provides some gain, but that\'s not sure what I want. here\'s what I have now:
https://photos.app.goo.gl/Zp2wmpdBcCtxxsUU7
For reference here is the circuit I\'m using now that works just fine, the only downside being the requirement for a negative rail:
https://photos.app.goo.gl/FCmdL4UFaEXsSFUV7

I\'m using an XOR phase detector (The double balanced mixer gave me no end of trouble) and so the output of that after filtering is 0->2.5V and experimentally I\'ve determined a gain of about 5 is what I want.
Correct me if I\'m wrong here but the problem is that the output of that circuit can\'t be zero because of the quiescent current through the resistor on the collector of the pnp (Rc2 in my schematic). I can bias the input voltage to be higher with a simple pullup on the base of the input pnp, but I really need the output to be 0V when the input is 0V, else my varactor will have too high of a voltage on it to begin with.
The output range also doesn\'t seem that crash hot (1.1->2.2V) either but I presume that I can fix that by fiddling with the current source and the output collector resistor.

Some appropriate bypass capacitor across the resistor to give a generous
PLL bandwidth without letting much of the RF get lost in the resistor.
Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...
 
Harry Dudley-Bestow wrote:
On Sunday, July 24, 2022 at 12:10:41 PM UTC-7, Phil Hobbs wrote:
Harry Dudley-Bestow wrote:
Thanks for the input.

Hul I am aware of rail to rail input amps :). What I would like
to do for my project is make something using discrete
components.

I will try out the PNP diff pair and see how it goes. I am not so
sure where this \"resistive pulldown is supposed to go but all
will become clear, I\'m sure.

Two PNPs connected emitter to emitter, with the bases connected
across whatever phase detector you\'re using. (It\'ll need to be
biased above ground if you want any significant output swing from
the amp.)

Resistor from the emitters to the supply to provide bias current.

One collector grounded directly, the other connected to ground via
a resistor that provides the output voltage to the varactor.

Thanks for the clarification Phil. That\'s more or less what I had
going in LT spice. The tip about biasing the input higher though got
the circuit to work in the sense that it provides some gain, but
that\'s not sure what I want. here\'s what I have now:
https://photos.app.goo.gl/Zp2wmpdBcCtxxsUU7 For reference here is the
circuit I\'m using now that works just fine, the only downside being
the requirement for a negative rail:
https://photos.app.goo.gl/FCmdL4UFaEXsSFUV7

I\'m using an XOR phase detector (The double balanced mixer gave me no
end of trouble) and so the output of that after filtering is 0->2.5V
and experimentally I\'ve determined a gain of about 5 is what I want.
Correct me if I\'m wrong here but the problem is that the output of
that circuit can\'t be zero because of the quiescent current through
the resistor on the collector of the pnp (Rc2 in my schematic). I can
bias the input voltage to be higher with a simple pullup on the base
of the input pnp, but I really need the output to be 0V when the
input is 0V, else my varactor will have too high of a voltage on it
to begin with. The output range also doesn\'t seem that crash hot
(1.1->2.2V) either but I presume that I can fix that by fiddling with
the current source and the output collector resistor.
Click to expand...

If you\'re OK using an XOR gate, just use an op amp and move on.

Alternatively, gain up the 5V swing, and use that directly with an RC
lead-lag filter. (For FM demodulation you don\'t care about reference
frequency ripple, after all.)

Otherwise, putting down the mouse and doing a little bit of algebra will
make things clear in a big hurry. Those sorts of questions are easy to
figure out with a bit of thought.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On 7/22/2022 9:55 PM, Harry Dudley-Bestow wrote:
Well time has gone by and I have got the PLL working!
Input to a VCO is green, output from the PLL control signal is blue:
https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6
Click to expand...

It wants to verify i\'m me. Just to view a photo. So screw it!


It locks from between about 10-11MHz and can be used to demodulate FM audio just fine.
For the final \"productionised\" version I would like to remove the negative rail I am using for my loop filter amplifier.
Currently the loop filter is a simple two transistor diff amp:
https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6
However in order for the input to go to 0V, this requires a negative voltages source which for a battery powered device is not ideal.

My question is: What is the simplest possible amplifier where the input can go to the negative rail?

From some reading in the \"Design of Analog CMOS Integrated Circuits\" I understand that something may be possible with a pmos diff pair, but I cannot get anything to work in simulation. @Phil Hobbs any ideas?
Click to expand...
 
Sorry about that John, I checked the link in an incognito window so it should work without a login.

Phil the XOR gate was just an experiment to try and get to a working design and iterate from there. I have now got everything working again with a two-transistor NAND gate. This is of course the same as an XOR gate only it goes from VCC/2->VCC instead of 0->VCC. Now I don\'t have any problems with trying to make an amplifier where the input goes to the negative rail :D. (I couldn\'t get that to work anyway, even with pen and paper).
This also means that a simple current mirror can be used to bias the diff amp that previously required a negative rail. The output of course no longer goes to the negative rail either, but a couple of diode connected transistors takes care of that!
https://photos.app.goo.gl/b3rB4Nvdydrg7cAo6

Overall the design isn\'t particularly elegant but it does work whilst sticking to my original principles, and that\'s enough for me at this point. I just want to finish the damn thing.
 
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