EMRFD Message Archive 5019

Message Date From Subject
5019 2010-09-03 07:32:27 Tim VFO construction technique stability observations
It seems that everyone else here is using DDS's or fancy-pants chips for VFO's these days but I have some observations regarding plain old vanilla LC VFO construction. These may be old hat to many but having tried some different configurations int he past couple weeks, and doing actual cooler-as-a-heat-chamber measurements, the possibilities have become more clear to me.

One thing I learned real quick with my russian surplus gear reduction caps, is that mounting the cap to the front panel using the front threaded holes, and then mounting the VFO board to the cap using the bottom set of holes, is the way to go. Any other scheme allows some sort of flexing to go on when you touch the knob that wiggles frequency at the more than a few Hz level. And real big knobs (nearly 3" diameter) on the VFO dial are a true win.

Dead bug construction seems to have a huge amount of "hit or miss" with respect to tempco. I can literally unsolder a part and solder it back on and the tempco changes by many ppm. My theory is that the residual stress on the capacitor or inductor in some mounting configurations, or maybe just the lead wires, are somehow responsible for introducing extra tempcos to effective inductance and capacitance.

Double sided G-10 PCB material, with one side being mostly ground plane seems to introduce stray capacitances with their own tempcos that can be larger than the tempcos in NP0 caps.

In the end the most stable approach I have found involves T50-6 toroids and having most of the tank capacitance in NP0 ceramic caps, and using simple (indeed crude, etched using nothing more than an Xacto knife to carve out copper) through-hole single-sided PCB. Tank inductor goes over a no-copper area of PCB and the toroid winding leads are tweaked to not put very much stress on them.

Will be doing more experiments over the weekend :-)

Tim N3QE
5022 2010-09-03 08:38:54 Ashhar Farhan Re: VFO construction technique stability observations
tim,

how does one mount the vfo toroid inductor in the ugly method? if i let it
'hang' from its pigtails, then it is unstable, if i stick it to the copper
clad, then the capacitance formed between the enameled wiring and ground is
makes is of a pretty bad quality and that the degrades stability too.

i am at the moment designing my first ever PLL/DDS. i have been a committed
Free running VFO buff until now. getting stable VFO is a purely craftmanship
according to me. it takes time and love to get a VFO going. it is almost an
exclusive preserve of the homebrewers with almost every commercial rig being
synthesized in one way or the other.

the thread on pll / dds design is extremely educative, making me run between
my 'hayt and kimberly', 'IRFD' and wolfram to understand (and only partly, i
guess) the theory behind it.

- farhan

5023 2010-09-03 08:44:50 victor Re: VFO construction technique stability observations
Tim,
I agree with all that you wrote on building stable free running VFO's.
One point I want to add is that my way to make stable oscillator tank inductors is to use regular ceramic coil forms and wind the turns while putting tension on the copper wire, so that the copper wire gets the thermal expansion characteristics of the ceramics. My constructi
5025 2010-09-03 08:55:31 Tim Re: VFO construction technique stability observations
In past dead-bug VFO's I have taken a small (say 15mm x 15mm) square of PCB, used an X-acto knife and soldering iron to peel away all but the edge of copper, and attached the toroid to that. Then this little daughterboard is soldered onto the main VFO board at a right angle. In some cases the toroid is held to the daughterboard with hot melt glue, in other cases with holes for wires, etc.

In other cases I use the X-acto knife to peel away an area of copper under where the toroid is going to go, and sometimes also carve up little mounting pads for the leads.

The above methods are a little awkward and I'm sure there are other and probably better ways.

I feel right now, that the flying leads and the way they support (maybe not very well) the tank components in dead bug probably don't always help the stability. I have found that simply rearranging and resoldering the flying leads in dead bug can make substantial changes to tempcos.

I think Dead Bug is a wonderfully useful technique in general. But for the VFO transistor and tank and the few other components around it, maybe a very simple through-hole secti
5026 2010-09-03 09:21:39 kb1gmx Re: VFO construction technique stability observations
5027 2010-09-03 09:36:27 kb1gmx Re: VFO construction technique stability observations
5029 2010-09-03 12:45:38 Tayloe Dan-P26412 Re: VFO construction technique stability observations
My experience with super glue is that it is a huge inductor Q killer. I
have had it kill oscillation in a regen receiver (super glue coated
coil), a 80m VFO (superglue coated coil), and a 20m VXO (gorilla glue
coated - a super glue derivative).

My experience is to avoid super glue for RF!

On the other hand, it seems to me there may be applications where the Q
killing capabilities of superglue could be very useful (readily
available and cheap).

- Dan, N7VE

________________________________

5032 2010-09-03 15:56:18 kb1gmx Re: VFO construction technique stability observations
5034 2010-09-03 17:28:49 ehydra Re: VFO construction technique stability observations
kb1gmx schrieb:
> I could believe gorilla glue as bad.
>
> [..]

BTW:
You can make a polystyrene glue very very cheap simply by dissolving
styrodur(polystyrene foam) in a glass bottle in acetone. Give a little
acetone in the bottle, then break the polystyrene foam in parts and
store them in the bottle, close the bottle very good. Store the bottle
from your desk away because the acetone is not really healthy in high doses.
Try to saturate the solution by repeating adding foam parts to the
bottle content. In a few days you get a translucent syrup-like glue. The
acetone 'eats' an astonishing mass of polystyrene.

Polystyrene is a very good candidate because it has a very low
dissipation factor including GHz range.

Use the glue very thin! Compact thicker Polystyrene is crispy and will
break. A thin film is elastic and transparent. The acetone evaporates
slowly.

The human body includes acetone by nature. The risk is low.
http://en.wikipedia.org/wiki/Acetone#Health_Information


- Henry


--
ehydra.dyndns.info
5037 2010-09-03 22:35:10 John Kolb Re: VFO construction technique stability observations
For mechanical stability, toroids are often mounted to the PCB with
washers and
screws through the middle hole. Plastic screws are great, brass
screws are OK,
whatever you do, do not use steel screws.

John (don't remember the reason now, just it's a big No No)

At 08:38 AM 9/3/2010, you wrote:
>tim,
>
>how does one mount the vfo toroid inductor in the ugly method? if i let it
>'hang' from its pigtails, then it is unstable, if i stick it to the copper
>clad, then the capacitance formed between the enameled wiring and ground is
>makes is of a pretty bad quality and that the degrades stability too.
5039 2010-09-04 02:26:27 joop_l Re: VFO construction technique stability observations
> I found the W1EL work: http://www.qrp.pops.net/w7el.asp
>

Nice tests. What exactly does 'RTV' stand for as used in 'Standard RTV'? The product is probably named different over here than in the UK.

Joop
5040 2010-09-04 03:05:45 w4zcb Re: VFO construction technique stability observations
Nice tests. What exactly does 'RTV' stand for as used in 'Standard
RTV'? The product is probably named different over here than in the
UK.

Joop

Room Temperature Vulcanizing. It cures by absorbing moisture from
the air.

W4ZCB
5042 2010-09-04 07:23:35 Leon Heller Re: VFO construction technique stability observations
On 04/09/2010 11:05, w4zcb wrote:
>
> Nice tests. What exactly does 'RTV' stand for as used in 'Standard
> RTV'? The product is probably named different over here than in the
> UK.

Silcone rubber stuff, made by Corning, IIRC.

73, Leon
--
Leon Heller
G1HSM
5043 2010-09-04 08:51:57 Dan Re: VFO construction technique stability observations
RTV= Room Temperature Vulcanizing

Dan W6DAN
----- Original Message -----
From: Le
5044 2010-09-04 08:54:11 kb1gmx Re: VFO construction technique stability observations
5047 2010-09-04 21:07:33 w7zoi Re: VFO construction technique stability observations
Hi Tim and group,

I have not generally had the difficulties that you describe when building "ugly" oscillators. My favorite topology is a Hartley, but I often build others. Stability and phase noise are usually fine with either, although compensation is almost always required.

One thing that I often do is to use one or two higher quality soldering terminals in the oscillator tank. The terminal types that I've use are shown in a new file that I posted under the name "Ugly Oscillator Hints.PDF." Also shown in an oscillator coil with mounting hardware consisting of ridged nylon washers and a 6-32 nylon bolt. A metal nut was used.

I have also had good luck with etched boards, although I've never found them to be necessary. My main station receiver at the present still uses ugly construction in the VFO. Single sided construction is, as you found, the preferred way to build.

One subtle thing was mentioned by Allison in her comments about your experiment. She commented that tank Q would decrease when the coil was mounted close to the board. That certainly seems intuitive, but I had never really done a measurement. Bob Kopski, k3nhi, has a saying, "To measure is to know." Following this, I did an experiment, which is also shown in the new file. The coil that I measured, which is typical of one I might use in a VFO, was mounted to a board and the Q was measured. The wire was not directly against the board foil, but was away by perhaps 50 mils, which is about the thickness of the washer ridge. The measured Q using the method of Ch 7 of EMRFD, was 264. When the same coil was measured in air, the Q went up to 270. The inductance also increased, but only by 0.2 %. This is well within the measurement error.

The starting point that I like to use for HF oscillators for stability is to use a -6 toroid wound with #26 wire, taking care to get the wire tight against the core. This is then resonated with mostly fixed capacitors. Most of the C is C0G type NP0, but about a quarter of the total C is polystyrene. I bought 100 or so of two or three values back "in the day" and am still working from that stash. I don't know if modern variations have the same TC. The ones I usually use were advertised as having a TC of -150 ppm/deg-C, but the error
5048 2010-09-05 02:56:49 Roelof Bakker Re: VFO construction technique stability observations
Hello Wes,

In march 1996, I have performed the same measurement and my findings agree
with yours.
The difference in Q of a toroid mounted flat against a copper clad circuit
board and the same toroid mounted vertical are negligible.

I used also tin plate boxes for mounting filters and found an increased
insertion loss when the toroids were mounted flat to the walls.
This is what I found:

Toroid T68-2, 18 uH, parallel C = 155 pF
Q mounted free standing: 256, F = 3091 kHz
Q mounted flat against single sided copper clad PCB, copper side: 254, F =
3097 kHz
Q mounted against tin plated wall: 193, F = 3081 kHz

With the latter method filter loss was increased by 3 dB.

73,
Roelof, pa0rdt
5049 2010-09-05 03:34:14 w4zcb Re: VFO construction technique stability observations
> Hello Wes,
>
> In march 1996, I have performed the same measurement and my findings
> agree
> with yours.
> The difference in Q of a toroid mounted flat against a copper clad
> circuit
> board and the same toroid mounted vertical are negligible.

Another one which surprised me, which was observed by Roy Lewallen
years ago, was that wire size on a powdered iron toroid was of no
practical consequence when considering coil Q. I found that difficult
enough to believe that I had to measure an experiment as well since it
certainly didn't follow my intuition.

He was right (again), and I stopped working my fingers to the bone
(or at least to the cramp) trying to use the largest wire size that
would fit rather than the smallest wire size that would handle the
current.

W4ZCB
5050 2010-09-05 04:50:27 Dino Papas Re: VFO construction technique stability observations
Hi Wes - where did you post your new ugly oscillator hints file? I checked the group files and your site but don't see it....maybe you just haven't gotten around to it yet....look forward to adding it to my "design file."

73 - Dino KL0S

On 5Sep2010, at 12:06 AM, w7zoi wrote:

> Hi Tim and group,
>
> I have not generally had the difficulties that you describe when building "ugly" oscillators. My favorite topology is a Hartley, but I often build others. Stability and phase noise are usually fine with either, although compensation is almost always required.
>
> One thing that I often do is to use one or two higher quality soldering terminals in the oscillator tank. The terminal types that I've use are shown in a new file that I posted under the name "Ugly Oscillator Hints.PDF." Also shown in an oscillator coil with mounting hardware consisting of ridged nylon washers and a 6-32 nylon bolt. A metal nut was used.
>
> I have also had good luck with etched boards, although I've never found them to be necessary. My main station receiver at the present still uses ugly construction in the VFO. Single sided construction is, as you found, the preferred way to build.
>
> One subtle thing was mentioned by Allison in her comments about your experiment. She commented that tank Q would decrease when the coil was mounted close to the board. That certainly seems intuitive, but I had never really done a measurement. Bob Kopski, k3nhi, has a saying, "To measure is to know." Following this, I did an experiment, which is also shown in the new file. The coil that I measured, which is typical of one I might use in a VFO, was mounted to a board and the Q was measured. The wire was not directly against the board foil, but was away by perhaps 50 mils, which is about the thickness of the washer ridge. The measured Q using the method of Ch 7 of EMRFD, was 264. When the same coil was measured in air, the Q went up to 270. The inductance also increased, but only by 0.2 %. This is well within the measurement error.
>
> The starting point that I like to use for HF oscillators for stability is to use a -6 toroid wound with #26 wire, taking care to get the wire tight against the core. This is then resonated with mostly fixed capacitors. Most of the C is C0G type NP0, but about a quarter of the total C is polystyrene. I bought 100 or so of two or three values back "in the day" and am still working from that stash. I don't know if modern variations have the same TC. The ones I usually use were advertised as having a TC of -150 ppm/deg-C, but the error on that was +/- 50 ppm/deg-C. I also have some -750 ppm/deg C ceramic caps that I got from Digi-Key or Mouser, but I can no longer find those.
>
> By the way, I was tickled to see you doing measurements versus temperature.
>
> 73, Wes
> w7zoi


[Non-text portions of this message have been removed]
5051 2010-09-05 05:27:03 Rick Commo Re: VFO construction technique stability observations
>> Hi Wes - where did you post your new ugly oscillator hints file? I
checked the group files and your site but don't see it....maybe you >> just
haven't gotten around to it yet....look forward to adding it to my "design
file."
>>
>> 73 - Dino KL0S

Try here:
http://groups.yahoo.com/group/emrfd/files/Ugly%20Oscillator%20Hints.pdf

-rick, K7LOG
5052 2010-09-05 06:00:28 Dino Papas Re: VFO construction technique stability observations
Well, of course if I had scrolled down far enough I would have seen it....got fooled thinking the list was chronological until I remembered that it isn't!

Thanks Rick...got it.

73 - Dino KL0S

On 5Sep2010, at 8:26 AM, Rick Commo wrote:

> >> Hi Wes - where did you post your new ugly oscillator hints file? I
> checked the group files and your site but don't see it....maybe you >> just
> haven't gotten around to it yet....look forward to adding it to my "design
> file."
> >>
> >> 73 - Dino KL0S
>
> Try here:
> http://groups.yahoo.com/group/emrfd/files/Ugly%20Oscillator%20Hints.pdf
>
> -rick, K7LOG.
>
>
>



[Non-text portions of this message have been removed]
5053 2010-09-05 08:41:55 kb1gmx Re: VFO construction technique stability observations
5054 2010-09-05 08:53:13 ehydra Re: VFO construction technique stability observations
It depends on the system impedance level. In a 50 ohms system or if
there is a typical 2K input impedance filter or a SA602 mixer this is
certainly more or less true.
If the impedance is very low (for example on an extremly short
electrical antenna), this is not true!

This was practice.

- Henry


--
ehydra.dyndns.info


w4zcb schrieb:
> Another one which surprised me, which was observed by Roy Lewallen
> years ago, was that wire size on a powdered iron toroid was of no
> practical consequence when considering coil Q. I found that difficult
> enough to believe that I had to measure an experiment as well since it
> certainly didn't follow my intuition.
>
> He was right (again), and I stopped working my fingers to the bone
> (or at least to the cramp) trying to use the largest wire size that
> would fit rather than the smallest wire size that would handle the
> current.
>
5055 2010-09-05 12:09:48 Shawn Upton Re: VFO construction technique stability observations
I could buy it. Some time ago I tried to visualize how current would flow in a wire wrapped around a torroid; I concluded that, much like how eddy currents keep current concentrated near the surface of the wire (skin depth), the current would be concentrated not only in the surface but in an area as far away from the other windings too. Perhaps wire guage has such a minimal effect because all the current winds up in basically a square area, say skin depth by skin depth? [Not sure about proving that, though.] Perhaps as long as the gauge is larger than that area, all the extra copper just buys you cooling area, for power applications?

Shawn Upton, KB1CKT
NAQCC 4723


5060 2010-09-06 05:07:59 Tim Re: VFO construction technique stability observations
Thanks for the comments and suggestions Wes!

I found your picture in "Ugly Oscillator Hints" to be very informative. That T50-6 toroid with plastic screw mount was way more sophisticated than my ugly mounting technique, which was at best to suspend the Hartley inductor toroid from its three leads.

I suspect my issues with stability have less to do with Q, and more to do with thermal expansion/contraction and how it affects my ugly-mounted component leads. What I saw in the thermal chamber (aka light bulb in a styrofoam cooler) was that tempcos had more to do with how I had tacked parts down for the run and less to do with the tempco of the actual L's and C's. I know some of this had to do with the toroid but usually that remained fixed while I tried out different C's.

What my crudely etched (e.g. X-acto knife carved!) through-hole PCB's allow me to do is swap parts around to find a good match of temperature coefficients with very little disturbance going on with the rest of the circuit. The terminals you suggest would help me do the same with ugly construction.

You talk a little bit about teflon/ptfe as if you suspect that the insulator electrical quality has a lot to do with stability. I think the huge step forward in those nice terminals is not so much how many gigohms of resistance they offer but simply the fact that they are stable supports. So many times while dinking around with subbing parts in my ugly construction, I find that those megohm resistors and especially 0.1uF bypass ceramics that often double as support terminals, will crumble and crack under the repeated mechanical and soldering stresses. Perhaps they aren't so good for this kind of experimenting with repeated substitution.

Of course I compare these component standoffs with the polystyrene coil forms and terminals that were around when I was a kid... I suspect I destroyed far more polystyrene insulators than I ever succesfully used :-)

On the subject of preferable terminals... in other, non-RF but high impedance projects I have used stake-in Keystone turret terminal in G-10 glass epoxy (no copper) with great success as standoffs. This is my preferred way of doing high voltage voltage multipliers etc. Example Keystone catalog page: http://www.keyelco.com/pdfs/M55p131.pdf
The individual Keystone turret terminals are quite affordable and I could see using them as RF component tie-downs by just carving a little copper away from the PCB.

Tim N3QE

5062 2010-09-06 10:39:40 w7zoi Re: VFO construction technique stability observations
Hi Tim, and gang,

Glad to hear that the posting was useful.

I have used one form or another of toroid mounting for years now. In some cases I just had a bolt (usually nylon) that held the toroid down to the board with a couple of flat pieces of circuit board material that had all copper removed. Those nylon washers with the ridges are nice, but are sometimes hard to find. I've used some much larger diameter nylon washers from the hardware store on some projects.

Even though my practice has always been to bolt things down, some friends will often build things "ugly" where no toroid mounting is used for a vfo. They have managed to mount the toroid to the usual quarter or half watt resistors with the toroid supported only on the leads. Three leads is a minimum. They have been able to realize adequate stability and to temperature compensate things. I tend to throw a lot of support "terminal-resistors" into my ugly construction, but some of the same friends use virtually no extra components.

I agree though that some of the 0.1 uF caps, etc, do tend to go to pieces when soldered more than one time.

Harold (w4zcb) brought up the issue of toroid inductor Q versus wire size. Large wire is always good for Q. We ran extensive experiments at Tektronix back "in the day" when we were designing the 100 kHz resolution bandwidth filters for the 492 spectrum analyzer. I was using T50-6 cores, but needed all the Q I could muster. We would use four tuned circuits (actually, a pair of double tuned circuits) to achieve a response that was 100 kHz wide at the 6 dB points. What we found was that with #19 wire (as I recall) we were able to get Qu between 290 and 300, and a bit higher once in a while. But we could do this only when the core was filled with wire and was machine wound such that the wire was right against the core. Dropping the wire diameter so something smaller, such as #22, dropped the Qu to the values quoted the other day where Qu was 260 to 270. This is a small change, but it was enough to cause us problems in realizing the needed filters. Incidentally, we had to temperature compensate those filters.

Oscillators where stability is important are a slightly different situation. If one is going to build an oscillator, you want to use a wire size that is on the smaller size, even if Q is compromised a little. What becomes important is to get the wire down on the core. If you wind a core with larger wire, a final inspection often reveals some loops in air. These will expand and contract with temperature changes and completely trash the intrinsic TC of the coil. But if you get the wire right on the core, you can often realize the 35 to 50 ppm/deg-C that is quoted for the -6 toroid material.

One reader ask me if I still do the w7el trick of boiling the coil in water. Yes and no. I don't usually use water. However, what I find is that the first run up and down from room temperature to 70 or 80 C and back will anneal the wire enough to stabilize it. The phenomenon is very real. This was reported in Roy's classic August 1980 QST piece.

Oh--another piece of old Tektronix practice: We used to hold toroid to PC boards with what we called "rat tails." These were a rubber band like piece of material in the form of an elastic string that was a couple of inches long. The ends were tapered. A couple of extra holes were included in the PC board, one aligning with the inside of the toroid and the other next to the outside. The rat tail would be threaded into the holes and around the toroid and would be expanded to hold the core in place against the board. The loose end would then be clipped off. This kept the toroid from coming off the board during vibrati
5063 2010-09-06 12:06:29 chuck adams Re: VFO construction technique stability observations
On Mon, 2010-09-06 at 17:39 +0000, w7zoi wrote:
>
> Hi Tim, and gang,
>
> Glad to hear that the posting was useful.
>
> I have used one form or another of toroid mounting for years now. In
> some cases I just had a bolt (usually nylon) that held the toroid down
> to the board with a couple of flat pieces of circuit board material
> that had all copper removed. Those nylon washers with the ridges are
> nice, but are sometimes hard to find. I've used some much larger
> diameter nylon washers from the hardware store on some projects.
>
...snip snip...

Two ideas for experimentation. Since the ridged nylon washers are
almost impossible to find in short order, I have tried two different
approaches.

1. Use two washers, the large one on the outside between the board
and the toroid and a smaller on the inside to add a centering force
to hold the system in place. Same for top.

2. If you can't find the right sized small washer for center support,
then take a drill and while rotating a washer (the same as the outer
washer) held centered with a screw and bolt use a file to create the
correct size washer as the inside centering support.

Both solutions cost more in material and some time, but will work
for some additional mechanical and electrical stability. Hope this
helps.

FYI

chuck



--
chuck adams, k7qo
http://www.k7qo.net/
chuck.adams.k7qo@gmail.com

/* no comment */
5064 2010-09-06 13:05:30 Harold Smith Re: VFO construction technique stability observations
For what it's worth, I've had some success mounting VFO (and other) toroids
with double-sided tape. It typically won't hold against severe vibration,
but is good enough to insure that the toroid won't move as the radio is
handled, such as when tuning.

de KE6TI, Harold


[Non-text portions of this message have been removed]
5065 2010-09-06 16:34:23 kb1gmx Re: VFO construction technique stability observations
I've used all the tricks at oen time or another but often in combination.

The last VFO I did was for a 10M rig with an offset conversion
PLL to get from 3mhz to 36mhz (if+). That used one of those
neat posts I had for years the lug on the tuning cap and ground
was the frame of the cap to support a typical tapped Hartley
T50-6. I did use #26 wire and it was dipped in Qdope. the end
affair was quite rigid and once I got the right amount of n750
cap in place it is stable. I might add it was not dead bug on
board but literally hanging off and mounted to the tuning cap
it self.

One source of drift that was plagued with initially was DC power
was via 6.8V Zener and the warm up drift was never ending. Replaced it with a 78L06 and that left me with some warm up drift and temperature related drift. when I measured the Zener it was many millivolts/degreeC worse than others in the same strip.

Also some trimmers have very unpredictable drift typical
50pf plastic types to sometimes be good but not reliably.
I prefer to use glass or ceramic types.

Also every VFO I've done is in a box and that box is in the box
Sometimes for RF isolation but also thermal isolation. It does
help.

Many of the better tube radios have good stable VFOs, and look
at the heat. But once warmed up they do stay put. It's all construction.


Allison

5066 2010-09-07 06:47:19 Tim Re: VFO construction technique stability observations
Interesting to go back and read Roy's "Optimized Transceiver" again. He assigns a lot of blame for instability onto PCB dielectrics. He avoids the issue by doing point-to-point construction.

I will have to take what I've learned from your pictures of toroid mounting and standoff terminals, and go back and see if I can improve my dead-bug techniques to the point where I get better than printed circuit board mechanical stability.

Really starting to think I should just skip all these PCB-board boxes and go straight to a die-cast VFO chassis with real standoffs like you suggest, instead of dead-bug or PCB for the tank.

Tim N3QE