Frequently Asked Questions - Wireless

FAQs - Wireless

In general, wireless receivers have a local oscillator that mixes with the incoming RF signal to produce a lower frequency signal at the Intermediate Frequency (IF) that is then processed in the rest of the receiver. For instance, in a 180 MHz, CR187 receiver, the IF is at 21.4 MHz. This is much easier to filter and amplify than the transmitter's 180 MHz carrier. To produce this signal, the incoming 180 MHz is mixed with a signal at 158.6 MHz to produce a difference signal at 21.4 MHz.

The mixer can also mix a signal at 137.2 MHz to 21.4 MHz since the difference between the local oscillator at 158.6 and 137.2 MHz is also 21.4 MHz. Therefore there are two signals that can easily produce 21.4 MHz by mixing with 158.6 MHz: the desired 180 MHz and the "image" of 137.2 MHz. The mixer is equally sensitive to either signal and without a front end, a receiver is just as sensitive at the image frequency as it is at the desired frequency. The RF front end, which is tuned to pass 180 MHz but stop the image of 137.2 MHz prevents any response to the image. This is why the 187 series receivers have multiple helical resonators in the front end. The front end has to smack the image down 100 dB or more. This image frequency should be taken into consideration when doing frequency co-ordination, though with modern receivers, the image is almost completely rejected.

Here is a RAMPS posting.

To the Group:

This post is in reply to an earlier post about "hot" SM transmitters. The SM puts out no more heat than a UM400; it just does it in a smaller volume. Since, despite the non stick finish, I'd never had any luck cooking omelets with an SM, I thought I'd run some crude tests.

I tested the temperature rise of an SMa transmitter, lying with its backside on a pad of paper with the temperature probe between the unit and the pad of paper. The air was fairly still with only the usual office air conditioning running. The pad blocked some air circulation to the backside and seemed to be equivalent to a unit on a belt. After 3 hours the temperature rise was 16 degrees, from 76 to 92 F. The battery was an Eveready lithium disposable, though the battery should make no difference. An SMQa (250 mW) under the same test rose 22 degrees from Seventy-four to Ninety-six degrees F.

I then firmly taped an SM transmitter to my leg with summer dress pants between the back of the unit and my leg. The probe was positioned between the pants material and the transmitter back. The unit was then covered in 6 layers of a Lectrosonics' jacket (polyester fleece) spread out over a 12 inch by 16 inch area around my leg. I think it is safe to assume that all the heat went into my leg. After the temperature stabilized with the transmitter off at 94 degrees F, I turned it on and at 45 minutes it was up to 104.5 degrees. At 66 minutes it was at 104.7 degrees and was effectively stable. The temperature rise (10 F) was smaller than the "belt" example (16 F) even though there was no heat loss to the air since heat was carried off by the circulatory system.

The 8 hr metal contact standard for Europe is 43 C or 109.4 F. and this correlates well with the fact that I never detected the smell of burning Larry. Another web tidbit was that oxygen gas sensors for neonatal units use a small metal plate warmed to 45 C (113 F) to increase the gas exchange rate from the skin of a baby to the detecting unit. These can be operated for up to 8 hours.

The point here is that if the SM is not touching the skin, the heat rise exists but is moderate. If the surrounding air temperatures are very high or the unit is in the sun, the unit might be uncomfortable to touch but that is only if it is not in contact with a cooling device (human being) and if it isn't in contact then there isn't a problem. If it is in long term contact with a person, then the person will cool the unit so that there is only a small temperature increase. The skin on a healthy person is going to be less than 98 degrees to begin with or they've got much worse problems than the 0.75 Watt heat source of an SM.

In spite of all this, if the talent doesn't like the warmth, then a larger transmitter that spreads the heat out (UM400a) is one solution or a pouch or some thin foam between the transmitter and the talent.

As far as potential burns, the safe touch temperature for unpainted metal for 10 seconds is 132 F. Ten seconds is more than long enough to remove the offending object to a safe place. If they have a good arm, it can be removed 50 feet or so.

Best Regards,
Larry Fisher

A customer sent us a SONOSAX SX-BD1 and indeed we found excess noise with this unit when used with a UH400a. We are 99% sure this is due to common mode noise on the Sonosax outputs particularly when the 40 dB output attenuator is selected. (We didn't have a schematic and we didn't want to tear up the customer's unit). When used with a good mixer, the common mode noise was mostly canceled out but that was not true with the UH400a.

We found that grounding the XLR pin 3 output of the SONOSAX cable to pin 1 of the same XLR removed the common mode noise by forcing the output into an unbalanced mode that matched the input of the UH400a. This can be done either in the SONOSAX male XLR itself or inside a barrel adapter available from Lectrosonics P/N 21750. This adapter is normally used to reverse the XLR polarity and is wired as pin 2 to pin 3 and pin 3 to pin 2. You can undo this by switching the wires at just one end of the XLR. To fix the SONOSAX SX-BD1 problem, simply wire pin 3 to pin 1 at the male end.

To wire a positive ground lavaliere mic (some older TRAM's and Sony's) to the new servo input used on the SM series and the new UM400a, LMa and UM450 transmitters, use the following wiring arrangements. (Positive ground lavalieres are also known as negative bias lavalieres.)

This is the simpler "servo only" wiring and is not compatible with older Lectro transmitters (UM200, UM400, etc.)

  • Pin 1 of the 5 pin TA5F is not connected.
  • Pin 2 goes to the shield or ground wire of the mic.
  • Pin 3 goes to the bias (audio) wire of the mic.
  • Pin 4 is not connected.
  • Pin 5 is connected to Pin 3 of the 5 pin TA5F.


The following is the compatible wiring and requires an external resistor but this wiring can be used with all Lectrosonics transmitters, old and new.

  • Pin 1 of the 5 pin TA5F is connected to one end of a 2.7k resistor.
  • Pin 2 goes to the shield or ground wire of the mic.
  • Pin 3 goes to the bias (audio) wire of the mic and to the other end of the 2.7k resistor.
  • Pin 4 is not connected.
  • Pin 5 is not connected.

Other value resistors can be used in a pinch from 2k to 4k, including the 3.32k resistor that we provide in the 5 pin wiring kit.

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 typically 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. Though these measurements were made on a UM200 transmitter and UCR201 and UCR210 receivers, the numbers should be comparable for the current UM400 or SM transmitters and the corresponding UCR401 and UCR411 receivers.

To see what kind of interfering levels would exist in a bag, 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 antenna 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. We then repeated the measurements for different frequency offsets between the transmitter and receiver. 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. The receiver was desensed by 2.3 dB. This means that with a real bag system having a 10 MHz offset in the two systems' frequencies and with the antennas 12" apart, the usable range from the talent to the bag would have been reduced to 77% of normal range. This is a pretty small reduction 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.) To simulate a worst case situation, we reduced the frequency separation to only 0.5 MHz with the talent transmitter and bag receiver still at 540 MHz and the bag transmitter now at 540.500 MHz. The desensing was now 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 measured values for a UM200 UCR210 system. I'll put frequency and then resulting range as a percent and also in actual feet, assuming 300 feet for a normal system.

Here are the results of UM200 and UCR210 at 12 inches apart:

  • 0.5 MHz separation results in 10% of normal range or 30 feet
  • 1.0 MHz separation results in 20% of normal range or 60 feet
  • 1.5 MHz separation results in 25% of normal range or 75 feet
  • 3.0 MHz separation results in 32% of normal range or 96 feet
  • 4.0 MHz separation results in 33% of normal range or 100 feet
  • 6.0 MHz separation results in 66% of normal range or 200 feet
  • 10 MHz separation results in 77% of normal range or 231 feet
  • 20 MHz separation results in 81% of normal range or 243 feet

Some users have wondered how the UCR201 would perform in a bag even though this was not 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.

Here are the results of UM200 and UCR201 at 12 inches apart:

  • 0.5 MHz separation results in 0% of normal range or 0 feet
  • 1.0 MHz separation results in 0% of normal range or 0 feet
  • 1.5 MHz separation results in 12% of normal range or 36 feet
  • 3.0 MHz separation results in 7% of normal range or 21 feet
  • 4.0 MHz separation results in 14% of normal range or 42 feet
  • 6.0 MHz separation results in 28% of normal range or 84 feet
  • 10. MHz separation results in 50% of normal range or 150 feet
  • 20. MHz separation results in 71% of normal range or 213 feet

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.

A quick measurement with the antennas between the bag transmitter and receiver at 18 inches instead of 12 inches as above, showed a reduction in interference power of 5 dB. This is a huge change, is faster than the usual square of the distance rule and would allow you to more than double the range for some smaller frequency separations.

The results for all of this are:

  1. Use a UCR211 or UCR411 for a bag system if possible rather than a 201 or 401.
  2. Try to separate the antennas of the transmitters and receivers by 18 inches if possible.
  3. Separate the frequencies by 5 MHz on a 211 or 411 system and by 10 MHz on a 201 or 401 system if possible.

More separation is better, particularly physical distance.

Here's the Lectro line on the SR receiver for bag use:

  1. We realize that our customers are going to use the SR in their bags no matter what we say, just like they did with the UCR401 and frankly, they have had pretty good success with the UCR401.
  2. The SR has a better front end than the 401 though not as strong as the 411. The input stage is more resistant to overload than the UCR401. However, the SR does not have front end tracking like the UCR411 (after all, which transmitter do you track?).
  3. The customer should take the same care with the SR as with the UCR401. For instance, don't operate in the same block as the bag transmitters, try to keep 25 MHz or more of frequency separation between the transmitters in the bag and the SR receiver frequencies and try to have as much physical separation as possible. Inches can make a difference.
  4. We are already planning to make a fourth bottom adapter for the SR that has two 6 foot audio cables and a 6 foot power cable that customers can cut to bag length and fit with their own custom connectors. Any of the different bottom adapters can be swapped in just a minute or so.
  5. As with all our digital hybrid receivers, the audio performance is absolutely equal to the UCR411.

With a little care, the SR should make a fine bag system receiver

The mic level input on the UM400a has the new servo input with much more gain for low impedance signals than the UM400 with the older input. The increased gain improves performance with dynamic microphones. However, switching back and forth between the UM400 and UM400a transmitters will require constant readjustment of the gain setting if you are using the MC40 cable. To make life simpler we have a MC41 cable that will work with either transmitter type and matches the gain to within a few decibels. If you want to build your own, the wiring of the TA5F is as follows:

  • pin 1 is ground
  • pin 2 N/C
  • pin 3 to a 3k resistor in series with the mic level wire.
  • pin 4 jumped to pin 1 (sets the servo bias to zero)
  • pin 5 N/C

This places resistance in the audio line and reduces the input to the UM400 by only a few decibels but reduces the UM400a by 20 dB and matches them very closely. The MC41 can be used with all the servo bias mics such as the SMa, SMQa, SMDa, UM400a, UM450, and LMa. It will also work well with any of the older style transmitters.

Here are some things we've found that will strengthen and protect the connection:

  1. If the mic cable is much smaller than the strain relief boot, use a very flexible sleeve to increase the effective size of the cable. Allow about an inch of this sleeve to stick out of the TA5F boot. What you are looking for is a gentle curve in the cable when it is pulled at right angles to the TA5F boot. A stiff sleeve will cause a sharp bend right at the sleeve and effectively do nothing. We use a soft silicone shrink tubing here. Silicone model airplane gas line is very flexible and will work well. There is a self fusing silicone tape that can also be used. Taper the thickness around the cable as it comes out of the boot so that the cable makes that gentle arc as described above. If you start the wrapping at a point away from the connector and then work toward the pins, the loose end will be trapped in the connector. This will also give you a natural taper if you decrease the wind angle as you get close to the pin end. It is not necessary or desirable to put the tape under the crimp tabs. You are only trying to increase the bend radius where the tiny cable exits the TA5F boot. See for the self fusing tape.
  2. The metal strain relief crimp tabs in the TA5F are very critical to the longevity of your connection. There are two sets of tabs. The tabs closest to the solder pins should be used to crimp over the shield of the cable but not the outer insulation of the mic cable. This shield grounding will also give the best protection against RF from the transmitter. (The shield still needs to be soldered to pin 1). The second set of tabs, farther back, should crimp over the insulation of the cable. This requires fairly exact placement of the cable but you can and should have slack in the inner wires of the cable before they are soldered to the pins of the connector anyway. By assembling the connector this way, you have strain transferred to the outer insulation by one set of tabs and strain placed on the inner wires by the other set. The tabs should be firmly mashed into the insulation and cables but not so much as to puncture the wires. There is some judgment required here. There should be some deformation of the insulation but not enough to cause failure. You want to crimp the strain relief tabs enough so that a good pull on the main cable does not move the cable past the strain relief.
  3. As mentioned in note 2 above, there should be some definite slack in the internal wires when the crimping and soldering is all done. The shield is normally a fairly stout wire and only a little slack is necessary but the other wires should have a definite "kink" so there is no chance of cable movement transferring force to the soft solder connection or to the small signal wires. All force should be taken up by the strain relief.
  4. When stripping the wires and the insulation of the cable, use a good round hole wire stripper or even a thermal stripper so that the wires and the shield wires are not nicked. If they are nicked they will break easily when even slightly flexed. Don't use too small a stripper setting for the same reason. When in doubt, flex the wire(s) a few times; if strands break off, the wire has been nicked. You might as well just start over.
  5. Don't overheat the shield. This is usually the largest wire in the bundle and can easily conduct enough heat to melt through the insulation of the audio and bias wires. Since all the wires have to be short to get them into the small connectors, heat can be conducted quite quickly up the wire, melting adjacent insulation. Pre-tinning the end of all the wires before attempting assembly will keep the ends from fraying and enable you to solder much faster. This will help prevent insulation melt through. Along the same lines, don't heat the metal strain relief in any way after the tabs are crimped down. We had one dealer that soldered a chip resistor to the strain relief tabs after they were crimped to the cable. This melted the insulation and caused many mic failures days and weeks after the connectors were wired.

If any readers have suggestions, post in RAMPS or email me at

The RM2 does not have an LCD readout and can not set the frequency of operation of the SM transmitter. The RM2 uses a potentiometer for the gain control setting of the SM where the RM has the LCD readout. This means that you can only set the gain to an approximate value where as on the RM it can be set exactly. The speaker on the RM2 is not quite as loud as on the RM so the maximum range is a few inches less. The lithium battery on the RM2 is inserted into a clip on the circuit board and requires removing the back cover to replace, though it should last for years.

From the Shoeps web site: "The SCHOEPS CMR microphone amplifier allows any SCHOEPS ”Colette” series capsule (except the BLM 03 C active boundary layer capsule) to be used with pocket transmitters." 

In the email below is the TA5F wiring that works with our older transmitters such as the UM200 and the newer servo style inputs such as the UM400a or the SM transmitters.

Hi Charlie,
The CMR adapter is on its way back to you. The rewiring was pretty simple. The problem was that the CMR adapter doesn't like any DC voltage applied to the audio line (our pin 3). The SM servo unit applies 2 Volts to pin 3 when pin 4 is not wired to ground. With the voltage switched off, the mic sounded great but had too much gain in my opinion. We added a 1.5 k resistor in series with the audio line (our pin 3 again) and now the mic has about the same gain as a COS-11 when used with a UM400a or an SM. It still works with an older transmitter such as a UM400 or UM200 with a few dB more sensitivity than with the SM. If you should want more gain with an SM, reduce the size of the resistor; 500 Ohms = +6 dB.

So here's the final wiring for full compatibility:

  • Schoeps shield to pin 1
  • Pin 4 jumped to pin 1 (shuts off servo bias)
  • Schoeps blue wire to pin 2 (+5 Volts)
  • Schoeps white wire to a 1.5k resitor in series with pin 3 (audio in)

Thanks for providing the mic adapter.
Best Regards,
Larry Fisher