[time-nuts] Odd-order multiplication of CMOS-output OCXO

[Mark Haun]

Hi Jim,

On Sun, 19 Jan 2020 10:35:42 -0800
jimlux <jimlux at earthlink.net> wrote:
> On 1/19/20 9:29 AM, Mark Haun wrote:
> > On Sun, 19 Jan 2020 09:37:39 -0500
> > Bob kb8tq <kb8tq at n1k.org> wrote:  
> >> Is your intended application tolerant of spurs at 16 and 32 MHz? If
> >> not, do they need to be in the 90 dB down vicinity (= the SFDR of
> >> the ADC) ?  
> > 
> > I guess you mean stray coupling between the oscillator, clock
> > conditioning circuitry and the analog inputs?  (Spurs on the ADC
> > clock input shouldn't matter as long as the zero crossings are
> > clean and jitter is low.)
> 
> Not exactly.  The sampler of the ADC is essentially a mixer, so if
> the clock has other signals on it, even at low levels, they can mix
> with input signals and show up in band.  I had a SDR receiver with a
> 49.244 MHz ADC clock that was contaminated by the 66MHz processor
> clock (at a very, very low level), and I saw mixing products when the
> input to the ADC was a clean sine wave at 112.5 MHz.
> 
> Analog Devices even has an app note on this.
> 
> https://e2echina.ti.com/cfs-file/__key/telligent-evolution-components-attachments/13-109-00-00-00-00-93-58/Impact-of-sampling_2D00_clock-spurs-on-ADC-performance.pdf

Hmmm, so in my case, other residual odd-order harmonics of the 16 MHz
input clock which make it through the multiplier will become
non-harmonic spurs of the desired 80 MHz, and therefore a potential
problem unless filtered out.  The analog amplifier scheme will
therefore require decent bandpass filtering, mainly against 16, 48, and
112 MHz.

One advantage of the Wenzel CMOS-based multiplier is that the threshold
behavior of the last inverter [mostly?] gets rid of everything but the
selected harmonic.

I'm still trying to understand the phase-noise pros/cons of that design
using, say, a pair of NC7SZ04 (UHS family) gates, versus a discrete
transistor amplifier tuned at 80 MHz, like the common-base design
quoted in the original post.

Regards,
Mark