Frequently Asked Questions - 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.
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.
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:
Here are the results of UM200 and UCR201 at 12 inches apart:
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:
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.
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.
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.
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.