Frequently Asked Questions - Wireless

FAQs - Wireless

The +4 dBm designation indicates that a full modulation signal will just reach +4dBm rms max. The line level input level at the transmitter can be attenuated by different factors depending on the pins selected (pin 4 or 5) and the resistor to ground (pin 3 or 4). The gain pot also can vary the applied gain also. To sum up, there is no fixed relationship between the input level to the transmitter and the output level from the receiver other than the +4 dBm setting is the maximum output level you can see. If the transmitter is set up so that you never activate the limiter, then you will never see the +4dBm. If you have 20 dB of headroom between your maximum input level and the transmiter limiter, then you will never see more than -16 dBm. My guess is that you are being pretty conservative on your input levels and could run them substantially higher; this will bring your output levels closer to the max indication and also improve the signal to noise ratios and noise ups.

There are three possibilities: One, batteries do not like to be cold. At low temperatures (32F) battery life can be one third of that at room temperature (72F). Two, some brands of batteries will not deliver the high currents used in our receivers and 100 mW transmitters. We use Eveready as our standard battery. Three, our units will operate to very low battery voltages and you may not be running the battery down far enough. Here's a reply to a UCR201 user that was replacing batteries every 2 hours or so.

At room temperature, the 201 should give you +4 hours of operation. Try an experiment when you have some free time with a fresh battery. Simply run the unit in the battery voltage display mode and see how long it takes to pull the battery down to 6.5 Volts. The system will operate perfectly to below 6.5 Volts since all internal voltages come from several switching power supplies. We have found a lot of variation in XXXX batteries and some batches will not provide the high currents the 201 draws. We have never found problems with Eveready 9 Volts. The XXXX batteries acted so weird I suspected they might be counterfeit. This was on several batches of XXXXXs from different parts of the country. Further testing found that other XXXXXs were almost equivalent to the Eveready's. We remain puzzled. My advice is that if you are getting short life, try the Eveready's as a standard before deciding the unit is defective.

If you are having short battery life in transmitters due to cold weather, keep the transmitter warm as long as possible before you have to use it. Belt pack transmitters can be also be put under the coat so as to be next to the nice warm human being.Alkaline batteries, though very good at room temperature, cannot deliver much current at lower temperatures. Battery life can be as little as one third normal on a cold day and even less if they cold soak for any length of time. Life can be as little as just a few minutes at -20 F.

If you must use disposable batteries (non rechargeable) then lithium batteries are the only good choice. They have shorter life at low temperatures but are still much better than alkalines.

In the AA battery size, NiMh batteries are a good cold weather choice. At low temperatures they have almost as much life as at room temperature and are rechargeable to boot. We recommend the Eveready NiMh batteries and 15 minute charger that we provide with the SM, SMD and SMQ transmitters. One precaution is that the batteries cannot be recharged if they are cold. They can be used without any problem but must be at about room temperature to be recharged. (See FAQ #087-WIRELESS)

In the 9 Volt battery size, NiMh batteries perform as well cold as they do at room temperature but they don't have much battery life (capacity) cold or warm. At one time they were the only choice for very low temperatures but LiPoly rechargeable batteries are now available that have more capacity than alkaline batteries and perform very well at low temperature. They are currently sold under the iPower brand and are available on the internet, from some dealers and from Lectrosonics.  (Also see FAQ #086-WIRELESS)

We make an adapter that converts the UH400 XLR input into a TA5M 5 pin equivalent for 2 or 3 wire lavaliere use. The model number is a MCA5X. You will need to set the UH400 for 5 Volt phantom power but it does have Zener protection if you forget and leave the UH400 on the 48 Volt setting. It's in this catalog: 

You can also do the following, but you MUST have the UH400 set for 5 Volts, not 18 Volts or 48 Volts:
For a 2 wire lavaliere, pin 1 of the XLR is ground and pin 2 is bias (audio). Pin 3 not used for a lavaliere. 

If you have a three wire mic like the Cos-11 it is a little more complicated but not bad. Hook shield to pin 1, a 2 k to 3 k resistor between pin 1 and the Cos-11 white wire (source load) and hook the remaining black wire to pin 2 of the XLR. This will give you a low distortion hookup comparable in gain to the UM series of transmitters. We strongly recommend putting a small 6.8 to 9 Volt Zener diode across pin 1 and pin 2 for voltage protection in case you misset the voltage. The cathode goes to pin 2 and the anode to pin 1.

The MCA5X is certainly an easier solution.

The UH plug-on has the same audio circuity as the UM belt pack with the exception of the variable bass roll-off pot found on the UM. The UH plug-on roll-off is fixed at 70 Hz, which is the best all around compromise, IMHO.

You can insert a "MiniCircuits" brand two way splitter into any of your RF distribution system's antenna outputs with a small loss of 3 dB. This loss will cut your range to 80% of normal and will probably be undetectable in the real world. You will need a total of two 2 way splitters, one each for diversity outputs from the distribution amp. For a diversity Quadbox, for instance, this will give both antennas 5 total outputs, 3 normal and 2 that are 3 dB down. You could change the attenuator that we use at the antenna outputs and reduce the attenuation by 3 dB to make up for the additional split, but I don't think it is worth the trouble and the Quadbox would then be non-standard. You can get the necessary short cables and two way splitters from us. The splitters are also available at the same price directly from MiniCircuits (a top notch company).

Additional Info:


ARG Series COAX Cables

Usually it won't. Our receivers' front ends are very quiet and within a few dB of the theoretical limits for noise. The range limitations are usually due to other interfering noise sources in the environment. Additional gain before the receiver will not increase range but will lead to increased RF intermodulation and RF overload. The only time you want gain before a well designed receiver is to neutralize cable and/or splitter losses. In this case, the gain must go before the cable or splitter loss and it must be a high quality amplifier such as our IFM50. Putting the gain after the loss is too late since the signal to noise ratio is now poor and gain won't improve it. An analogy for RF is the same as for music; lots of amplification can't overcome poor source material.

This was a general question from the RAMPS group about various wireless transmitters generating low frequency noises when struck.

In general, there is mechanical coupling from the case into the inductors in the main oscillator in the transmitter. A thump on the case moves or bends the inductor, changes the inductance value by a tiny amount and changes the frequency of oscillation. Since a changing frequency is just FM, the FM receiver picks it up as a low frequency thump. There are various ways of reducing the mechanical sensitivity. Most involve very rigid coil assemblies such as inductors wound on ceramic forms. In our case, we use solid quarter wave ceramic resonators. 

The cutest trick I've seen, was a (brand) unit that used a miniature Teflon insulated coaxial line as a resonator. They wound the coax stripped off the outer insulation in a tight cylindrical coil with about 6 turns. The entire shielded coax coil was then soldered on the outside into a solid mass. This made a nice rigid assembly with the center conductor acting as the inductive element since a short coax line with one end shorted looks like an inductor.

The other way to generate a thump is to use a capacitor in the audio circuity that is sensitive to mechanical stress. The wrong kind of ceramic capacitor with DC voltage on it can really generate a lot of voltage when stressed. NPO ceramic capacitor types are as good as most film caps or tantalums but X5R types are bad and Y5Z are horrible. NPO's have the least capacity for a given size and the other types have 5 to 50 times more capacity in a given size and that's why they exist. I tried 50 Volt Y5Z type capacitors in the design of the 48 Volt phantom supply for the UH200C. You could get about as much audio talking into the transmitter PC board as you could using a microphone. Fortunately some small 50 Volt tantalums came on the market that would fit in the same space and saved my bacon. I knew the problem existed, but the severity surprised me.

My advice is to whack the case of a transmitter with both your finger and with a pencil sized object. If you know how a transmitter is going to react to mechanical shock, you can prepare for it.

On the subject of mechanical stress and audio, try the same thing with your electret mic cables. Some are much worse than others. If you tap the cable close to the mic (6") you will get mechanical noise transmitted directly to the mic element. In the middle of the cable, it is due to flexing of the mic cable. Phantom powered are sensitive to this since there is DC voltage on the cable and flexing the cable changes the dimensions and the capacitance of the cable. The pro mic manufactures have taken this into consideration in the choice of cable.

The 100 series does not have a pilot tone in the transmitter so this won't work because the IFB receiver requires a 30 kHz pilot tone to be present before producing audio (unsquelching). If you are trying to run 100 series transmitters on the talent, a 100 receiver feeding a recorder and then monitoring on the IFB receivers simultaneously, I'm afraid you are out of luck. 

The newer LM and UM transmitters can run in the IFB mode and the newer units will have that capability marked on the case. You could run the transmitter in IFB mode and then IFB receivers and 100 receivers would both work. However the sound quality in the 100 receivers would not very good.

The pre-emphasis of the 100 series and the IFB series is about the same. The problem comes from the fact that the 100 series has a dual band compander that treats the bass and treble part of the signal separately. If the audio signal is balanced above and below 1 kHz, the single band compandor in the IFB and the dual band compandor in the 100 will track pretty well. If the bass and treble are not about equal after pre-emphasis, then the weak band will be reduced even lower compared to the stronger band.

The power supply for the electret mic bias turns off immediately rather than delaying till the receiver mutes. It is a design error. All LMs shipped after Jan 2005 have been modified. We have a clean fix for earlier units and will modify your LM at no charge. Contact our service department for help.

This is a long posting made to the RAMPS news group. There is a clearer explanation here: see FAQ 105

Some weeks ago we made measurements of a simulated bag system to see what having a 100 mW transmitter 12 inches (30 cm) away from receivers would do to the receivers' sensitivity. If you read the previous post, you will remember I was somewhat surprised at the performance of the UCR210 receiver in this test. I thought the older, helical resonator front end, single frequency UCR195 would carry the day. In fact, the UCR210 performed as well and in some areas was a bit better. Some users on the group were wondering how the UCR201 would perform in a bag even though this was never our intended use for the 201. This time the numbers are more in line with what I would guess, since the 201 is definitely weaker in this test. The first set of numbers are for the UCR210 and the second set are for the UCR201. I'm not going to try to put them on the same line since I think various news readers will mangle the formatting. To see how the test was run, I included all the text of the previous test below the new numbers. Again, I'll put frequency separation in MHz, then dB of desensing, then resulting percentage of range with a percent and then a comma as a line separator.

Again, the explanation of the test setup and procedure are in the text after this posting. 

(UCR210 from the previous test)
0.5M 20dB 10%,
1.0M 14dB 20%,
1.5M 12dB 25%,
3.0M 10dB 32%,
4.0M 9.8dB 33%
6.0M 3.6dB 66%,
10.M 2.3dB 77%,
20.M 1.8dB 81%

(UCR201 from a new test)
0.5M 89dB 0%,
1.0M 77dB 0%,
1.5M 18dB 12%,
3.0M 23dB 7%,
4.0M 17dB 14%,
6.0M 11dB 28%,
10.M 6dB 50%,
20.M 3dB 71%,
30.M 2dB 80%

As can be seen from comparing the numbers, the UCR201 needs twice the frequency separation before the ranges are comparable. The 3 MHz number looks funny but that's what we measured. The 50% of normal range is reached by the UCR210 at 5 MHz of frequency difference while the UCR201 needs 10 MHz of separation. I would recommend separation of at least one of our blocks (25 MHz) between the 201 receivers and transmitters in the bag. On the other hand, the UCR210 can operate inside the same block with a little care. The difference is due primarily to the tracking front end in the 210 and secondarily due to the higher power level of the first RF transistor in the front end of the 210. Once the signal is past the front end, both receivers are essentially the same.

We ran the same tests with reduced transmitter power (30 mW equivalent) and found a one to one reduction in desensing. The 5 dB reduction in power (100 mW to 30 mW) reduced the desensing by 5 dB. So you could take the above numbers and reduce the desensing by 5 dB which would be equivalent to a 30 mW system at 12 inches or a 100 mW system at 18 inches (45 cm). Note that the levels drop faster than the square of the distance at very close distances. A 10 mW transmitter would improve the desensing numbers by 10 dB. The reason I mention 10 and 30 mW is those are the power levels of some other popular transmitters on the market. I don't think the matching receivers will be quite as tough as the UCR201 but with the lower transmitter power, the upshot is the 201 numbers probably will describe what other systems would do. The one exception, to the best of my knowledge, is the small Sennheiser receiver that does not have an RF amplifier but takes the antenna input directly to the mixer. It has a very good third order intercept but does not have the sensitivity of other Sennheiser receivers. It would be interesting to measure it and see if the resistance to desensing would counter balance the lack of an RF stage. I'm sure it would. A good universal rule of thumb is that 10 MHz of separation with reasonable quality systems will give you 50% of your range. You will need at least 5 MHz even with the UCR210.

Larry Fisher

[[[ Below is the text of the original measurements and a complete description of the setup and parameters. The new measurements above were made in the same manner. We tried to make sure we were comparing apples to apples.]]]

We did some interesting RF measurements on a simulated two way bag system to see how much the bag transmitters would affect the bag receivers' sensitivity. A two way bag system will at least consist of multiple receivers to receive audio signals from the talent, a portable mixer to mix the audio and one or more transmitters to retransmit mixed audio to the video cameras. The immediate question is "If the receivers and transmitters are on different frequencies why should the transmitter reduce the sensitivity of the receiver?" One obvious answer is that the RF front end of the receiver is not a perfect filter and can let strong, nearby frequencies pass through and overload the first amplifier. In addition, transmitters do not produce a single sharp frequency but have some noise 5 Mhz or more from the carrier. The levels are very low but bag systems have antennas that are very close together. In the same way, the local oscillator in the receiver produces some noise many MHz away from the desired frequency and acts the same as having noise in the transmitter. Instead of trying to calculate all this stuff it is simpler to just measure a simulated system.

We put a transmitter 12" (30cm) away from an antenna mounted on a power meter and measured an average signal of -5dBm (.5mW) from a transmitter with 20 dBm output (100 mW). This is a very strong signal to bleed into a receiver but will be very typical of a bag system with 12" of separation. We used this level for the interfering transmitter for all the sensitivity tests. We then checked the receiver sensitivity with the transmitter off and then on and measured the reduction in receiver sensitivity for different frequency offsets between the transmitter and receiver. So to simulate a bag system where the talent's transmitter is on 540 MHz and the bag is re-transmitting mixed audio to the camera on 550 MHz, we would inject a 550 MHz signal at -5dBm into a UCR210 receiver set at 540 MHz and see how much that affected the receiver's ability to pick up the desired 540 MHz signal. We attenuated a block 21 UM200C transmitter set at 550 MHz down to -5 dBm and combined it with a weak 540 MHz signal from a signal generator, set the receiver to 540 MHz and checked the sensitivity with the transmitter off and then on. With the transmitter off, the receiver had a normal sensitivity of -107 dBm for 30 dB SINAD.(Same as "signal to noise ratio" at these values) With the transmitter on, the sensitivity fell to -104.7 dBm for a decrease in sensitivity of 2.3 dB. Or the receiver was desensed by 2.3 dB. This means that in the real world, a 10 MHz offset in the two systems' frequencies with the antennas 12" apart, the usable range from the talent to the bag would have been reduced by 23%. This is pretty small and surprised me. I thought it would be much worse. (There is no reduction in the distance from the bag to the camera since the receiver at the camera is not near a transmitter.) At 0.5 MHz separation with the talent transmitter and bag receiver still at 540 MHz and the bag transmitter at 540.500 MHz, the desensing was much worse at 20 dB. This would reduce the talent to bag range to 10% of normal and is a good reason to never operate with only 0.5 MHz frequency separation. Here's some more values for a UM200 UCR210 system and I hope the news readers don't totally mangle the formatting. I'll put frequency, then dB of desensing, then resulting range with a percent and then a comma as a line separator. Something should get through.

0.5M 20dB 10%,
1.0M 14dB 20%,
1.5M 12dB 25%,
3.0M 10dB 32%,
4.0M 9.8dB 33%
6.0M 3.6dB 66%,
10.M 2.3dB 77%,
20.M 1.8dB 81%

The above numbers are for a system with a tracking front end with relatively high power RF amplifiers both of which help. The relatively high power transmitter of course hurts. If you didn't need much range, bag to camera, you could attenuate the transmitter. The better solution would be to move the transmitter away from the receiver. On the other hand, 5 Mhz of frequency separation would leave you with 50% of your range and that would still be a gracious plenty in 99% of the cases. This is all a good reason to have the transmitters and receivers in a bag system separated by a frequency block (25 MHz). But if you have to operate in one TV channel, 6 MHz, off set the frequencies as much as reasonable.

I had postulated before we tried this experiment that the helical resonators in the fixed frequency UCR195 receiver might work better in a 2 way bag system than the higher power RF amps and wider tracking filters in the UCR210. Here's the numbers for a UCR195D at 536.250 MHz and a block 21 UM200C. The measurements were made the same as above,frequency, then dB of desensing, then resulting range and a comma.

0.5M 30dB 3%,
1.0M 28dB 4%,
1.5M 26dB 5%,
3.0M 25dB 8%,
4.0M 7.8dB 41% 
6.0M 4.4dB 60%,
10.M 3.1dB 70%,
20.M 2.8dB 72%

The numbers are much worse for separations less than 4 MHz. Basically the dual gate GasFet is wiped out in the UCR195D until the helical resonators can start filtering at 4 MHz off frequency. Note the rapid change in the numbers between 3 and 4 MHz. This is where the helicals really begin to filter. Overall, the higher power RF amps in the UCR210 carry the day, though the helicals are roughly equivalent at 4 MHz and higher. If I can say something in self defense here, that is why the newer Lectros are rightly accused of eating batteries; they burn it in the front ends.

Having been major wrong about the UCR195 and the helicals, I thought we'd test out my hypothesis that the "rock" transmitters (crystal controlled UM195) would have less noise off frequency than the synthesized transmitters such as the UM200. Here we used a UM195 on 536.250 and varied the frequency of a block 21 UCR210 to check sensitivity versus separation. Note the frequency offsets are a little different.

1.5M 28dB 4%,
2.0M 25dB 8%,
4.0M 17dB 14%,
6.0M 7.0dB 45%,
10.M 4.2dB 62%

Notice that the range is much more affected by the UM195 transmitter. The receiver is the same UCR210 used in the first measurements yet the receiver is now much more affected. The only explanation is that the UM195 is noisier off frequency than the UM200C which is not what I would have expected. By the way, this "noise" would only show up under these bag conditions. Twelve inch separations from transmitters to receivers are not usual. We will measure some different UM195's but at the moment the synthesized radios seem to perform easily as well as the fixed frequency units in bag system conditions. Now where's the crow seasoning.

Larry Fisher