1. Designed supply voltage for 202001, 202004 and 202005 Micromodem Power Amplifiers.
Micromodem Power Amplifiers have magnetics (a transformer and inductor) that are matched to a particular transducer at a particular supply voltage so that the maximum acoustic source level can be produced. Our standard magnetic matches are designed for a supply voltage of 12V, 24V or 36V for a given transducer.
Here is a short list of details that should be observed:
- Any power amplifier specification (e.g. 12V, 24V or 36V) can be operated a lower supply voltage than designed. However, the acoustic source level will decrease in proportion to the supply voltage.
- None of the power amplifier specifications should be operated a higher supply voltage than designed. Operating the power amplifier at a higher supply voltage may cause the magnetics to overheat and fail, and or overdrive the transducer which may overheat and fail.
- Avoid switching the power supply providing power to the power amplifier on and off too rapidly. Although the power amplifier has many protection devices designed into it there have been a couple cases of power amplifier failure due to excessive power supply switching.
2. Supply voltage for 202006 and 202009 Micromodem Power Amplifiers.
The 202006 and 202009 Micromodem Power Amplifiers differ from the older designs in three ways.
- The input can now accept a wide range of supply voltages. Refer to the hardware documentation for specific values.
- The voltage supplied to the transmit FETs is a regulated by an onboard DC/DC converter that is only switched on during the transmission.
- The transmit voltage can be adjusted via an external set of I/O lines, an on-board DIP switch, or via the Micromodem 2.
3. Required hardware for PSK.
All Micromodem systems can transmit FSK and PSK signals by default.
All Micromodem systems can receive FSK signals by default.
Only Micromodem systems that have a 203004 Micromodem c67 Coprocessor can receive PSK signals.
International Customers: Export restrictions allow us to only provide PSK hardware domestically at this time.
The Micromodem 2.0 will eventually support native on-board PSK in a future firmware revision. Once native PSK support is added the 203004 coprocessor will no longer be needed on Micromodem 2.0 systems.
4. Arrays and PSK performance.
PSK Micromodem systems that utilize a 4 element spatial diversity array typically out perform single channel systems in the same environment. We have performed tests that showed that arrays can be mounted in several different configurations and provide equivalent results relative to each configuration. Please contact us for more information.
5. Data Rate Explanations.
It is important to realize that there is a difference between packet burst rate and throughput, which is a function of many things, including serial traffic in and out of the modem and the time for the modem to decode the PSK data. These details are described below.
When referring to the FSK or PSK data rate we are talking about the burst rate. For example: A Rate 5 PSK packet has a Burst Rate of ~5000bps as that is the user content in bytes divided by the total time for the packet.
Packet Burst Rates:
|Burst Rate||Frames / Packet||Frame Size||Packet Length||Signal Length(Probe + Null + Packet)|
|PSK Rate 6(5kHz BW)||490 bps||6||32 bytes||2.95s||3.25s|
|PSK Rate 5(5kHz BW)||5388 bps||8||256 bytes||3.04s||3.34s|
|PSK Rate 4(5kHz BW)||1301 bps||2||256 bytes||3.15s||3.45s|
|PSK Rate 3(5kHz BW)||1223 bps||2||256 bytes||3.35s||3.65s|
|PSK Rate 2(5kHz BW)||520 bps||3||64 bytes||2.95s||3.25s|
|PSK Rate 1(5kHz BW)||498 bps||1||64 bytes||3.08s||3.38s|
|FSK Rate 0(4kHz BW)||80 bps||1||32 bytes||3.9s||5s (with CI)|
Communication System Throughput:
Total throughput depends on many other factors such as serial port speed (on each side of the communication system), travel time in the water, polling time, and processing time on the coprocessor (which varies between single channel and multi-channel operation) etc. We can provide throughput numbers for specific situations (x channels, x baud rate).
Some example aggregate receiver throughput numbers:
- Downlink with no ACK.
- Serial Port Baud Rate: 115k on the modems and host on both sides.
- Times: Times include serial input to the modem (data request interaction) on the TX modem and the serial port character output from the RX modem.
- Rate 5 PSK with 5400 bps packet burst rate
For a single channel receiver at rate 5, such as what would be used for a deep vehicle talking to the surface, the overall timing and rate are actually limited by the transmitter. The speed of the host computer sending data to the modem is what controls the overall throughput. The best-case throughput (with a host computer that can respond immediately to each data request), the time to transmit a packet is 4.9 seconds (and the receiver can process it in 4.6 seconds), so the data rate is 3343 bps.
TX Side Limited PSK Rate 5 Timing (1) Channel: 4.9s
PSK Rate 5 Throughput (1) Channel: 3343 bps
For the multi-channel case at rate 5, such as what would be used in shallow water, the modem is no longer running in real-time because of the additional processing required to use the 4 channels. The time per packet and resulting continuous throughput is:
- RX Side Limited PSK Rate 5 Timing (4) Channel: 10.1s
- PSK Rate 5 Throughput (4) Channel: 1622 bps
If the time for an ACK is added, then, depending upon the distance, the total cycle time grows to about 15.5 sec (for 1500 m range), and if the system is polled, then an additional cycle-init packet plus travel time is necessary, increasing this by another 2 seconds, for 17.5 minimum.
Note that It’s unlikely that any modem manufacturer adds any of the polling and ACK cycles to their throughput numbers because that adds range to the calculation.
Suggestions for Installation and Testing:
1. Wear hearing protection when working near the acoustic transducers.
Many of the operating frequency bands that the Micromodem use are within the audible hearing range and can be loud enough to cause damage to your ears after prolonged exposure.
2. Test with transducers in water.
Transducers are designed to be operated in water. Although the modem systems may sometimes work in air. There are sometimes that they will not work which will lead to confusion. It is always best to put all transducers in water when performing end to end tests.
Here is a short list of the primary benefits:
- Transducers remain properly loaded resulting in the correct amount of current draw from the modem during transmit. In some cases transmission in air will result in greater current draw which may cause the magnetics on the power amplifier to over heat.
- Transducers will remain cooler. The ceramic in the transducer gets hot during transmission. If the transducer does not cool properly it may overheat and the potting will delaminate from the ceramic, destroying the transducer.
- Transducers will properly couple to the water for proper transmission and reception of signal.
If there is absolutely no option to put the transducers in water during an end to end test, it is recommended that the transducers make contact with each other to improve coupling, and that the duty cycle of the transmissions be kept to (1) per minute.
3. Smart packet handling.
It would be nice to live in a perfect world where every packet that is sent from modem A is received at modem B and vice versa. However, we don’t live in a perfect world and have to deal with lost packets occasionally. There are several ways to deal with lost packets here are just a few:
- Use the modems ACK feature to verify that the receiving modem successfully received and decoded the packet. If the ACK states that the packet was not successfully received resend the packet.
- Some of the higher PSK data rates are more susceptible to interference than the lower PSK data rates. It may be necessary to develop a system that works with the ACK functionality to record statistics for a particular environment then step through the available data rates until a particular packet is successfully received.
4. Noise tests.
The preamplifier circuitry on the modem is very sensitive and has a great deal of gain in order to receive very small signals in the water. Any noise that may couple into the preamplifier circuitry via EMI or acoustic noise will negatively affect the modem’s performance. Some major culprits of noise in typical systems come from fans, switching power supplies, and other electronic devices. It is highly recommended that the modem be installed far from these sources of noise and that good wiring practices are implemented throughout the system. Performing a spectral analysis on every installation will assist in identifying and isolating interference.
5. Your results may vary.
We extensively test the Micromodem in the field and work with customers to obtain statistical benchmarks for modem performance. It is important to remember that that every acoustic channel is different and may work for or against you in terms of maximum achievable range.
Some typical ranges that we have seen in the field:
- 25 kHz FSK and PSK: 2.5km-4km in both shallow and deep water horizontal channels using omni-directional transducers.
- 15 kHz FSK and PSK: 4km-6km in vertical channels using direction transducers.
- 10 kHz FSK and PSK: 8km-18km in deep water horizontal channels. 4km-5km in vertical channels.
6. Vehicle effects on transducer beam patterns.
The shape of many underwater acoustic vehicles can negatively affect the acoustic beam pattern of an installed transducer. Typically what is seen is that there are acoustic nulls at the front and back of the vehicle and that the beam pattern of the transducer is bent downward when viewed from the side of the vehicle. Acoustic testing that has been performed by us on 9″ O.D. underwater vehicles has revealed that standing the transducer off of the hull by as little as 1″ can alleviate some of the effects cause by the hull shape. Standing the transducer off by as much as 6″ nearly eliminates the problems at certain angles.
7. Where to install the transducer on an underwater vehicle.
Typical transducer placements are on the top or bottom of the hull. There are advantages and disadvantages to both depending on the operating circumstance. For example a transducer mounted on the top of the hull will work great when the vehicle is at depth but will not be able to communicate acoustically when at the surface since the transducer will be out of the water. A transducer mounted on the bottom of the hull will be able to communicate acoustically at the surface but will be susceptible to damage during handling.
Careful consideration should be made when installing the transducer. We have tested many systems and can provide as much assistance as possible.
8. Always protect the transducer with a cage.
Transducers are delicate and as such should always be protected by a cage. Regardless of what the local hydrodynamic expert may say about the drag caused by the cage, a vehicle that has a broken transducer will not be able to relay the required data back to the user.
1. Modem system checks out during and end to end test in water but does not get particularly good range in the field.
- Make sure AGN=250. A low preamplifier gain will result in reduced range.
- Make sure AGC=1 for PSK systems. Turning on automatic gain control will make sure the modem is receiving the signal at a proper gain setting.
- Make sure the supply voltage is correct for the modem being used. Lower supply voltage than what the power amplifier magnetics were designed for will result in reduced source level.
- If your system uses and acoustic array for multi-channel PSK make sure PCM=15 such that the array is turned on and used.