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

Mon Oct 25 13:08:21 UTC 2010

Hi Nigel,

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. 

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.

One additional note and that the original PSU also generates a lot of interference to the external power source.

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 have the operating manual of the Rapco. I think will be very difficult  to have the schematic diagrams of this equipment.

For radio use, I will recommend this high dynamic amplifier 

 http://yu1aw.ba-karlsruhe.de/bfg135aeng.htm

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

Ciao, Luciano

Luciano P. S. Paramithiotti
IZ5JHJ

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of GandalfG8 at aol.com
Sent: domenica 24 ottobre 2010 2.14
To: time-nuts at febo.com
Subject: Re: [time-nuts] RAPCO 1882M-2 autochance over distribution amplifier

In a message dated 22/10/2010 14:09:13 GMT Daylight Time, luciano.paramithiotti at hp.com writes:

I bought  a distribution amplifier Rapco model 1882M-2 auto changeover on ebay. This  distribution amplifier was intended to be used in conjunction with a GPSDO to  distribute 5MHz and 1PPS.

It has two inputs for 5MHz and and two for  1PPS, and have nr14 5Mhz outputs and two 1PPS. Switching between the two  couple of input is automatic. 
Normally each master input works, when the the  master drop,the input source is switched to the second input.

This  equipment uses wideband amplifiers  with ft100 MHz and I verified that it  works perfectly with no attenuation at 10 Mhz. The gain of the Rapco is 1 and  is not adjustable.

I noticed, however, that without the input signal,  at the exit, there is noise due to the two internal switching regulators that  generate -6 and +11 volts from +24 vdc input. The noise at the 5 Mhz output is  60mV pp(sum of the two switching noise).

I reduced the noise to 10mVpp,  cuttingin two tracks on pcb, from power output from the switchings and  inserting an inductance 300uH (.8 A)than putting an electrolytic 470UF 16V.  from inductance to ground (respecting their polarity).

---------------
Hi Luciano

Thanks for the information, I'd been planning on replacing those two switching regulators with linear but that's not so straightforward for the negative supply so might experiment with your suggestion.

>From the description in the manual I guessed in advance these might be wideband and took a gamble on that, once confirmed on my first one I  bought a couple more:-)

Aside from frequency distribution I was also curious to try them as  LF/MF antenna distribution units.
I knew they were likely to be quite noisy at microvolt levels but  there's quite a bit of noise at LF anyway and the op amp noise actually  wasn't too bad.
However, when feeding into a radio with a 30MHz wide spectrum  display the effects of those switching regulators are immediately obvious.  They're running at 100KHz and the harmonics generate a nice picket fence that's  clearly visible up to at least 20MHz.

I'd still like to get rid of those switching regulators altogether but could really do with a schematic before trying that.

I assume you got a copy of the basic user manual from the seller? If not let me know and I'll send it to you.

regards

Nigel
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