[time-nuts] PN measurement of SSINE to CMOS converters

[Bruce Griffiths]

The technique I used to measure the PN contribution of the LTC6957-4 at 
10Mhz
Low noise 10MHz sinewave OCXO (Trimble 37265 removed from Thunderbolt 
(Thunderbolt has rather noisy output amplifier)). OCXO PN measured 
against a pair of similar OCXOs via correlation using the TimepodConnected 
to low noise RF amplifier (push-pull transformer feedback CB Norton 
amplifier NF ~ 2dB Z10043 from Clifton Laboratories) with a gain of 
~12dB.Output (~19dBm) from amplifier is split by a minicircuits 1:2 splitter 
(Minicircuits ZFSC-2-4)3dB 50 ohm passive attenuator reduces one output 
to ~ 14dBm this is connected to the Timepod reference input.
The other output from the splitter is attenuated with a passive 50 ohm 
attenuator to the desired input signal level for the DUT.
The DUT output drives a minicircuits 1:2 splitter via a lowpass filter.Each of 
the splitter outputs is connected to either Timepod Ch0 or CH2 via an RF 
amp (in my case a HELA10-D) with a gain of around 10dB.
Cross correlation ensures that the relatively high noise of the HELA10-D's 
isn't too significant, the trade off being that more time is required to 
average down their contribution to the measured noise.
I use an RF power meter to measure the output of each HELA10-Dto 
ensure that the Timepod input signal doesn't exceed the maximum rated 
input for the Timepod.
Alternatively if one has a sufficiently low noise amp like the Q-bit QB188 (NF 
~3dB Gain ~ 15 dB) a single amp connected directly to the Timepod signal 
input via a passive attenuator should suffice.
For most useful comparators or CMOS gates an ouput circuit like that on 
the LTC6957-4 evaluation board should suffice.

Bruce