10 MHz Distribution Amplifier
Introduction
The 10 MHz Rubidium Frequency Source (RFS) signal is used in a number of areas. The need arises to provide multiple connections to various devices. As the requirements are fairly simple here at HawkRAO - merely needing some form of buffering of the RFS output - an alternative to the relatively expensive distribution amplifiers was sought.
The 10 MHz RFS signal can now connect to different devices, and to some extent, it is safeguarded by isolating its connection from the rest of the equipment.
Implementation
Some investigation into building a distribution amplifier from scratch was done - but while looking through my collection of miscellaneous electronic gear I came across a 'Video Enhancer". While not thought suitable for the purpose of a distribution amplifier itself, it occurred to me that the bandwidth of video distribution amplifiers should just be sufficient. A search on eBay turned up a candidate - advertised as "Car Video Booster Amplifier Splitter Box 1 Input To 4 Output RCA Easy Mounting". I ordered one at a cost of AUD$ 17 to investigate. NOTE: these are 'analog' video units - not digital.
Testing for Suitability
This device is designed for use in a car for running a number of displays from one video source. As such, it is powered by 12V DC. Upon powering up it was found the power LED did not come on - and only 3 out of the 4 outputs showed life when the 10 MHz RFS signal was input.
Disassembling and inspecting the unit found 2 dry solder joints which - when repaired - restored correct operation. Seizing the opportunity, a number of other solder joints were redone where the connection looked suspect. It was noted that there were several instances where the track had lifted from the substrate and solder blobs were used to 'beef up' the connection. (Update: several more of these units were purchased and found not to have these issues).
Testing showed that the gain starts to drop at frequencies higher than about 7 MHz - which is what would be expected for a video amplifier. Nonetheless - the RFS 10 MHz input signal was passed to the outputs with a modest amount of gain (~1 dB) and acceptable wave shape as shown in the oscilloscope trace below.
Additional Noise
A check for introduced noise was done using the 10 MHz output of this distribution amplifier as the primary reference into a clock multiplier which, in turn, is the reference clock for a MAX2870 Signal Generator set to output 1.5 GHz. This output signal was examined for noise sidebands. No significant increase in noise was observed.
Modifications Done
While entirely suitable for the purpose of a 10 MHz distribution amplifier, a number of modifications were done to the unit.
The RCA connectors are not suitable for the HawkRAO testing setup. At first it was considered to pass small diameter coaxial cable, with flying SMA end connectors, through the RCA connectors and hardwire the coaxial cable internally. Instead the simple - although costly - alternative of using RCA-plug-to-BNC-socket adaptors was chosen. Ironically the cost of the 5 adaptors exceeded the cost of the amplifier. Nonetheless, the total cost remained below AUD$40.
The outputs are AC-coupled through electrolytic capacitors. As there is no load placed on the output connections on the PCB, the leakage current through the electrolytic capacitor meant that the DC voltage on the outputs sat around the 12V supply level when unconnected. This meant that at the moment of connection of an external device it would be presented initially with 12V - not good. To prevent this, a 56 ohm resistor was placed across all the outputs, which discharges any leakage current from the capacitor whilst not loading down the output significantly.
The voltage regulator IC is un-heatsinked, and so gets very hot (~70 C) even when the cover is off. This temperature is a little too high for comfort. The only purpose of this regulator is to bring the designed-for automotive 12V supply voltage down to around 8V. As this unit will likely be running 24/7, the energy used by this voltage drop is unnecessary waste. To eliminate this wasted energy - albeit minor - the regulator was removed and a bridging link added in its place. Now the power supply directed into the unit is 9V.
The clearance between the bottom of the PCB and the base - although well painted - was a little too close for comfort. A suitably sized piece of thin cardboard was placed in the base upon reassembly.
Conclusion
Although not perfect, the use of a video booster intended for automotive applications was found to be adequate for purpose at HawkRAO as a 10 MHz distribution amplifier.