One of the important components of my GPS-disciplined oscillator project is the signal distribution board.  The oscillator sends its pure signal through an isolation / buffer amplifier, then through an RF splitter.  The splitter sends one copy of the signal to the digital control circuits and another copy to the distribution board.

This board has two Linear Technology LT6551 quad video amplifier chips which are configured for 50Ω impedance instead of the usual 75Ω.  The amplifiers offer good performance at 10 MHz and provide good isolation between channels.

The input signal comes in through an SMA board edge connector and leaves through eight tiny U.FL connectors.  Pigtail cables will carry the signals to BNC female connectors that can be panel mounted in the overall enclosure.

Later, I'll machine an aluminum housing out of aluminum bar stock to shield the board from other subsystems in the main enclosure, namely the digital board.

I got 3 printed circuit boards made with gold plating.  I included pads in the input signal path to install an optional matching pi network, but there is also a 50Ω input resistor (actually, two 100Ω resistors), so either can be used.  The pi network was overkill, but it cost nothing to leave the provision other than a 0Ω jumper to bypass the inductor position.

Bare Board
Bare Board
(click for a larger view)

Board #1 Under Construction
Board #1 Under Construction
(click for a larger view)
Completed Prototype
Completed Prototype
(click for a larger view)
Board #1 Under Test
Board #1 Under Test
(click for a larger view)

I measure about 67 dB of adjacent port isolation at 10 MHz and better than 0.5% total harmonic distortion although the THD test needs to be redone with a preamp because the harmonics were at the noise floor of the instrument.  Real THD is probably much less than 0.5%.

Gain Compression
Gain Compression
(click for a full view)
Adjacent Port Rejection
Adjacent Port Rejection
(click for a full view)
Total Harmonic Distortion
Total Harmonic Distortion (Preliminary)
(click for a full view)

I'm pretty happy with the board.  One change that I'll make is use a different LDO regulator on the final version.  Fortunately, that's not a design change -- just a part substitution.  The prototype has a 5V regulator that was intended to be supplied with 12V, but the power dissipation is way too high.  7V is much better.  By using a 3.3V regulator and driving it with 5V, it should run happily.

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