Group SD0606

From ECE Department Wiki

Contents 1 Project Overview 2 Key Project Areas 3 Completed Circuits 4 Members 5 Key Terms 6 Links

Project Overview

• Our project is called the Improved RF Amplifier II. This project was suggested by Dr. David Rogers. He originally found an article in the December 2005 QST which featured an amplifier with automatic voice activated switching, SWR monitoring, and filtering capabilities all included in one enclosure. This article was the inspiration for our project. Many amateur radio operators like to operate in a mobile setting, whether it be driving down the road, hiking, camping, or numerous other applications. The radios built for portable operation are typically small and will output a maximum of 5 watts. This is great for rural settings, but in an urban setting, 5W does not generate the long distance contacts that many hams like to make. The Improved RF Amplifier will allow the user to set up in their own home and broadcast at up to 100W if they wish. The amplifier must also offer the user the flexibility to broadcast at high power, or easily broadcast at lower radio power as well without the need to unhook or move the amplifier unit. This is accomplished using the VOX Control circuit. The amplifier unit is designed to work on bands between 1.8MHz and 30MHz.

• There are however restrictions for such a unit. The Federal Communications Commission (FCC) governs the Amateur Radio Service in Part 97 of their rules. More specifically, our project is governed mainly by two sections as follows:

FCC Part 97.315:

(a) No more than 1 unit of 1 model of an external RF power amplifier capable of operation below 144 MHz may be constructed or modified during any calendar year by an amateur operator for use at a station without a grant of certification. No amplifier capable of operation below 144 MHz may be constructed or modified by a non-amateur operator without a grant of certification from the FCC.

FCC Part 97.307:

(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission. For transmissions installed on or before January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must not exceed 50 mW and must be at least 40 dB below the mean power of the fundamental emission. For a transmitter of mean power less than 5W installed on or before January 1, 2003, the attenuation must be at least 30 dB . A transmitter built before April 15, 1977, or first marketed before January 1, 1978, is exempt from this requirement.

• The first rule, FCC Part 97.315(a) only allows for a licensed amateur to build one amplifier unit of one model per year if that amplifier is capable or to be used at any frequency below 144MHz. A non-licensed person or business is NOT allowed to build such a unit without special permission from the FCC. This is why hams who wish to amplify in the HF bands must build their own unit.

• FCC Part 97.307(d) tells us that for operation below 30MHz, any harmonics generated must be at least 43dB below the fundamental frequency. That is the motivation behind filtering. This rule helps maintain clear and easily understand transmissions and prevents a bunch of harmonic noise clutter up the amateur bands. To comply here, the filter aspect was included in our final product.

• SWR monitoring is something that can normally be done externally. Having an on-board SWR monitor that will prevent the amplifier from operating at an SWR which is two high is a nice addition that nearly eliminates the need for a separate SWR meter to be constantly in the signal path. The fewer things connected in the signal path, the cleaner the signal and the higher the efficiency of your radio transmissions. Causes of high SWR can include broken antennas, bad cables, and bad connections.

Key Project Areas

Amplifier

• The amplier we are using is the Motorola AN762 supplied by Communication Concepts. The AN762 is a kit build project, allowing the user to hand build it. If need be, it will output up to 140 Watts. We will only be asking it to give us 100W.

Filters

• In general, most transceivers (radios) emit a very clean signal. A filter on the radio signal would be virtually worthless. The amplifier does introduce even more non-linearities which in turn cause not only the fundamental to be present, but harmonics will exist as well. To prevent noise and interference, as well as to comply with FCC rules, we built passive low pass filters that cover the HAM bands for 1.8 to 30 MHz. There are some cases where two bands lie close enough together to allow us to use the same filter for each band. We had looked into using active filters but they were to cost prohibitive to implement. We also were not able to find ones that would meet the power requirements.
• We picked out the filters by looking at what other Amateur radio users have used for similar cases. The values that we decided on,are shown to the left in the chart. They were designed to meet the FCC requirements that all harmonics would be 43 dB below the fundamental. The inductor cores were purchased from Amidon. For each inductor core Amidon lists an equivalent inductance/turn. Through this we were able to figure out how to choose our inductance for each filter. The low pass filters are a cheby type one because there is ripple in the passband.
• We verified the filters by testing them two ways. We first tested them by using the equivalent ideal model for each filter set in Matlab. We found that they meet our requirements. Next we constructed a filter on a PCB and tested it on the network analyzer. The analyzer showed that the filter meet our needs and there was sufficent suppression outside the fundamental. We were not able to construct it on a breadboard because at the frequencies that we were working at the breadboard would introduce unwanted properties. The final circuit with the all the filters on one board was also tested. I found that at about 75 MHz the filters started increasing in magnitude. This was not predicted by the model and it appeared that it was the relays that were the culprit because this is what they all had in common.

Filter Control Board

• We used an a PIC16F786 to control which filter was being used. The user is able to turn a knob on the front panel and it will then change filters based on the A2D reading. This method was used was because it would keep the wires being run to the front of the case to a minimum. We had some problems initially with interference on the PIC. On certain bands when we would transmit it would switch the filters off or switch to a different filter. We narrowed this down an interference on the five volt supply. What was we saw was a substantial ripple in the voltage that the PIC was not able to handle. It would cause the A2D reading to change or even caused the PIC to reset. This was solved first by putting a ground plane on the PIC board. We also used all shielded wire and moved the voltage regulator to the outside of the case.

SWR Protection

• Using a PIC micro controller to measure the (S)tanding (W)ave (R)atio, the circuit will be used to determine safe operating conditions for the amplifier. By having the SWR measurement, the circuit will be able to determine if the connected transmission line is closely balanced to the output impedance.

VOX Control Board

• The VOX control board helps make the Improved RF Amplifier much more user friendly to use. VOX stands for Voice Operated Transmission. The VOX control takes three different inputs. The first is a manual switch, mounted on the front of the amplifier enclosure. The switch is for the user to turn the amplifier ON or OFF. If switch is off, the amplifier will not be used while broadcasting. If the switch is ON, the VOX then looks for a voice transmission. The amplifier will be switched out of the signal path if their is no voice transmission present. This ensures that a signal being received will not be filtered or amplified before it comes into the radio. Once the user is transmitting with the radio, the VOX will switch the amplifier and filters into the signal path, provided the SWR is at an acceptable level. The SWR input is the third input with which the VOX Control makes it's decision. If the SWR is too high, the VOX Control will switch the amplifier and filter out of the circuit, allowing only the radio power (maximum 5 watts) be transmitted into the antenna, rather than the amplifier's maximum of 100W.
• The VOX Control uses two 2-Input AND gates connected as a single 3-input AND gate to make decide whether or not to switch the amplifier into the circuit. The amplifier and filter modules are considered normally off, that is they are not used in operation unless the three conditions are satisfied: The manual switch is ON, the radio is broadcasting, and the SWR is at an acceptable level.
• The manual switch is a single pole double throw (SPDT) switch with the center contact connected to one of the AND gate inputs, and one outer connection is connected to +5VDC and the other to Ground. This configuration is required so that the AND gate only sees a true logic 1 or 0. At first a single pole single throw (SPST) switch was tried. This caused the AND gate input to "float" when the switch was off, making the AND gate see something other than GND or logic 0. The SPDT switch fixed the problem.
• The "Voice Operated" part is accomplished using three wire wound ferrite core transformers connected as a directional coupler. The directional coupler configuration used is actually considered a bi-directional coupler. The coupler has two input ports and two output ports. The two input ports are directly connected and put into the main line of the circuit. The output ports are where the forward (or reverse) traveling signals are coupled to. Since this operation only used the forward traveling wave, the reverse traveling wave was simply coupled to a 50 Ohm resistance. The forward wave was rectified using a diode and capacitor configuration and then input into an op-amp comparator. The non-inverting input was connected to the rectified input, and the inverting terminal was connected to an input of 0.25 VDC. The op-amp, when used as a comparator, will saturate to the power supply of whichever input is higher. In this application, if an input was detected on the non-inverting terminal that was larger than the 0.25VDC that was connected to the inverting terminal, the op-amp saturates to it's positive supply voltage of +5VDC. If there is no input present on the non-inverting terminal, then the 0.25VDC on the inverting terminal is more than sufficient to saturate to the negative power supply, which in this case is grounded. The purpose is to digitize the presence of an input. Their is either a radio input (+5VDC from the op-amp, logic 1) or there is no radio input (0VDC from the op-amp, or logic 0). A standard LM741 op-amp was tried at first, but was not successful. The 741 is not designed to be used on a single sided supply. The final version used was the MCP602, which comes in a dual op-amp package from Microchip. A voltage divider was needed to bring the inputs within usable ranges for the MCP602, but that was a small matter and the MCP602 turned out to work great for this application.
• The third input to the AND gate is from the SWR meter. The SWR meter would send a logic 1 when the SWR is high, and a logic 0 when the SWR was OK. An logic inverter is used from the SWR meter to allow the 3-input AND configuration to be used. As of the writing of this entry, the SWR circuit had not been implemented into the final product, and thus the SWR input for the AND gate was simply grounded to make the circuit work using only the switch and the VOX circuit.
• The actual switching that takes place is done using electromechanical relays. Once the AND gate output is true (logic 1, +5VDC), the signal is used to energize two relays, causing them to switch the amplifier and filter into the circuit. Each relay is a 12VDC relay. The switching is accomplished using an NPN BJT transistor for each relay. When the AND gate output is low, no current is driven into the base of each BJT, causing them to be "off." When the AND gate output is high, the base of each BJT is driven by an op-amp voltage follower which then allows the relays to be switched on. Resistors are used connected to the base to set the collector-emitter current to the optimal relay current as recommend by the relay data sheets. An MCP602 Dual op-amp package is used for the voltage followers. These are used as "current buffers." The AND gate cannot supply much current, and to be safe, the op-amp buffer was implemented to preserve the AND gate longevity.

Imedence Matching

• Impedance matching was not an issue. There was not enough loading on the main line to require a matching circuit to be made. Amplifier performance may be enhanced by extra tweaking on each operating band, but the impedence was close enough to 50 Ohms to allow normal operation.

Completed Circuits

Members

• Dan Johnson
• Andrew Roberts
• Kevin Novacek
• Dr. David Rogers

Key Terms

• RF(Radio Frequency)
• HF(High Frequency)
• Amplifier
• SWR(Standing Wave Ratio) Protection
• VOX
• Directional Coupler
• Electromechanical Relay
• Impedence Matching
• c = λf

Links

• Communication Concepts supplier of our Amplifier kit
• Amidon supplier of the inductor cores we are using
• Amatuer Radio Relay League
• FCC Part 97 The FCC rules that regulate the Amateur Radio Service, as well as our project