[time-nuts] Re: Efratom MBF 5MHz distrib amp schematics & conversion to 10MHz

[Bob kb8tq]

Hi

As you go to ever narrower filters, the phase shift / group delay of the
filter goes up. Just how much very much depends on the filter type, 
filter order. 

With something like a 5 or 10 MHz Rb, narrow filters can get you into
trouble as the temperature changes and it. The edge of your output can 
shift by quite a bit. This degrades the timing accuracy of the signal. 
With narrow filters, it can impact the frequency stability.

Bob

> On Dec 13, 2021, at 9:34 AM, Wilko Bulte <wilko.bulte at xs4all.nl> wrote:
> 
> hi,
> 
> As a FYI:
> 
> Over the weekend I have checked 5 of these 5MHz MBF amplifiers. I think I can
> conclude that Efratom/Ball has individually matched C41 and C42 to the 5MHz
> center frequency. In other words: check what your amp has installed, even 1
> capacitor might be > 180pF. Happened to me on one unit.
> 
> Sofar all amps I could get centered on 10MHz with 150 or 180pF total
> capacitance.
> 
> Maybe even better 'peaking' can be achieved with a differen toroid coil but 
> I decided that that was too much hassle (these things are attached to the
> PCB globs of silicone sealant "goo").
> 
> Wilko
> 
> Quoting Wilko Bulte, who wrote on Mon, Nov 08, 2021 at 05:43:33PM +0100 ..
>> hi,
>> 
>> With many thanks to my ham pal Robert PE1MJA for patiently reverse
>> engineering it please find attached the schematics for the Efratom MBF 5MHz 
>> distribution amplifier.
>> 
>> Note that the MBF distrib amps come in different versions, the schematics 
>> show the 5MHz in / 5MHz out version.
>> 
>> Given that we want to use them for 10MHz a small change proved to be
>> required: 
>> 
>> - in the 1-st stage removing the 750pF capacitor (center bottom) made the 
>> center frequency of the amp move to 10MHz. That cap is in parallel with an
>> 180pF cap, leave that one in place. The cap to be removed is marked as C42
>> on the PCB silkscreen.
>> 
>> The amps are quite narrowly 'peaked' on their design frequency, in its original
>> form the MBF was not much of an amp for 10MHz.
>> 
>> We have both MBF with BNC outputs, as well as units for internal
>> distribution via their DIN41612 rear connectors. Both versions have 
>> slightly different parts values in places.
>> 
>> In two of my own units I found 2 different values for C42. Regardless,
>> ensuring that the total capacity is at ~180 pF peaked the amp at 10MHz. So,
>> also do inspect the cap in parallel with C42.
>> 
>> Yet to be resolved is the red Fault LED remaining lit with 10MHz, for now I
>> am ignoring that one.
>> 
>> Wilko
>> PA1WBU
>> 
>> NB: I also attached an OpenDocument spreadsheet with the connector pinouts 
>> for the various modules.
> 
> 
> 
> 
> --- End of quoted text ---
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