Wolfgang Sauerwein DL 8 JT
this article, DL8JT describes his home made Z3801A distribution
amplifier using commercially available Maxim evaluation kits
and parts from surplus PC Ethernet boards.
He has also found an interesting note in an old HP
"Bench-brief" about ground-loops.
of us have more than one piece of equipment to be synchronized
at 10 MHz to the Z3801A "in house standard". Commercially
available 10 MHz-distribution-amplifiers are too expensive (even
on Ebay). Building it with many outputs with discrete components
is a frustrating job. Fortunately, Maxim-IC
has an IC (MAX
4135 with a gain of 2 or MAX4136 with variable gain) as
a video distribution amplifier. Each device has 6 analog outputs
with a bandwidth from DC to 185 MHz!
these chips are available only as "hard-to-handle" SMD versions
and Maxim recommends GHz proof layout for the board (no prototype-boards)
But now, best of all, Maxim offers an evaluation
kit for a very reasonable price (about US-$ 65. in Europe),
which includes a completely assembled and tested board. It
is designed for 75 Ohm impedance and is equipped with the fixed-gain
MAX4135, but you can get a free sample of the MAX 4136 (variable
gain) with your order. The board requires + and - 5 Volts only.
I don't understand why they don't use the MAX4136, which would
make it easier to adjust the output level, especially because
the board is designed for the MAX4136 as well; you just need
two SMD-resistors or a pot. (Nevertheless,
I will not try to replace the 24 pin SMD chip due to missing
SMD experience after 40 years as an electronic engineer !)
The gain of two will just compensate the loss in the serial
output resistor of 75 Ohms with the termination-resistor of
75 Ohms in a coaxial video systems. Input and the six outputs
are gold-plated SMA-connectors.
If you download the data sheets, use caution with the size of
the board layouts on the MAX4135 Kit data sheet, they are scaled,
although they appear to be 1:1 for quick reproduction and I
was fooled by that !
While I was waiting for my shipment from Maxim I started to
build a prototype board (100x160mm) with transformers and isolated
BNC output connectors, assuming the size (74x59mm) of the layout
on the Maxim data sheet, but now the actual boards are larger
(95x76mm) and I have some trouble sandwiching the two boards
as intended !
I planned to sandwich the two boards to save 6 each coaxial
cables with SMA connectors by making a direct vertical connection
between the SMA center pins and the transformers on the connector/transformer-board.
This will save 6 costly SMA cables.
photo shows how I originally intended to mount the Maxim kit
on my prototype board. I will probably cut my Maxim boards between
the SMA input connector and the channel enable DIP switch. Then,
solder the unnecessary enabling lines to ground and drill two
additional mounting holes, or just let the Maxim board extend
Caution: When you receive your kit, all outputs
will be disabled; set all DIP's to "ON" to enable! For this
extreme accurate 10 MHz reference, I decided to isolate each
output from the others to prevent ground loops (like on your
stereo equipment). This requires isolated BNC jacks and 10 MHz
RF transformers (i.e. Minicircuits T 1-1). These components
cost about US-$ 10.- per channel, but I found
a very cheap solution!
Try to get obsolete 10 Mbit Ethernet PC boards from any source
(i.e. your company, any company near your location or Ebay). I
was able to purchase 19 unused, brand new boards for a total
of US $ 5 only on Ebay, including a BNC-T connector and a diskette,
because 10 MHz only is absolutely obsolete. Modern
10/100 MHz boards are available now between 5-10 dollars, but
these do not have the BNC output any more!
By Ethernet specs, all outputs have to be isolated by transformers
and almost all designs are more or less identical. The BNC coax
connection uses a floating IC (typically a 83C92P) which is
connected to the rest of the logic by an IC like chip containing
3 each 1:1 10 MHz wideband-transformers (One for transmit, receive
and collision signals, similar to the Minicircuits T1-1). Data
sheets from various manufacturers are available on the internet.
I have found that that some, but very few of the Ethernet transformers
contain internal termination resistors, especially those for
the RJ-45 jack. This will degrade the output of the 10 MHz signal
drastically. Check your component specifications via the Internet. The only
missing spec is the maximum power of these transformers, but
it should be adequate for our applications. Two of these triple
transformer chips can isolate the six outputs of the MAX 4135/6.
Most of the boards contain many more "goodies" !
A necessary power source for the isolated IC is a DC/DC converter.
This is typically the largest module on the board and is a DC-101
from YCL. It converts 5 V to 9V DC at about 280 mA. If your
boards have an additional RJ-45 twisted pair connector, you
will find another chip with 2 each RF-transformers with integrated
baluns and a 2/3-pole 17 MHz low pass filters for receive and
transmit (typically YCL 20F001). These can be used instead of
the triple transformers if you want some low pass filtering
of your output or if you need the 10 Mhz signal at a remote
site via twisted pair copper cable.
The isolated BNC-Connectors are typically from AMP and most
boards have a 20.000 MHz crystal plus other components like
capacitors and SRAM's usable for other applications. For a total
of 12 outputs to feed all of my equipment, I will use two sets
of boards in a sandwich style construction.
Due to the uncritical low frequency of 10 MHz I do not care
about impedance differences between 50 Ohm and 75 Ohm of the
MAX-4135-EV-kit. In the past we used silver-plated cavities
at frequencies above 144 MHz; today PC's are clocked over 2
GHz on epoxy boards designed by digital oriented engineers!
of the commercial equipment like counters and synthesizers specify
an input impedance of about 1 kOhm for external 10 MHz sources,
but even with a specified 50 ohm everything should work well.
If you run out of ports, several units should be able to share
one channel in parallel. However, this would be against the
ground loop isolation requirements!
My installation which will be housed in an "cannabilized"
HP cabinet. I will connect the inputs of two distribution
amplifiers in parallel to the output of the Z3801A. The twelve
output connectors will be on the back. On the front I will have
an additional 10 MHz and an 1 pps output derived from the differential
(quasi ECL signals) available at the RS 422 / RS 232 connector.
This DB-25 connector will be extended to another DB-9 or DB-25
connector at the rear-panel of the cabinet.
To prevent overloading the input of the MAX4135 with the output
of the Z3801A, I will replace the input-termination-resistor
R7 of the kit with a miniature variable CERMET-variable-resistor
of 100 Ohms, (don't use wire-wound versions !) which has about
the distance between it's outer wires as the SMD-termination-resistor
R7; cut the trace between the printing "R7" and connect the
input (Pin20) to the wiper of the pot.
This will allow me to adjust the input until clipping of the
output occurs. Unfortunately I have no schematic drawing software
to draw complete diagrams for you, but the information above
should be understandable by most anyone involved in electronics.
I have not finished my project yet, and additional ideas from
other users are highly welcome!
Sauerwein DL 8 JT