[time-nuts] RAPCO 1882M-2 autochangeover distribution amplifier

GandalfG8 at aol.com GandalfG8 at aol.com
Mon Oct 25 16:51:31 UTC 2010

In a message dated 25/10/2010 14:09:08 GMT Daylight Time,  
luciano.paramithiotti at hp.com writes:

I have  two distributors of frequency, one to be mounted in a rack where I 
have all my  frequency reference and the other one in my laboratory to 
distribute the FX to  the measuring instruments. 
Unfortunately domestic restraints here, and the fact that another  room is 
already racked out for equipment and parts storage, means that one  room has 
to contain time and frequency, radio, and computing racks, as well as  me, 
my desk, more shelving and my development/test/calibration area.
I've just about squeezed in all I can and it's quite a tight fit, next step 
 might have to be a serious crash diet:-)

I will limit myself to the described modification for the  laboratory one, 
while in the rack that I'm going to completely replace the  power supply 
with another constructed by me, always well a switching psu but  well filtered 
and in a small metal box that helps shielding  interference.
I'm on a bit of a crusade right now, trying to eliminate as many  
switchmode PSUs from the lab area as possible.
Radiated and conducted noise is not generally so much of a problem  at 
signal levels normally associated with frequency distribution but it  plays 
havoc at sub microvolt levels as I'm sure you also know only too  well.
There's a limit to how far I can move antennas away from the house and the  
noise levels from some switchmode wall warts in particular are horrendous.
Even well constructed and shielded switch mode PSUs can play havoc at  LF.

One  additional note and that the original PSU also generates a lot of 
interference  to the external power source.
That doesn't surprise me. I've got some toroidal transformers that might  
just fit that 1U case and they've enough windings to allow a fully linear 
setup,  so might give that a try.

My goal 'to have a noise output of less than 1 mV pp  measured without 
input signal. This' to avoid amplitude modulation that would  increase the 
uncertainty of measurement for the phase noise and  TIC.
I can't remember what the general noise floor was, as opposed to the  
pulsed interference, and the kit's all unpowered again right now, it was quite  
hight at LF but I would have expected 1mV to be attainable at 5 and 10MHz.

I have the operating manual of the Rapco. I think will be  very difficult  
to have the schematic diagrams of this  equipment.
Fortunately the input and output buffer stages are highly repetetive so  
tracing the circuitry shouldn't be too difficult. The biggest aggravation  is 
probably going to be getting at the underside of the PCB, I suspect that 
will  entail an almost complete strip down of the case.

For radio use, I will recommend this high dynamic amplifier  


(single +12 VDC)  followed by a passive splitter (ex.: minicircuit). 

Thanks for the link.
That's not the route I'd normally follow at LF/HF, I  have commercial and 
ex-mil antenna distribution units if I need  several signals from one antenna 
but generally try to just use minicircuits  splitters without amplification 
where levels allow, or with a Norton amp if  needs be, but will take a 
closer look at the device specs later.
My antenna tests on the Rapco were really just for a bit of fun, and more  
out of curiosity than anything, but at least with an outdoor active vertical 
the  LF noise floor was already comparable and, aside from the switchmode  
pulses, it did better than I was expecting
With a decent loop it would probably be a different story.

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