[time-nuts] Distribution amps and isolation transformers

Bob Camp kb8tq at n1k.org
Mon Nov 24 17:54:37 EST 2014


If you decide to optimize the transformers, it’s pretty easy to do:

Bought in bulk (like by the pound) ferrite binocular cores (two hole ferrite beads) are quite cheap. The last batches I got were sub 10 cents each. You can get them in a variety of sizes and compositions. Obviously bigger is going to be more expensive than smaller. 

A surprisingly small number of bifilar turns through one of these is plenty to make a good broadband transformer. Think in terms of 4 to 8 turns. 

Yes, if you get the ones that are too small to see or work with, they are cheaper still. They also don’t handle a couple of watts of RF plus a few hundred ma of DC. The hand made ones are pretty rugged ….

You can put them on a network analyzer to see what’s going on. A signal generator and a power meter is perfectly fine for doing the testing. A tracking generator on a spectrum analyzer also is great. For most uses they fall apart for loss before much else goes wrong. 


> On Nov 24, 2014, at 2:39 PM, Joseph Gray <jgray at zianet.com> wrote:
> Although my current project doesn't have the strict requirements that
> many of you have, I thought I would mention part of it, due to the
> recent talk of distribution amps.
> I am putting one of the FE5680A's that I bought a while back into a
> permanent home. I have a nice box from a gutted HP microwave mixer
> that is just about the perfect size. I also have two surplus RGB video
> distribution amps rated for 100 MHz. After changing out the 75R
> resistors for 50R, I will mount the bare board in the HP box, along
> with the rubidium. Since the video amp BNC's are not isolated, I
> thought I'd better do that while I was at it.
> Being frugal, I used a heat gun to remove several transformers from
> old 3Com Etherlink III ISA cards. As most of you know, there is a
> single 1:1 transformer for the 10Base-2 and a dual 1:1 transformer
> module with low-pass filters for the 10Base-T.
> After removing the parts, I thought I'd characterize them on the
> spectrum analyzer. It was a good thing I did. Some of the transformers
> were obviously bad and had wacky curves. (the Ethernet cards were
> pulled from a scrap pile.)
> All of the dual transformer modules were within about 2 dB of each
> other. The -3 dB point being around 15 MHz and dropping to about 20 dB
> down or more at 30 MHz.
> The single transformers were of two types: square and oval with two
> flat sides. All of the oval transformers were almost flat out to  50
> MHz (I didn't test higher). The square transformers were odd. Their -3
> db point was around 8 MHz and then things gently sloped downward about
> 30 degrees for quite a ways. Being that both styles of transformer
> were being used for the 10Base-2 connection, I assumed that they
> should behave similarly. Rolling off at 8 MHz doesn't sound good for a
> 10 Mbit speed.
> The finished project isn't going to be anything fancy. I just wanted
> to make a semi-portable standard. I will divide things down so I also
> have 1, 5 and 10 MHz TTL outputs, in addition to the 10 MHz sine.
> Later, I may also incorporate a DDS that is locked to the 10 MHz to
> give me programmable sine and TTL outputs.
> I also spent all weekend tweaking both of the rubidiums via the serial
> port. Both of them had a zero offset programmed. Using a scope and
> triggering on my Z3801A, I slowly dialed things in until I got as
> little drift as possible. One rubidium needed to go low and the other
> high. BTW, using the calibrator program from VK4XV made things much
> easier when talking to the rubidiums serially.
> Joe Gray
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