VCO in a PLL

An old engineer told me that although in theory you get improvements in demodulating FM via a PLL, in practice you cannot make a VCO which has low enough phase noise to compete with say a Quadrature Detector. 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?

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.

> 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"?


-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

On 12.1.14 22:27, 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.
>

That was a different thing. It was the PLL-based frequency
synthesis as the first heterodyne oscillator.

-- 

-Tauno Voipio

On Monday, January 13, 2014 9:27:51 AM UTC+13, 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.
> 
> 
> 
> > 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"?
> 
> 
> 
> 
> 
> -- 
> 
> 
> 
> r b-j                  rbj@audioimagination.com
> 
> 
> 
> "Imagination is more important than knowledge."

A PLL VCO need only be a square wave since the higher harmonics get filtered.

Pll may be best for linearity but not so good for thermal noise.

In a pll you typically apply the input to a comparator which feeds the phase detector, and whenever you have a thresholding operation you will sample the noise and this causes aliasing. By contrast , a detector based on tuned LC circuits does not suffer from aliasing, but it typically relies on the open- loop phase or amplitude response as a function of frequency, which is not really a straight line. So the distortion tends to be high but the noise is quite good. 
There's actually a lot of depth in the design of fm radios which is largely lost these days. I should have studied a bit more before trying to tune the IF stage of my old Advent receiver using the  "seat- of- the- pants" method!

Bob

A PLL based demodulator can  enjoy a few dB of threshold extension. The phase noise of the VCO is not usually an issue.
Many analog video satellite receivers used a PLL demodulator for threshold extension.
See Gardner and search terms threshold extension.
Mark

On Monday, January 13, 2014 11:38:45 AM UTC+13, mako...@yahoo.com wrote:
> A PLL based demodulator can  enjoy a few dB of threshold extension. The phase noise of the VCO is not usually an issue.
> 
> Many analog video satellite receivers used a PLL demodulator for threshold extension.
> 
> See Gardner and search terms threshold extension.
> 
> Mark

yes that's the theory and Gardner is an old book. As i said, industry prefers the Quad detector since the noise floor is lower (even though the PLL should give a lower noise floor).

On Sunday, January 12, 2014 6:01:14 PM UTC-5, gyans...@gmail.com wrote:
> On Monday, January 13, 2014 11:38:45 AM UTC+13, mako...@yahoo.com wrote:
> 
> > A PLL based demodulator can  enjoy a few dB of threshold extension. The phase noise of the VCO is not usually an issue.
> 
> > 
> 
> > Many analog video satellite receivers used a PLL demodulator for threshold extension.
> 
> > 
> 
> > See Gardner and search terms threshold extension.
> 
> > 
> 
> > Mark
> 
> 
> 
> yes that's the theory and Gardner is an old book. As i said, industry prefers the Quad detector since the noise floor is lower (even though the PLL should give a lower noise floor).

One very significant discriminator between ADC-FPGA and analog IC is cost. 

radams2000@gmail.com writes:

> In a pll you typically apply the input to a comparator which feeds the
> phase detector, and whenever you have a thresholding operation you
> will sample the noise and this causes aliasing. 

Bob,

If we were doing everything in the digital domain, couldn't we convert
the input into a complex envelope and, assuming the VCO was also
complex, do a real phase difference, thus avoiding thresholding?
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com