On Tuesday, May 17, 2016 at 3:12:58 AM UTC+12, rickman wrote:
> On 5/13/2016 11:44 AM, Tim Wescott wrote:
> > On Thu, 12 May 2016 23:05:39 -0400, rickman wrote:
> >
> >> On 1/12/2014 3:27 PM, robert bristow-johnson wrote:
> >>> On 1/12/14 3:05 PM, gyansorova@gmail.com wrote:
> >>>> An old engineer told me that although in theory you get improvements
> >>>> in demodulating FM via a PLL,
> >>>
> >>> dunno what that theory is.  but i *do* remember that "PLL front end" or
> >>> something like that was a selling point for stereo receivers back in
> >>> the 70s.  i think the PLL was also part of the AFC function.
> >>>
> >>>> in practice you cannot make a VCO which has low enough phase noise to
> >>>> compete with say a Quadrature Detector.
> >>>
> >>> dunno.  have to see the specific circuit, system, and specs on the
> >>> parts.  i dunno what phase noise is in a VCO.  i think of "phase noise"
> >>> as a term applied to roundoff applied to the index to a table-lookup
> >>> wavetable oscillator.
> >>
> >> Doesn't phase noise also apply to analog components?
> >>
> >
> > <snip>
> >
> > Indeed it does, and analog PLL's both reduce it in places and increase it
> > in places, which makes the design challenging.
> 
> Indeed...
> 
> -- 
> 
> Rick C

That's why PLLs are out of fashion in many FM receivers. They use Quad detectors instead.

On 5/13/2016 11:44 AM, Tim Wescott wrote:
> On Thu, 12 May 2016 23:05:39 -0400, rickman wrote:
>
>> On 1/12/2014 3:27 PM, robert bristow-johnson wrote:
>>> On 1/12/14 3:05 PM, gyansorova@gmail.com wrote:
>>>> An old engineer told me that although in theory you get improvements
>>>> in demodulating FM via a PLL,
>>>
>>> dunno what that theory is.  but i *do* remember that "PLL front end" or
>>> something like that was a selling point for stereo receivers back in
>>> the 70s.  i think the PLL was also part of the AFC function.
>>>
>>>> in practice you cannot make a VCO which has low enough phase noise to
>>>> compete with say a Quadrature Detector.
>>>
>>> dunno.  have to see the specific circuit, system, and specs on the
>>> parts.  i dunno what phase noise is in a VCO.  i think of "phase noise"
>>> as a term applied to roundoff applied to the index to a table-lookup
>>> wavetable oscillator.
>>
>> Doesn't phase noise also apply to analog components?
>>
>
> <snip>
>
> Indeed it does, and analog PLL's both reduce it in places and increase it
> in places, which makes the design challenging.

Indeed...

-- 

Rick C

On Thu, 12 May 2016 23:05:39 -0400, rickman wrote:

> On 1/12/2014 3:27 PM, robert bristow-johnson wrote:
>> On 1/12/14 3:05 PM, gyansorova@gmail.com wrote:
>>> An old engineer told me that although in theory you get improvements
>>> in demodulating FM via a PLL,
>>
>> dunno what that theory is.  but i *do* remember that "PLL front end" or
>> something like that was a selling point for stereo receivers back in
>> the 70s.  i think the PLL was also part of the AFC function.
>>
>>> in practice you cannot make a VCO which has low enough phase noise to
>>> compete with say a Quadrature Detector.
>>
>> dunno.  have to see the specific circuit, system, and specs on the
>> parts.  i dunno what phase noise is in a VCO.  i think of "phase noise"
>> as a term applied to roundoff applied to the index to a table-lookup
>> wavetable oscillator.
> 
> Doesn't phase noise also apply to analog components?
> 

<snip>

Indeed it does, and analog PLL's both reduce it in places and increase it 
in places, which makes the design challenging.


-- 
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work!  See my website if you're interested
http://www.wescottdesign.com

On 1/12/2014 3:27 PM, robert bristow-johnson wrote:
> On 1/12/14 3:05 PM, gyansorova@gmail.com wrote:
>> An old engineer told me that although in theory you get improvements
>> in demodulating FM via a PLL,
>
> dunno what that theory is.  but i *do* remember that "PLL front end" or
> something like that was a selling point for stereo receivers back in the
> 70s.  i think the PLL was also part of the AFC function.
>
>> in practice you cannot make a VCO which has low enough phase noise to
>> compete with say a Quadrature Detector.
>
> dunno.  have to see the specific circuit, system, and specs on the
> parts.  i dunno what phase noise is in a VCO.  i think of "phase noise"
> as a term applied to roundoff applied to the index to a table-lookup
> wavetable oscillator.

Doesn't phase noise also apply to analog components?


>> This is also the feeling in industry. I would have thought that a VCO
>> could be made near perfect with an FPGA nowadays - or am I missing
>> something?
>
> do FPGAs have analog components inside?  i generally think of them as
> all-digital parts.  or do you mean the digital counterpart to a VCO,
> usually called an "NCO"?

FPGAs have internal PLLs, but they are intended for clock generation and 
I don't think would be suitable for an FM receiver, unless it was a SDR, 
all digital receiver.

A digital PLL can be designed with as low phase noise as required by the 
application.  Ultimately the limit will be the phase noise of the 
reference oscillator or if the output is analog, the DAC.

-- 

Rick C

On Wed, 11 May 2016 10:44:52 -0700, mariam876am wrote:

> HowtodemodulatedtheFMsignalwithhighfrequencysuchas1GHZ,

1: If you have a new question, ask it in a new thread.

2: use spaces.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

HowtodemodulatedtheFMsignalwithhighfrequencysuchas1GHZ,

On Wednesday, January 15, 2014 1:45:08 AM UTC+13, radam...@gmail.com wrote:
> Again I'm not really an RF guy but my understanding is that your "classic" IF strip (circa 1970) in an FM radio has multiple stages of LC band pass filters with distributed gain and near the end of the chain the signal is limited pretty severely to keep AM noise from turning into FM noise and also to enhance the capture ratio. 
> 
> I don't remember if the AGC is also done in the IF strip, or in the front- end. 
> 
> 
> 
> Bob

That is pretty much right. The limiter attempts to stop AM getting into FM. This works ok for high SNRs but not for low however.

On Tue, 14 Jan 2014 04:45:08 -0800, radams2000 wrote:

> Again I'm not really an RF guy but my understanding is that your
> "classic" IF strip (circa 1970) in an FM radio has multiple stages of LC
> band pass filters with distributed gain and near the end of the chain
> the signal is limited pretty severely to keep AM noise from turning into
> FM noise and also to enhance the capture ratio.
> I don't remember if the AGC is also done in the IF strip, or in the
> front- end.

I posit that you don't remember where the AGC was done because a lot of 
them didn't have AGC - it isn't needed with the typical limiting IF chain.

The NE604 datasheet shows a typical circuit with five cascaded emitter 
coupled pairs (with an external IF filter between gain stages 2 and 3).
An emitter coupled pair has (approximately) a tanh transfer function.  
Cascade a bunch and you have a pretty good limiter.

Regards,
Allan
I'm not really an RF guy either.

Again I'm not really an RF guy but my understanding is that your "classic" IF strip (circa 1970) in an FM radio has multiple stages of LC band pass filters with distributed gain and near the end of the chain the signal is limited pretty severely to keep AM noise from turning into FM noise and also to enhance the capture ratio. 
I don't remember if the AGC is also done in the IF strip, or in the front- end. 

Bob

On Tuesday, January 14, 2014 1:40:31 PM UTC+13, radam...@gmail.com wrote:
> Tim
> 
> 
> 
> RF is not really my domain, so I'll take your word about the noise aliasing. What I was imagining was an unfiltered sinusoid with wideband noise, applied to a comparator feeding a type 2 phase detector, but in reality the signal is very narrowband by the time it arrives (and also heavily limited as you mention) so maybe this prevents the noise from turning into jitter at the pll input. 
> 
> Regarding the LC detectors I am moderately familiar with the S-curve method that dates back to the 50's I would guess. I recall it was in the Heathkit receiver I built in the 60's, and you were lucky if you got 0.5% THD. I found a good reference on FM detectors, below. 
> 
> 
> 
> http://www.radio-electronics.com/info/rf-technology-design/fm-reception/fm-demodulation-detection-overview.php
> 
> 
> 
> Bob

Limiters are not filters. They only convert amplitude fluctuations into phase noise. In the case of FM this is not a good thing since the derivative of phase is needed to demodulate it! Hence it is known that for poor carrier to noise ratios it is often better to get rid of the limiter altogether since it only amplifies the noise.