Motorized tuner project
Craig Tull
Posts: 7
Greetings to all from Papalote, TX
I need some (ALLOT) of help with a project.
I am trying to re-vamp an old antenna tuner by adding a motorized vacuum capacitor to the existing motorized inductor. This is an 80 vintage piece of equipment and I think there must be an easier way of getting this done. The original tuner (simplified) uses a binary input through analog multiplexers to select one of 32 multi turn pots and uses that through a comparator (op amps) comparing the reference voltage from the feedback pot on the motor assembly to the selected channel pot it then drives an H bridge to move the motor one way or the other until the voltages are the same, then it switches to a directional RF bridge that produces a bias voltage and an error voltage to fine tune the unit when RF is applied using the same type of comparator circuitry, it also uses op amps to produce a upper and lower limit control to keep the motor from over running the limits of the inductor. All in all a fairly large collection of 4000 series chips, now I need to reconfigure it with an additional motorized vacuum capacitor. It would be necessary to reproduce three boards (two 16 channel program boards and one servo board) to get this done using the old technology, only one control circuit needs to use the RF Bridge to fine tune the unit.
I would think that a microcontroller could be used for this purpose, selecting the binary channel input, manually driving the motors into the correct position and then writing it into memory. Unfortunately my old analog brain is not able, even after going through several tutorials, to grasp this programming thing Also this does not seem like wise first project anyway, so I am soliciting help on this, I am pretty competent with the development and implementation of the hardware and circuits I just get lost on the programming end.
Thanks
Craig A Tull
I need some (ALLOT) of help with a project.
I am trying to re-vamp an old antenna tuner by adding a motorized vacuum capacitor to the existing motorized inductor. This is an 80 vintage piece of equipment and I think there must be an easier way of getting this done. The original tuner (simplified) uses a binary input through analog multiplexers to select one of 32 multi turn pots and uses that through a comparator (op amps) comparing the reference voltage from the feedback pot on the motor assembly to the selected channel pot it then drives an H bridge to move the motor one way or the other until the voltages are the same, then it switches to a directional RF bridge that produces a bias voltage and an error voltage to fine tune the unit when RF is applied using the same type of comparator circuitry, it also uses op amps to produce a upper and lower limit control to keep the motor from over running the limits of the inductor. All in all a fairly large collection of 4000 series chips, now I need to reconfigure it with an additional motorized vacuum capacitor. It would be necessary to reproduce three boards (two 16 channel program boards and one servo board) to get this done using the old technology, only one control circuit needs to use the RF Bridge to fine tune the unit.
I would think that a microcontroller could be used for this purpose, selecting the binary channel input, manually driving the motors into the correct position and then writing it into memory. Unfortunately my old analog brain is not able, even after going through several tutorials, to grasp this programming thing Also this does not seem like wise first project anyway, so I am soliciting help on this, I am pretty competent with the development and implementation of the hardware and circuits I just get lost on the programming end.
Thanks
Craig A Tull
Comments
Craig
Posting a simple block diagram along with the number of turns over the tuning range of the coil and capacitor would be a great help. From what you posted I am guessing that 2 pots (or optical encoders), a 4 channel ADC chip, a quad op amp, 2 motors, and 2 motor drivers would be the basic components to do this.
If you can deal with ladder logic, someone, in another forum, posted a program that allows you to draw ladder logic and program a micro with it.
I will try to post some pictures, but all the BS would be doing as far as the hardware is concerned is sending a up or down signal to the two H bridges while receiving the voltage feedback from the multi turn pots on the gear reduction drives and comparing it to the stored info for the "channel" selected.and then after that is done, comparing the bias voltage to the error voltage to fine tune. this should time out after a reasonable time for me to send a tune signal, in the original tuner it was always waiting for an error voltage and that sometimes caused the servo motor to "fibrillate" during high power SSB voice peaks. Above 11 mhz it was disabled as the bandwidth of the preset channel settings provides an adequate match for the transmitter. at 1.6 mhz one can only move a few khz before the reflected power gets too high.
the inductor needs about 20 turns and the cap about 10, but this should not be a concern as the BS would not be counting turns.
It would be real cool if it could decipher the frequency and automatically tune the cap and inductor but that would be a very complex issue given it operates from 1.6 mhz to 30 mhz, I could however see it storing preset info by frequency but that also would be much more complex than what I have in mind.or really need
Craig
Craig
Counting turns, with an encoder and an end-stop switch, would be far more accurate than a potentiometer. One other potential way to feed-back the capacitor value would be to use a light and a linear photodiode array. Note, I've never seen a Vacuum Capacitor in person, only pictures online, so this is just a guess.
What I had in mind was using 2 10 turn pots geared to match the coil and capacitor turns range. The ADC would measure the pot voltages and the microcontroller would use that measurement to control the motors. When the voltage from the pot matches what the micro has stored for that “channel” it stops the motor. There may be some overshoot to correct for but it would be pretty much the same as what you posted.
After the coarse tuning the micro would use the ADC channels connected to the bias and error voltages for the fine tuning. The software could either time out, use thresholds, or peaks/dips to decide when tuning is finished. Once finished tuning it could stop until the “channel” was changed, retune if the bias or error voltage exceeded a threshold, or both.
If you were using a propeller instead of a BS it would be simple to measure the frequency and use that for tuning. The counters on the propeller can handle 30MHz easily, and only needs to have the RF squared and limited to 3.3V p-p. I am not certain but I think you might still need the pots and ADC for determining the capacitor and inductor tuning position. It depends on how the error and bias signals react to being off tune. If they are always low (or high) when tuning is low (or high) it could work without the pots and ADC.
Having it "auto tune" seems to be too big of a project, the L and C interact differently at various frequencies, the RF coupler only looks at the impedance presented to the transmitter and tries to match it to 50 ohms, when I tune the antenna I use an antenna analyzer that looks at impedance and reactance and calculates the most efficient power transfer to the antenna, the impedance alone can be satisfied at several different values of L and C but at only one set of values is the power transfer the most efficient, the RF coupler is only designed to compensate for small variances in the antenna due to weather, humidity, salt spray on ship antennas for example, The original application had one setting for each frequency in a "channelized" scheme, I use it so I can move frequency above or below the pre-tuned setting. The coupler generates a voltage on the bias and error leads in proportion to the RF power, if the impedance is 50 ohms they are the same, lower or higher than 50 ohms the error voltage is ether higher or lower than the bias voltage.
Even having the microcontroler read the frequency and going to programmed setting would be a daunting task, on the higher bands say the 17 meter band it would be relatively simple, it could count 18.110 to 18.168 mhz (the phone portion of the 17 meter band) would only be one setting and the RF coupler would not even be active, But get down to the 160 meter band (1.8-2.0 mhz) it is a completely different story It may require for example 1.800- 1.810 for one setting 1.811-1.820 for the next etc. I figure by limiting myself to 32 settings I should be able to cover most of what I need to.
In the past I have been able to "walk" the tuner by moving frequency in small increments to portions of a particular band I was not set up for, too large of a change at one time and the RF coupler can not handle it.
I do however like the cost of the propeller, and that is a consideration as this little jewel will be living at the base of a 320 meter loop of wire mounted at 65 feet on 4 telephone poles in S TX, this means it WILL get whacked from time to time, I have in the past been directly hit twice in less than two weeks, on one of these I un-hooked the coaxial jumper from the equipment and after the storm I found the N connector that was on the end of the jumper that was laid on the floor blown off and embedded in the wall!! (I do wish I had a high speed video of that!) This is another reason to move towards a microcontroler, I am not looking forward to troubleshooting all those logic gates and op amps after a hit, there would be much fewer components in a microcontroler setup to deal with.
The tuner must me able to be programmed (tuned) in place, as I envision this, one would select the "channel", put the tuner in programming mode with a switch, have momentary switches to manually move the L and C into the correct position as monitored by the antenna analyzer for that particular frequency, then writing it into ram with another momentary switch, Hopefully after all the work is done I could remove the microcontroler or download directly and save the ram information to blow it into another microcontroler as a backup without going through all the initial setup again.
Below is a picture of the prototype "balanced balanced" tuner it is very simple, not seen in the picture is the Choke Balun it is about 75 feet of 1/2 Heliax tight wound around the tuner support pole under the tuner, this converts the 50 ohm unbalanced signal to 50 ohm balanced and feeds it to the two identical and synchronized inductors, up to the C component then out of the tuner on 6" home brew ladder line, 75 feet to the feed point at one corner of the loop. A vacuum cap will be necessary because the conventional capacitor does not have a low enough minimum capacitance to tune the higher bands, the microcontroler will be mounted in a metallic box next to the tuner and the motor, resistor and coupler leads will be RF de-coupled using ferrite beads to keep any "noise" generated by the processor from getting into the antenna. The tuner itself is mounted in an all non metallic environment so the Eddy currents can flow unobstructed around the inductors.
I am using some MDS spread spectrum radios(RS 232 up to 9600 baud) to carry the commands out to the tuner to completely isolate that aspect from the shack, and as I said now I am using a pic and 232 transceiver to convert the ASCII to binary, simply sending a 00 (cr) to 31(cr) using a terminal program but the pic etc. may not be necessary using a microcontroler, but it would be very nice to have a readout of what channel is selected, I planed on using a binary to seven segment display in the original scheme to accomplish this, this would make it easier to be sure what channel was selected at the tuner during setup, also a way to step through the selections during setup would be nice, some feedback from the tuner would be nice to indicate it received and executed new commands etc.
Craig
Looks like we are thinking along parallel lines. I suggest the following:
Use an MCP3204/8 ADC (4/8 channel 12 bit ADC) and have the same power supply for the ADC and the pots then you will get a binary value between 0 and 4095 for the servo position.
Tuning and storing channels would similar but not quite the same as it is now.
1. Select a channel (no reason you can't have more than 32).
2. Tune the capacitor and coil using buttons to move the servo motors up or down.
3. Press the Store Channel button to save the current channel, coil voltage, and capacitor voltage from the servo pots into ram.
4. Repeat steps 1 to 3 for as many channels as you want or will fit in memory.
5. When finished press the Save Table button to store it in eeprom and/or upload it.
If you want to use the wireless modules it would be possible to sit in your shack and do the tuning from there asuming your analyzer can be connected there as well.
I know what you mean. The ICP Spectrometers only had 2 positions (start and run) and sometimes it could take hours to find the optimum tuning points. So many peaks and dips and only one optimum peak.
I'm not sure this would be as hard as you think. Using a frequency count range is not that much different from using a channel number but limiting it to a fixed number of channels does make the software simpler.
This is pretty much the way I pictured the initial tuning being done. Once you have the pot voltage readings done for a specific band it might be possible to extrapolate the settings between those points.
That is the benefit of the propeller. It is relatively inexpensive, uses a separate eeprom that can be copied, and does not need a lot of support circuitry.
Pretty much as I outlined near the top of this post
If the antenna analyzer can be used in the shack you could do the tuning right there and have all of this displayed on your PC or by a second propeller in your shack. The propeller in the shack could drive a video display, small LCD display, or seven segment display and send commands to the propeller in the antenna base to perform all the functions discussed so far.