It is surprising to me how many times I have seen this done incorrectly in the field. Summing a stereo source, whether it is balanced or unbalanced is not simply twisting a couple of wires together. This will effectively reduce the impedance of the outputs by one-half. With newer, active balanced outputs, this may cause damage to the output amplifiers.
The parallel resistance formula is thus:
Therefore a 600-ohm stereo output tied together would look like this:
Rt = 1/(1/600+1/600) or 300 ohms.
It also creates an impedance mismatch with the next piece of gear, which will affect the common mode noise rejection of the circuit.
The best way to sum is through a resistive network. That way stereo separation is maintained, the impedance of the output circuits is maintained and the output amplifier will not current cycle. That looks like this:
resistive summing network
Pretty easy to fabricate in the field. It is good to do things the right way, it sounds better on the air too.
This is a neat piece of kit, designed to save those late-night/early morning callouts, which is the ultimate goal of all broadcast engineers, or at least it should be. This seems like a really good idea, however, BE has discontinued the product line, and the last manual update is from 2000.
A small four-port coax switch is located next to the power supply transformer. This is controlled by the circuit board. The circuit board senses a loss of excitation by detecting a forward power level below the threshold set on the board. The power sample comes from the exciter forward power remote metering terminals. Thus, it can be used with any exciter(s) that have a remote forward power sample.
BE FW30 exciter switcher block diagram
The idea is to use the RF fault function output of the FX-30 (later FX-50) exciter to automatically switch from a faulted exciter to one that is working. Finally, it can be hooked to a remote control for manual switching. The unused exciter is muted and routed to a dummy load mounted on the back of the unit.
Broadcast Electronics FW-30 front, mid 1980’s BE blue
An alternate configuration would be to route the backup exciter to the backup transmitter instead of the dummy load. This would create the best redundancy on a limited equipment budget. It also has a battery bank designed to hold the last state of the unit through a power outage. As we have a good-sized UPS powering the remote control, STLs, and satellite receivers, the batteries are not needed.
On the face of it, a pretty good idea. I have had a few exciters fail over the years, which normally means the backup transmitter is placed in service by remote.
I did download the manual, but since it is currently listed on the BE website, it’s probably not a good idea to post the schematic. Suffice to say, it is a tad bit complicated what with all the CMOS logic and that. It is very possible to duplicate the functions of this equipment with a simple RF forward power sample and set a failure threshold with a comparator circuit. Hook that to a small four-port coax switch and a couple of RF mute/un-mute commands to each exciter and: Viola! Automatic exciter switching!
The old version of the software, that is. I like the graphical interface, just one glance is all that is needed:
Burk auto pilot
I have not had a chance to fool around with the newer version, the screen shots on the Burk website look a little bit different.
The setup and programming of macros is pretty easy; power/pattern change times, Pre-sunrise, and post-sunset functions, automatic tower light monitoring, AM Directional Antenna readings, and automatic transmitter restoration routines. If programmed correctly, the software can eliminate many of those late-night/early-morning phone calls, which is always a good goal.
ROHS stands for Restriction of Hazardous Substances Directive. It is a mainly European effort to reduce lead (Pb), Mercury (Hg), Cadmium (Cd), Hexavalent Chromium (Cr+6), Polybrominated Biphenyl (PBB), Polybrominated diphenyl ethers (PBDE) and Acrylamide in electronics and consumer goods.
The main effort appears to be in the reduction of lead in circuit boards and solder. Generally speaking, the reduction of pollutants is a good thing. Lead is toxic, especially to young children. Mercury is a potent neurotoxin. Those other elements and chemicals don’t sound good either.
There are all sorts of green logos and other nice-looking things attached to products that meet the standard.
Typical ROHS label
I feel better, don’t you?
Now for the other side of ROHS. According to Lead Free Electronics Reliability (large .pdf) by Dr. Andrew Kostic, the effort had been hugely expensive with very limited results:
A huge (~ $14B annual revenue) semiconductor manufacturer estimated the annual worldwide Pb reduction per 1,000,000,000 integrated circuits was only equivalent to ~100 automobile batteries.
Wow! That is simply amazing on the face of it. Over the years, I have probably found and carted at least 10 old car batteries to the recycling center for a few dollars each. According to the Kostic paper:
(Computer chip manufacturer) Intel’s efforts to remove lead from its chips have so far cost the company more than $100 million and there is no clear end in sight to the project’s mounting costs
Wouldn’t $100m be better spent on other, more pressing pollution issues? Fukushima, springs to mind.
Further, the replacement metals used in electronics have some problems of their own. They may be better for the environment, however, they lack testing and are
Not optimized for high reliability, severe stress, long life applications
Further, replacing parts in legacy equipment using ROHS parts and solder may present problems with bonds between dissimilar metals. Thus, making field repairs, or any repairs impossible.
Many of the newer solders and circuit boards use Tin (Sn) as the finishing metal. There is a problem with tin, known as Tin whiskers. This was first noted at the Bell Labs in 1947. Small hairs grow out of the surface of the metal, acting as short circuits, and at higher (above 6 GHz) frequency RF, antennas. This happens with other metals such as Zinc, Silver, and Gold.
Silver Sulfide Whiskers on circuit board
As you can probably deduce, this can have certain detrimental effects on the performance of the circuits in question. I can imagine all sorts of strange behavior from controllers and other bits and parts of equipment.
I don’t know how prevalent this is in Europe where the directive has been in effect for 6 years or so. It would be interesting to find out. I also wonder how many US manufacturers are adopting RoHS as the de facto standard in order to do business in Europe.
