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RF Attenuators

Signal reflections are an inevitable occurrence that arise when a high speed signal meets an impedance blip on it's passage from transmitter to receiver. These reflections are one of the many sources of added jitter in every digital system. Reducing the strength of these reflections should result in a reduction in the jitter generated by the transport to DAC connection. When used in digital audio, these RF attenuators can reduce jitter by reducing cable reflections. By using these attenuators on a digital cable the strength of the signal is reduced & as a consequence the signal reflections are reduced by twice the amount so a 10dB attenuator will reduce the signal by 10dB but the reflections by 20dB. 


To use these attenuators the reduced SPDIF signal has to be still capable of working in your system. This is certainly the case for the Hiface which has a high SPDIF signal to start with & may apply to other transports/receivers. They only come in BNC format but people with RCA connectors can still use them by getting a BNC/RCA adapter. The signal will still be improved even using this adaptor. I have tested a RCA SPDIF cable (with a BNC to RCA adaptor) with & without one of these attenuators & the sound was noticeably smoother & without edge with the attenuator in-line.

 

These attenuators are like small in-line adaptors that can be connected at the start or end (or both?) of your SPDIF cable. Here is the cheapest place I have found on the internet for these attenuators at $12 each! I believe this is cheap enough to allow experimenting with the range of values available.  http://217.34.103.131/products/attenuators_coax_fixed.shtml - Scroll down to the bottom of the page - it's the BNC 75ohm variety that you want. They come in 3, 6, 10, 15, 20dB versions - the 3 & 20dB ones could probably be ruled out as too low & maybe too high an attenuation. What you want to aim for is enough attenuation to suppress signal reflections BUT not too much to reduce the signal to too low a level. You will know if you have done this as the DAC will not lock to the SPDIF signal. Read a white paper on minicircuits about these attenuators.  


Look at their international sales reps for the nearest sales outlet. 


Have a look at some end-users impressions of using these RF attenuators & also an on-going experiment in their use  


Why buy expensive digital cables when using the high output Hiface & similar transports - these simple & cheap devices can be used to achieve a better solution. It's better solution because of the fact that a good digital cable if well made (expensive!) will have controlled impedance from end to end & will be a close match to the desired 75ohm. Outside of the cable at the DAC end or the transport end the cable cannot ameliorate any impedance anomalies that are encountered by the high speed signal so these will give rise to reflections travelling down the cable. The attenuators, attenuate these signals!


Some scope plots of the reduction in these reflections when using the RF attenuators follow

   An oscilloscope shot of the stock Hiface without attenuators showing the reflections on top of the square wave (click for larger image) 
         An oscilloscope shot of the modified Hiface with 16dB attenuators - note the flat top of the square wave (click for larger image)