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

[Magnus Danielson]

Ed,

On 7/9/22 22:26, ed breya via time-nuts wrote:
> I've been following the thread about Erik's DIY PN analyzer, and 
> wondering if it might be easy enough to use a frequency discrimination 
> method. I'm opening this in a different thread to avoid muddying the 
> water on the original (and long) one.
>
> What I'm picturing is putting the DUT's output into a quadrature power 
> splitter that optionally has a voltage-tuned slight phase shift 
> feature. The I and Q outputs would go into the DBM and produce the 
> nearly-zero DC plus baseband signal for analysis as in the original 
> story.
>
> If the quadrature is precise and stable enough, the DC out should be 
> close to zero, and since the baseband is ultimately AC coupled to the 
> analyzer, small offset should be OK, within reason.
>
> If this is not sufficient, then having a phase tuning feature could be 
> used to form a PLL to hold the DC at zero. The big difference here is 
> that instead of locking a separate reference source to the DUT, the 
> relative phase at the mixer just has to be fine tuned to maintain the 
> output DC. The same sorts of PLL requirements are encountered to get 
> the results, but no external reference (and its noise and lock range 
> etc issues) is needed.
>
> The downside is that a different quadrature splitter would probably be 
> needed for each DUT frequency to be applied - I'm picturing ones for 5 
> and 10 MHz initially. Those 90 degree broadband splitters that Mike 
> mentioned seem very interesting too.
>
> There is still the necessity of calibration, either way.

There is no need for it. Using the PI loop, it will drive the phase 
detector into quadrature and as it does this, the DC component of the 
detector is cancelled as it is integrated into the integrator path I.

There is however use for quadrature splitter as you do a Costas loop, 
which is needed for some modulation schemes. Then again, you really do 
not need to use a quadrature splitter to achieve the needed quadrature 
pair, but there is other tricks to achieve the same thing. The Tayloe 
detector comes to mind, which uses a frequency 4 times higher, divides 
it down and then drive the detector for a S/H style of mixer. See for 
instance Elecraft KX3.

Cheers,
Magnus