Tuesday, February 12, 2013

Cheap HD/SDi parts, Return Loss and the danger of short cables

Return loss is the loss of signal power resulting from the reflection caused at a mismatch with the terminating load or with a device inserted in the line. It is usually expressed as a ratio in decibels (dB);
$RL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}$

where RL(dB) is the return loss in dB, Pi is the incident power and Pr is the reflected power.

Return loss is related to both standing wave ratio (SWR) and reflection coefficient (Γ). Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss. In the case of newer budget HD/SDi equipment return losses can be as bad as 12dBs (I've measure Blackmagic boxes thus) whereas the spec for 3G is 16dBs and in the case of proper broadcast manufacturers 18dBs or better is often measured (Sony, Tektronix).

I recently tested some real budget SDi parts from a reseller who is thinking of importing them from the Far East. Here are a couple of eye patterns of a 1.5G signal at the input and via the high-impedance looping output of a distribution amplifier.
If you take the first measurement as the base (it's not brilliant but that probably due to the poor signal generator) and then see what you get when the feed is connected to the DA's input and measured at the Hi-Z loop-through you see the effect of return loss - the ability of the sending equipment to drive the line impeded by reflections at the receiving piece of of equipment because of sub-optimal termination.

One thing to look out for is using short cables with budget SDi parts because the reflected portion of the signal doesn't have the chance to dissipate and interferes with the incoming signal. Replacing the short coax cable with a longer one solves the problem. This seems counter-intuitive as all engineers are painfully aware that signals get more compromised by long cables. We all carry a set of figures around in our heads; Gigabit Ethernet over cat6; 100m, 3G HD/SDi over Belden 1694; 60m, 10gig Fibre Channel over OM3; 600m etc etc.

Matt, Wes and I were left scratching out heads with the following setup;

HD/SDi MTX, optical o/p -> 100m OS1 line -> optical->SDi converter -> SDi-HDMI converter -> TV

We were getting nothing at the TV. Checking for SDi just pre the HDMI adapter using a Tektronix WFM5200 (no eye patterns, unfortunately!) was fine, but the giveaway with sticking a signal generator just pre the HDMI converter and using the same short-length cable showed no signal at the TV. Replacing the short cable with a longer one (15cm with 1m) made it all come good.
Clearly the Tektronix had a much better return loss on it's input and so wasn't phased by being fed with the short cable. The cheap HDMI converter was a lot more sensitive. Interestingly switching between 270MBit/s SD and 1.5GBit/s HD made no difference.

the short cable was the bogeyman!