[time-nuts] Re: What about the frequency discrimination method? (offshoot from DIY PN analyzer)

[Magnus Danielson]

Hi Ed,

On 7/10/22 20:26, ed breya via time-nuts wrote:
> Hi Magnus,
>
> I know what you mean about not needing a quadrature splitter - if you 
> have a very wide phase or delay tuning range - but I'm picturing 
> getting most of the way to quadrature with a fixed structure for a 
> given frequency, and only fine-tuning the phase over a narrow range, 
> in order to minimize the PLL's overall noise contribution. This should 
> also keep it monotonic - too wide a range may let it get stuck on the 
> humps.

Well, the traditional method is to lock it up. When you do that 
Quadrature (PM) comes for free, but In-phase (AM) does not. The lock 
require either oscillator to be frequency steerable to achieve lock. If 
that is not what you want to do, well then you can use a non-synchronous 
method and then a quadrature splitter is the way to go. This is 
essentially what the modern oversampling measurement devices do anyway. 
They just sample the waveforms, convert the IQ into amplitude and phase, 
and then decimate down from that. Comparison with reference is then done 
in that context after the fact. Frequency errors can be handled within 
some fairly flexible range.

Similarly, just mixing unlocked sources with I/Q setup produces some 
beat-note and if you get that as I/Q you can do more or less the same thing.

>
> For amplitude calibration, I'm picturing rearranging the splitter 
> ports or guts somehow (as simply as possible) to present the DUT 
> signal to the mixer at 0 or 180 degrees, which should give a maximum 
> DC out.

For amplitude measurement, you want the in-phase component rather than 
the quadrature component, if they are locked that is. Using a normal 
lock, the direct lock phase-detector actually gives you the quadrature, 
so you need the splitter to get the In-phase too. For non-synchronous 
receivers, you need to track the beat-note to sort out what is the 
in-phase and quadrature for AM and PM respectively. An abs/arctan 
function (such as CORDIC) will convert I/Q samples nicely and the 
frequency error will be just a phase-ramp on the detected phase.

>
> BTW I had never heard of the Tayloe detector, but it appears to be the 
> method (4x f multiplier then digital quadrature divide) used in 
> lock-in analyzers, and I have used the same in a number of projects.
Yes. It is fairly widely used these days. Seems to have good enough 
properties for many things.
>
>
> Azelio wrote:
> "How can you measure something, any type of measure, not only PN,
> without a reference? Voltmeters need voltage references, "timemeters"
> (and frequency meters) need time references."
>
> Azelio, this is a well known technique - I haven't described anything 
> new, just a particular implementation I've been pondering.

Indeed. There is many ways to measure without an actual reference. There 
might be other oscillators, but they are not "reference" in the way of a 
voltage reference for instance.

Cheers,
Magnus