Controlling DDS with a Variable Inductor - Page 1
Forum section: electronics / projects

author: NY2KW
author_rank: Regular Contributor
timestamp: March 11, 2021, 05:54:45 pm
content: I have an old radio that uses a permeability variable inductor (~ 1uH - 3-uH) within a vacuum tube VFO (PTO). it offer very smooth, precise tuning over a full 360 rotation that covers 100 kc band spread. I want to replace the tube oscillator with a DDS but retain the permeable inductor to retain the old design as the electromechanical tuning interface. The new DDS/PTO would cover the HF band between 1.8 - 2MHz and that then goes into a mixer stage. I am looking for a circuit concept that would allow the inductor range of 1 - 3uH to linearly vary a voltage from e.g., 0 -5vdc TTL to be read by an Arduino controlled DDS. The perm inductor is in a shielded, temp compensated enclosure within the radio. Any advise where to start would be much appreciated.Jerry

author: Kleinstein
author_rank: Super Contributor
timestamp: March 11, 2021, 07:07:46 pm
content: May be keep the idea of a variable osciallator and let the µC measuren the frequency. Not to get the frequency drift back to the DDS as one would get it with a simple 1:1 transfer, one could use some velocity algorithm: so change the DDS frequency only if the sensor frequency changes fast and the faster the more. This would be a little like used on some trackpads. So there would be no direct 1:1 correspondence of knob position to the frequency, which could be a bit confusing.With a fixed frequency raster one could to a more 1:1 mode, just with rounding to avoid slow drift - still a chance to get some jumps.

author: mawyatt
author_rank: Super Contributor
timestamp: March 11, 2021, 07:20:50 pm
content: Use the inductor in a one shot and use the Arduino to measure the PW.Best,

author: NY2KW
author_rank: Regular Contributor
timestamp: March 11, 2021, 07:23:07 pm
content: Quote from: mawyatt on March 11, 2021, 07:20:50 pmCould you explain what you mean by "... in one shot///"Jerry

author: NY2KW
author_rank: Regular Contributor
timestamp: March 11, 2021, 07:26:22 pm
content: Quote from: Kleinstein on March 11, 2021, 07:07:46 pmMy objective is to retain the old radio look and feel for tuning. The large tuning knob shaft is mechanical tied to the permeability inductor shaft. I want to avoid redesigning a new mechanical linkage and feel for the radio. The permeability inductor is really a thin g of electromechanical beauty and when lubes properly makes the tuning knob feel like supersmooth control with no backlash, etc.

author: Renate
author_rank: Super Contributor
timestamp: March 11, 2021, 07:47:13 pm
content: It sounds like you're going forwards and backwards.If you want to use the inductor it would be best to use it in the original VFO.Then you'd end up measuring a frequency to generate a frequency.Why not get a nice optical encoder to put on the back end of the knob?Feed a TCXO or ovenized or whatever to an Analog Devices DDS, you're done.You'll need a µC in any case.You can make it variable slew; turn it a lot and it moves quickly, turn it slowly and it moves slowly.

author: mawyatt
author_rank: Super Contributor
timestamp: March 11, 2021, 07:53:17 pm
content: Quote from: NY2KW on March 11, 2021, 07:23:07 pmOne shot or mono-stable multivibrator, where output PW is controlled by inductor.Best,

author: NY2KW
author_rank: Regular Contributor
timestamp: March 11, 2021, 08:11:14 pm
content: Quote from: Renate on March 11, 2021, 07:47:13 pmI understand what you are saying and I have made a DDS VFO for other radios. The smooth, high end encoders i have seen on some nice radios are all relative rotation, they have no end stops. The radio I want to update has the main tuning knob tied to an old fashion sliding pointer on a imprinted transparent frequency scale. Nothing digital... all 1940-50. Once calibrated the relationship between inductor value and mechanical frequency readout is excellent. As to rebuilding the VFO, yes that is possible with some silver mica caps and some NP0 ceramics but that is not the project.I guess I should have framed the question more theoretically without mentioning the radio.....what is the most stable circuit to use a 1-3uH RF inductor to vary a DC voltage between 0.0 - 5.0 vdc?? I am not an EE :-)

author: Renate
author_rank: Super Contributor
timestamp: March 11, 2021, 08:46:35 pm
content: You want a stop? Put in a stop.You just need a biphase encoder and a reference mark, either index or end stop.Throw in a coin cell and you won't have to re-home it.

author: NY2KW
author_rank: Regular Contributor
timestamp: March 11, 2021, 09:16:44 pm
content: One shot or mono-stable multivibrator, where output PW is controlled by inductor.Sounds perfect, i can breadboard that easily. Thanks

author: fourfathom
author_rank: Super Contributor
timestamp: March 12, 2021, 06:37:02 am
content: Obviously you are free to chose whatever design concept you prefer, but why do you want to go with a DDS? If you are looking for better stability, I predict that any variable inductor-tuned one-shot will have worse stability than the original LC oscillator. If you want to get rid of the vacuum tube, why not retain the inductor and associated capacitors and use them in a transistor oscillator?

author: KE5FX
author_rank: Super Contributor
timestamp: March 12, 2021, 08:23:36 am
content: If all you're trying to do is get rid of drift, consider a"huff and puff"stabilizer, maybe. What you're describing sounds like extreme overkill.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 12, 2021, 10:40:12 am
content: Why do you want to battle such a design, are you the one who wanted to do a similar mod with a variable capacitor ?I have a 1957 Philips multibands radio from my deceased father, changed the lamps, redone the dial cord, changed the caps while the same physical caps where available, and it still works, i can hear from Canada some Chineese Scottish and other countries at nightYou would need an arduino to act like an lcr meter, the read values to be converted into the dds interface ... one of my friend tried it, it wasn't perfect, ended with an dial encoder with more notch / clicks / pulses than a regular one (cant say where he got it) but the encoder was around 1.5 to 2 inches in size ?? not sure if it was an optical oneAn optical encoder could be made ?? if you have a fine printing say at 1200 dpi, an optical wheel could be easily done

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 12, 2021, 01:46:38 pm
content: What about find a way to add a potentiometer? Could also couple it with making the knob touch sensitive so that it stays put when the user is not touching it.

author: NY2KW
author_rank: Regular Contributor
timestamp: March 12, 2021, 02:54:13 pm
content: Quote from: KE5FX on March 12, 2021, 08:23:36 amIts not about drift. The radio design HF frequency is based on mixing a crystal oscillator with a PTO. The crystals (a set of 10 for ham bands) are getting hard to find and as I said repair of the Collins PTO requires special jigs, test stands that the average Collins radio owner does not have nor makes it worthwhile. I think when there were designed/made in the 1950's it was cheaper to replace the entire PTO 3"x3"x3" box than service it. However, around 10 years ago, some entrepreneurs started selling an external box that replaced both the Collins XO and VFO. These were all DDS based with nice digital displays, optical encoder tuning knob, etc. They work great, I own one of them. But for those who like old radios its the touch and feel not the electronic functionality which can be achieved with modern designs. To keep that original 'front-end', what I envisioned was what most ham radio guys can do... remove the the original PTO, take out the glob of passive components, retain the electromechanical tuning design which was exquisitely designed by Collins and mechanically tied to these one permeable inductor. Fools mission? I'm sure many would say yes except those crazy like me which I know is not a small number :-)

author: iMo
author_rank: Super Contributor
timestamp: March 12, 2021, 03:17:35 pm
content: Your basic problem is the thermal coefficient (TC) of your variable inductor is xx times higher than TC of the crystal in the DDS. So ANY great solution will always be as stable as the TC of the inductor.Unless you decide your DDS tuning step will be something like 100-500Hz, thus the TC of the inductor will not have much influence on the DDS tuning (perhaps you will hear some +/- tuning step jumpsfrom time to time..). You have to measure the L and based on that tune the DDS.It could be easily done with a single 16F84A (or any modern alternative) and LM311 - the great famous precision LC meter (google it - you'll get zillion references).. The PIC can measure the L and drive the DDS directly..google "16F84 16F628 16F84A 16F628A LM311 precision LC meter"PS: with several measurements per second you may calculate a "median" of L measurements thus you could "smooth" out the influence of the L thermal fluctuations on the tuning step, imho..

author: NY2KW
author_rank: Regular Contributor
timestamp: March 12, 2021, 03:41:08 pm
content: Quote from: imo on March 12, 2021, 03:17:35 pmYes, understood but isn't that the same situation in the original PTO design? There is no temp compensating feedback circuitry. The perm inductor is no lightweight and not the typical cardboard wound coil/slug. Its the chunkiest component in the tightly spaced PTO box and has surprising mass. These radios always require 20 minutes for the cabinet/chassis temp to cook until stable. My wishful thinking is that the perm inductor thermal mass after a while will be provide as consistently a stable inductance value as in the original design.... whcih works well for its intended tuning function. Am I being too simplistic in that regard?

author: NY2KW
author_rank: Regular Contributor
timestamp: March 12, 2021, 03:49:23 pm
content: I think I can answer my own question. In original design, the perm inductor's function to set the RL constant for the oscillator could be relatively insensitive to small thermal fluctuations as the specs where not fractions of a Hz but +/- few Hz were OK. If I use the perm inductor to generate a range of PW signals to control a DDS, can I dampen (if that's the right term) the noise from small thermal changes even after reaching some temp steady state? Averaging? Maybe add Thermistor as a calibration for the PIC to adjust the PW?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 12, 2021, 05:15:00 pm
content: A variable frequency osciallaor is a conventient way to read the variable inductor by the µC. One can use averaging to reduce noise - though this changes the feeling / response when tuning. It may take more than just simple averaging, more like averaging as long as there are no significant changes and faster reaction if there is a significant rate of change.An extrea temperature compensation would also be possible, by additionally reading the temperature. It may not be needed if one has a good way to switch between an adjustment mode (reactor fast and also to small changes) and a just keep the old value mode (keep the old set value for the DSS until a significant movement is detected). This would also eliminate the need for explicite averaging.

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2021, 05:22:23 pm
content: The original PTO design is a “normal” LC harmonic oscillator, which, in general, will be more stable than a relaxation oscillator (multivibrator) using the same inductor. Good PTOs, such as in the famous Collins equipment, often employed open-loop temperature compensation using capacitors with an opposing temperature co-efficient. If you want to improve on the performance with a digital synthesizer from the same knob, I think you will need to use a mechanical connection to a rotary encoder, as suggested above.

author: NY2KW
author_rank: Regular Contributor
timestamp: March 12, 2021, 05:30:44 pm
content: Quote from: TimFox on March 12, 2021, 05:22:23 pmI will tinker with fitting an encoder into PTO box and get back to the group. Thanks for all the constructive comments

author: iMo
author_rank: Super Contributor
timestamp: March 12, 2021, 06:06:15 pm
content: Ok, so imagine following:1. you have got a stable PTO2. you don't need the PTO for your rig as you are going to use DDS3. thus feed the output of the PTO into the pic16F628 (or any alternative, call it "pic16") frequency meter/counter - there is a plethora designs available on the web4. the pic16's input can be fed with up to 50MHz without a prescaler, no problem (I did it many times)Now the idea:5. I would change the capacitors in your PTO such it outputs say 30-35MHz within the dial scale range (or any other high-enough frequency)6. the PTO's L (1-3uH) is just a part of the L_total, so I do assume it will be stable "enough"7. the pic16 will do, say, 20 measurement per second, you will get (35M-30M)/20= from 0 to 250.000 counts in each measurement (it means the 250.000 freq steps in your dial scale)8. based on the counts you will drive your DDS, ie. such it does tune from 1.8-2.0MHz with a step of 200.000Hz/250.000=~1Hz (theoretically, there will be noise say +/- 10.. That will be, say, 10.000 "clean frequency steps")9. all you need is a pic16 counter - again - dozens of designs are available (pic16, 8MHz crystal, 7805, and perhaps 1 transistor for the input signal conditioning, plus 3-5 resistors and maybe 6 capacitors, etc.).10. the pic16 may do smoothing or any math tricks with the measured data and then after some (A-B)*C/D it can feed the calculated DDS word into your DDS VFO.11. moreover - it can measure temperature and make TC compensation - etc, etc.12. total HW costs - say $5 plus fw

author: mawyatt
author_rank: Super Contributor
timestamp: March 12, 2021, 06:09:07 pm
content: One issue to consider is getting the frequency to agree with the scale on the receiver. Since this was developed for a LC oscillator, the frequency can be approximated as omega ~ 1/Root(LC), omega = 2*pi*F, where L and C are the equivalent total values which include circuit parasitics and other elements.F = 1/(2*pi*Root(LC)), since you are interested in the change of F wrt to the inductor L, thendF/dL = (1/(2*pi))*d(1/Root(LC))/dL = (1/(2*pi))[-1/2*1/RootC*1/(L)^3/2]dF/dL = (1/(2*pi))[(1/Root(LC)* 1/L)][-1/2]dF/dL = (-F/2)/Lor Delta F = -(F/2)[(Delta L)/L]So a change of 3X in L produces a -3/2 change in frequency.Hopefully I didn't mess up the math.Best,

author: NY2KW
author_rank: Regular Contributor
timestamp: March 12, 2021, 06:19:25 pm
content: I will tinker with fitting an encoder into PTO box and get back to the group. Thanks for all the constructive commentsAttached is a picture of the PTO to show relative size.... note the permeability inductor 'tuning' shaft is located far off-centered on the face of the PTO because it is aligned and hard-coupled to tuning knob shown in the 2nd picture. The shaft is so far off center it comes off of the enclosure face only about 1/2" from the enclosure floor. I have been looking at some nice quality encoders and their mounting size is too large for the inside of the PTO enclosure and cannot be mounted outside the PTO enclosure because the PTO is bolted to the top of the radio chassis. Maybe I should go back to the original LC oscillator approach but make it into a solid state PCB with SMT C0G quality components and leave the permeability inductor for tuning and the variable cap also seen on top of the PTO for end-spread/calibration adjustments. No DDS. For those interested on how incredible these PTO were designed back when...http://www.collinsradio.org/pto-overview-specifications-history/

author: NY2KW
author_rank: Regular Contributor
timestamp: March 12, 2021, 06:23:37 pm
content: Quote from: mawyatt on March 12, 2021, 06:09:07 pmYes, 2 complete rotations of the perm inductor provides a linear range of 200.0 kc. Collins chose to wind the inductor logarithmically to provide linear dial scale readout.

author: iMo
author_rank: Super Contributor
timestamp: March 12, 2021, 06:24:40 pm
content: What is you PTO output frequency - from - to?Scale linearity vs. PTO freq vs. DDS output - it does not matter - you may do any math inside the MCU..The goal is to get at least 0..250000 counts 10-20x per second (see my above calc). That will make the DDS tuning smooth, with good resolution withing the scale.An example (see above):Code:Scale PTO  MCU delta counts (20/sec) DDS word-----------------------------------------------------------------------MIN 30.000.000 0 X + f(0)MAX 35.000.000 250.000 X + f(250.000)

author: mawyatt
author_rank: Super Contributor
timestamp: March 12, 2021, 06:44:31 pm
content: The one-shot use of the inductor can be estimated using a unity step response of a series LR, with step provided to L and R is to ground. A little circuit analysis will show the voltage across R as:Vr =Vstep (1-exp(t/tau)), where tau is the LR time constant, soVr/Vstep = 1 -exp(t/tau), assuming the trigger as 1/2, thenVr/Vstep = 1/2 = 1 -exp(t/tau) and solving for tt = -tau*ln(1/2), or t = 0.69*RL, then the change in time or PulseWidth wrt L is,dt/dL = 0.69*R, soDelta PW = 0.69*R*(Delta L),(Delta PW)/PW = 0.69*R*(Delta L)/(0.69*R*L) = (Delta L)/L,So the change is PW percentage is directly proportional to the change in L percentage, or a 300% change is L causes a 300% change is PW.Again, hope I didn't mess up the math....been a long time since I was an adjunct prof teaching grad schoolBTW those old Collins radios are masterpieces of engineering and workmanship, like the old Tektronix scopes and HP equipmentBest,

author: fourfathom
author_rank: Super Contributor
timestamp: March 12, 2021, 06:58:17 pm
content: Quote from: NY2KW on March 12, 2021, 06:23:37 pmThat is a nice trick. But I suppose not that much different than the presumably logarithmic plate profile on the old "365 pF" AM radio tuning capacitors.