[time-nuts] 35601A as stand-in for 11848A ?

[Bruce Griffiths]

SAIDJACK at aol.com wrote:
> Hi guys,
>  
> does anyone have any experience with "home-brew" quadrature-PLL PN  
> measurements? Renting/buying a E5052A or TSC5120A does get to  be expensive.. 
>  
> I am using different mixers (Mini Circuits ZFM-150 and ZFM-4) and the noise  
> floor coming out of these seems much higher than I expected. Any  suggestions 
> on mixers anyone? How about the low pass filter and amplifier?
>  
> I built an LC 8th-order 1.5MHz lowpass, and I wonder if the reflected  energy 
> (at 10MHz, 20MHz, etc) is causing noise and  intermodulation in the diodes 
> etc.
>  
> I am using an EL2125CS opamp (<1nv/rtHz noise) for the PLL and the Audio  
> amp, with 40 or 60dB gain for the Audio amp.
>  
> Tried different power into the mixer (0dBm, +10dBm, +13dBm etc), this does  
> not seem to affect the noisefloor, which I believe to be at least 25dB too  
> high.
>  
> Thanks for any hints,
> bye,
> Said
>  

Said

What is the actual  mixer noise floor?
What are the RF and LO port signal levels.

For lowest noise at offsets of up to a few kHz terminate the mixer IF
port in a shunt capacitor.

For a suitable low noise amplifier see:
http://www.ko4bb.com/~bruce/LNMP.html
<http://www.ko4bb.com/%7Ebruce/LNMP.html>

NB this amplifier has a gain of 60dB so may not be suitable for noisy
oscillators.
It has much lower noise than the EL2125Cs at low offset frequencies.

The mixer IF port dc load should be small (large R) for maximum signal
and lowest noise.
Both LO and RF mixer ports should be saturated for maximum output and
minimum noise..

Improving the mixer RF and LO port VSWR by using resistors in series
with the RF and LO ports can be helpful.
 
Bruce