EMRFD Message Archive 12584

Message Date From Subject
12584 2016-04-03 08:25:08 bob_ledoux VFO Ovens for a Stable VFO
My frequency counter/timer has a crystal oven to maintain a precise frequency standard.

Why not do the same with an LC VFO?

A portable rig requires a stable VFO over a broad range of temperatures. Keeping stability over such a temperature range is hard.

But if a mains powered VFO sits in a temperature controlled shack a different design may be reasonable.

What about setting the VFO in a well insulated box with temperature maintained above anticipated ambient? Increased mass may be a benefit as temperature change might follow a slower variance curve.

Temperature control, say to within 0.5 degrees F might take some thought.  You might get fancy and use some type of PID controller.  But you also might get by with a simple heat source on a variable control, manually monitored and adjusted for minimum variance.

I used to develop color photographs.  We used a water bath to keep developer to within half a degree F.  That is a thought to consider.

This approach may be inconsistent with a pint-sized rig, but maybe not.

The little pinpoint thermocouple temperature sensors found on some multimeters have a very rapid response curve.  Those with digital readout would permit the setup for some interesting experimentation.


12585 2016-04-03 15:58:13 kb1gmx Re: VFO Ovens for a Stable VFO
power consumption.

Insulation will limit rate of change as the VFO changes temperature.

The solution is to go with DDS or NCO if stability is criteria against higher 
power consumption (same for heater).  Note a heater has to run at 
something over 40C (much warmer than ambient) and be able to do 
that down to minimum temp.  The last case can seriously suck power.

Another solution is to temperature stabilize the VFO.

Generally Varicaps have very noticeable temperature vs capacitance 
change for  a fixed voltage.  One solution is to use a good (mechanically)
dual bearing capacitor and add termerature compensation (NPO, n150,
 n750 caps)   Doing that excellent stability can be had.

Also its possible to measure VFO temperature and correct for changes 
(use a small change varicap).

Of course everything in the VFO must be mechanically stable and 
also temperature stable over temperature.  That means -7 iron toroids 
if used or air core ceramic.  Stiff leads, Oscillator design being DC 
stable with temperature and coupled to the tuned circuit as lightly 
as possible to minimize its influence on the LC.  FYI PCB materials 
can be a problem over temperature.

The process is make a chamber and put the unit under test in it.
EMRFD describes how to do it. and much time and patience.

If you do you can net a good result.


12586 2016-04-03 16:21:41 Tayloe, Dan (Noki... Re: VFO Ovens for a Stable VFO
Actually I found very stiff leads can vibrate when the case is shocked  (struck) creating a "twang" to the VFO. Lightweight, "limp" leads like #30 guage seem to be more self damping for shock.

- Dan, N7VE

Sent from my Galaxy Tab® A

-------- Original message --------
12587 2016-04-03 17:33:52 bob_ledoux Re: VFO Ovens for a Stable VFO
Allison, did you even read my post before commenting?  Your comments suggest you didn't.

Otherwise, why did you argue for the importance of temperature stable components when the VFO is supposed to remain within  half a degree F of a fixed point?

I don't understand why 40C is so important.  Would you explain why? 

I mentioned a crystal oven as a method to provide precise frequency control.  Maybe I lost you there.

From there I wanted to suggest that a fixed temperature VFO might be preferable to testing a VFO over a broad temperature range. Which is followed by component replacement to reduce swing.

My shack never gets over 75F so why do I need to go much above that?  I would much prefer to "age" my components at 22C rather than 40C.  Especially, if the entire VFO is maintained  at that temperature.

12588 2016-04-04 01:43:57 Hans Summers Re: VFO Ovens for a Stable VFO
Hi all

You might be interested in my OCXO/Si5351A Synth kit, http://qrp-labs.com/ocxokit.html - it's priced $16 and is a Si5351A board (triple synth 3.5kHz to ~300MHz) with 27MHz OCXO reference, all kit-built. The OCXO is made from a PCB broken into 16 pieces, with PCB boxes to provide an oven compartment and some thermal insulation. The temperature controller is proportional-integral. Oven temperature is 40-45C to match the crystal flat point. Other crystals could be substituted e.g. 10MHz.

There are some design details here http://www.hanssummers.com/ocxosynth

To the best of my knowledge this is the ONLY kit-built OCXO available - maybe ever. Of course it is not as high performance as a proper commercial OCXO but then again, it is a kit, and a fraction of the price. Very educational too!

73 Hans G0UPL
12589 2016-04-04 07:27:17 bob_ledoux Re: VFO Ovens for a Stable VFO
When I started this thread I assumed the reader would understand that I was looking for a very low noise oscillator.  So I took the DDS off the table.  Because it was variable frequency I was looking for a broader tuning range than a VXO.

The application was indoors where I had access to AC power and fairly consistent indoor temperature.

Given these constraints I don't have to test a VFO over a broad temperature range, replacing components to minimize drift.

Rather, I can consider building an LC VFO that is maintained at a precise temperature.  This concept isn't new. For example,  Collins used heated PTO's in some of their products.

I'm not dealing with a crystal so I don't need the 40 degrees C oven.  I can set my stable temp at any convenient level.  I'd choose slightly above my highest expected indoor temperature.

How about an 8640 Jr wide band oscillator in its own temperature stable case?  It might not be stable enough to evaluate crystals for filters, but it might otherwise approach DDS frequency stability.

12590 2016-04-04 09:15:43 IEN Re: VFO Ovens for a Stable VFO
What about using HUFF PUF?
----- Original Message -----
12591 2016-04-04 09:28:22 Dana Myers Re: VFO Ovens for a Stable VFO
12592 2016-04-04 09:43:15 Thomas S. Knutsen Re: VFO Ovens for a Stable VFO
I have been following the thread along and want to way in:

First, don't assume. Thats how lore gets started. State your goals and work out the parameters you need. The equations for things like phase noise and intermodulation dynamic range should give you some pointers on what you need in order to obtain the wanted preformance.

When I started this thread I assumed the reader would understand that I was looking for a very low noise oscillator.  So I took the DDS off the table.  Because it was variable frequency I was looking for a broader tuning range than a VXO.

Well, modern DDS chips should be at -150dBc/Hz at 1MHz out (AD9957), this is better than most published VFO's, and most crystal oscillators in the litterature.  One could do a bit better than this by implementing it into a FPGA, but the amount of work would be extencive.
People seems to accept that DDS systems are noisy, because they were that in the 1980. without doing any evaluation of modern components.

 The current low noise approach seems to be phase locking a VHF helical or cavity resonator and using frequency dividers. Regenerative dividers should be a interesting experiment to avoid the noise added by the divider chain.  Rubiolas book on oscillator noise and stability have some excellent design advice. 
Of course, the rest of the radio must be high preformance. This is not a place to use NE612 as the first mixer if you want the dynamic range to be high. It may also be neccessarry to design the IF chain so that extreme narrowband IMD isn't a problem, and thats a whole different can of worms.

The application was indoors where I had access to AC power and fairly consistent indoor temperature.

Given these constraints I don't have to test a VFO over a broad temperature range, replacing components to minimize drift.

Rather, I can consider building an LC VFO that is maintained at a precise temperature.  This concept isn't new. For example,  Collins used heated PTO's in some of their products.

I'm not dealing with a crystal so I don't need the 40 degrees C oven.  I can set my stable temp at any convenient level.  I'd choose slightly above my highest expected indoor temperature.

Constant power to a oscillator should help the drift. Crystals have some sweet spots depending on the crystal cut. A SC cut crystall will have different point of minimum drift than a AT cut.
The temperature you choose should be so that any change in the environement around you don't affect the oscillator, say if a window is opened, or the xyl vacuums. The simplest way around this is to increase the temperature, just make sure the control loop used to control the temperature don't oscillate (speaking from experience) and that its isolated from the environement.

The aforementioned Collins PTO's are a excelent alternative if the tubes are replaced with J-Fets. If you can get them, that may be a good start.

How about an 8640 Jr wide band oscillator in its own temperature stable case?  It might not be stable enough to evaluate crystals for filters, but it might otherwise approach DDS frequency stability.

That statement make no sense at all. A DDS is based on a high accuracy reference, and should be more stabile than any reasonable LC oscillators.
A stabile wideband oscillator will be more difficult to build than a narrowband one. Why do you think the older radioes from the 50's, 60's and 70's cover approx 500KHz in each band segment?

73 de Thomas LA3PNA.


12593 2016-04-04 09:51:11 bob_ledoux Re: VFO Ovens for a Stable VFO
My approach is to use a variable cap.  I'm thinking about an 8640 junior with an ARC-5 capacitor.

The huff and puff frequency shift corrector employs a varicap that adds noise to the system.

Is there another way to electronically alter an LC circuit resonance frequency without contributing noise?

I remember reading a British account of inductor value being altered by a DC magnetic field.  That might be a low noise solution. 

So what if some of these ideas aren't practical?  Thinking outside the box is a good way to promote experimentation.

12594 2016-04-04 10:27:10 Alberto I2PHD Re: VFO Ovens for a Stable VFO
12595 2016-04-04 18:20:48 kb1gmx Re: VFO Ovens for a Stable VFO
Yes I did.  You asked a very wide question.  

First 40C is warmer than where you'd like to sit.   After all you don't want the oven cooler 
than the possible environment for the obvious reasons.  You can go lower but you still 
want to be warmer than possible swings or heating due to RF i the coil and components.

Ok, you go for a 76F oven and the AC quits in the summer and its 90... or the power amp gets that
area warmer than the room what then?   Of course if the room is stable why do anything?
Hint: the oscillator has high circulating current even in the milliwatt power level and the tuned 
circuit caps and coils do heat as well the active device and things then shift.

Now temperature stable parts, critical as they are more stable but also
change less (NPO/COG).  If you not heating then that stability is more 
important.  Further not all parts used will be temperature neutral.  Most 
iron powers are not drift free, forms can change mechanically with 
temperature.  Copper wire has a positive temperature coefficient.

Most crystal ovens run very warm.  I've worked with them back in the T44 
though Motrac days and then they went to TCXOs.  TCXOs with aged 
crystals were better as they didn't fail and burn up.

FYI: if you use a thermister and use that with the proper scaling you can dial out 
some amount of temperature change.  The problem is often the change with 
temperature is not linear but with an opamp and a bit of effort that can be near neutral.

A good VFO is doable, otehr ahve done it and I followed their work.  I have one 
running at 14mhz range that drifts maybe 10hz an hour after 10 minutes.  Another 
at 5mhz is within 100hz linear dial and maybe moves around 10 hz for a 
20 degree change.  Testing them over temperature to find the thermally 
weak parts and also doing the correct amount of temperature compensation 
using N150 and N750 parts get to stable.   Why bother?  Most VFOs are far
cleaner than those digitally derived ones.  Many are far lower power.

12596 2016-04-04 18:25:35 kb1gmx Re: VFO Ovens for a Stable VFO

I've done both. stiff and limp.  Stiffness or not is also length dependent.
Its hard to twang short leads.  Then again if your using long leads in 
a VFO other problems may present themselves.  Then again in many 
cases other mechanical issues tend to mask that.

There is a lot of lore, less fact and few notes on good practice and art.


12597 2016-04-04 18:55:11 kb1gmx Re: VFO Ovens for a Stable VFO
Rather, I can consider building an LC VFO that is maintained at a precise temperature.  This concept isn't new. For example,  Collins used heated PTO's in some of their products.<<<

Those were temperature stable before heating them and heating them was to compensate for the tubes around them (warm up time).  The later solid state ones were heated as the environment required it (close cases and other warm parts),  I still have one.  Besides being a good linear PTO then are also mechanically and electrically stable.

Ac power means a power supply that can generate heat.  If its a transceiver then there are finals and driver 
that can contribute heat.  For that even the audio amp can add heat as well as a stiff RF preamp running at
50ma at 12V (.6W).   If you put watts of power into the box you get BTUs of heating.  Heat is a fact of life 
even in cool solidstate and IC designs, its there maybe far less but not totally gone.

That's why you have to compensate against drift with heating even if only a little.

And yes a good VFO is far better than most PLL, NCO and some DDS.  A DDS is as good 
as the crystal clock source can be very good) for phase noise, their weakness is spurs at 
random frequencies due to quantization errors though the newer ones are very good.
That problem is solved by using more D to A bits in the part.

>>How about an 8640 Jr wide band oscillator in its own temperature stable case?  It might not be stable enough to evaluate crystals for filters, but it might otherwise approach DDS frequency stability.<<

With a little care it can be very stable. Again start with a good oscillator.  By time you divide it by 4 or 8
the drift also drops by the same factor and the phase noise does as well.  Its why the real 8640 is 
such a big deal.  A UHF oscillator that tunes an octave and divided by 2/4/8/16 as needed to lower lower ranges.   The stability in the lower HF range is very good.  Then again the UHF oscillator assembly is 
maybe 30% of the units total weight.  The 8640JR is a good idea but for stability the build quality has 
to be there as well.  Stability to 10 hz is more than enough to measure crystals if the frequency display 
is real time.

As to DDS stability.  You start with a oscillator of known accuracy, low phase noise, and stability 
(a locked crystal oscillator) OCXO slaved to GPS (Rubidium standard) and you can get 1^10-11 and far 
better frequency stability with fraction of a hz accuracy.  No VFO can match that, most would not build 
a dial that can read to .1hz with a VFO as setability is mechanical problem that nearly exceeds stability.

There are things one or another technology can do or can't easily do.  Then there are practical limits
imposed by physics and construction.


12598 2016-04-04 18:59:40 kb1gmx Re: VFO Ovens for a Stable VFO
huff&puff gets stability but there is a lot of short term drift as teh distance between 
huff and puff is usually in the 10 to 50 hz range.

Not very useful for very narrow band modes like PSK31 or WSPR.

I tried it and its like slow FMing of the oscillator.  Less painful if the oscillator is 
stable but at some point if there is drift you hear the warble.  It shows really 
well on a PSK31 signal or a carrier displayed on a waterfall.

12599 2016-04-04 19:13:02 kb1gmx Re: VFO Ovens for a Stable VFO
Bob in all cases start with a stable design like Vackar, or Seiler.  Correctly built 
with good parts they are stable.

Huff&Puff has the FMing issue though a varicap is not required.  You can tune 
using a magnetic field that's old school.  Its also mechanical issue and the DC 
control for the magnetic field must be quiet as that would introduce FM noise.
The magnetic filed is low noise but the control of that field must also be low noise.

I believe Chris Trask wrote a paper on an wide tuning oscillator using magnetic
control with varicaps to get extended range VCO.  The basic info will be there.

The ARC-5 (or BC221) cap is a mechanical marvel if you can get one in good 
shape without dead grease or corrosion on the wipers or bearings.
If you get the ARC 5 cap then you also want the ceramic coil forms,  Both are 
big but mechanically stable.  Just remember its only 500 turns to go end to end 
on a BC221 cap and that's a QSY killer.

If your doing RIT/XIT (for split operation) you still need a varicap.  Just use the 
smallest range you need and add a N750 cap in there to temperature compensate it.
It will not degrade the phase noise if done right.  

12601 2016-04-05 05:27:03 Hans Summers Re: VFO Ovens for a Stable VFO
Hi all

Bob N7SUR mentioned the magnetic tuning of VFOs to avoid the noise introduced by a varicap, in a Huff Puff correction circuit. I built one of these in a Huff Puff-locked VFO and documented it here:

I also have on my website, PDF copies of nearly all the articles ever written about Huff Puff VFO stabilisation. See http://www.hanssummers.com/huffpuff/library.html

In my opinion, if the Huff Puff is properly designed and uses one of the more recent "fast" circuits, an already good VFO, and adjusted for minimum sized "huffs" and "puffs" then the FM should be very small - I don't think there should be any audible warbling. I used mine in a receiver http://www.hanssummers.com/polyphase that I used extensively for CW and I would have easily noticed if there was any warbling. 

I also agree with Thomas LA3PNA n that you can beat a good modern DDS. An good DDS with clean crystal reference will have better stability, lower phase noise, and lower other unwanted things than any LC VFO I think, at least on HF. 

And finally just for fun - and because Allison mentioned the beautiful BC221 variable capacitor - I used one in an all-tube VFO. After a lot of work, this VFO has a drift of only a few Hz per hour at its 6MHz operating frequency. See http://www.hanssummers.com/tubevfo.html . It is intended to be used in a heterodyne VFO for a 15m superhet, mixing with a 20MHz crystal oscillator. As it stands now (incomplete), the 6MHz LC VFO is more stable than the 20MHz crystal! Time for an oven.

73 Hans G0UPL

12602 2016-04-05 06:58:23 bob_ledoux Re: VFO Ovens for a Stable VFO

Thanks for your recent posts.    You've presented ideas I haven't read before.

Its nice to see that these classic circuits still have a place in our days of digital.

12603 2016-04-05 07:36:52 jwharding3 Vackar Low Noise
Just posted a file EMRDV Vackar 4.7.pdf. An unfinished project (one of many) Includes original article by YO3DAC/VA3IUL. Editable schematic is accessible at Digikey's Scheme-It http://www.digikey.com/schemeit/#2vus. It includes oscillator, slightly different buffer (not mine) and the power supply. The scheme was converted to Robot Room, exported as Gerber and the board made. As far as it has gotten. Tuning cap was BC221 (they just don't make them like that anymore.)
Lot of effort, might help someone else with a lot more focus.
Be well


12604 2016-04-05 10:25:38 kb1gmx Re: VFO Ovens for a Stable VFO
The do indeed fit well in the digital world.

The biggest advantage is the analog stuff doesn't create wide band switching
noise for the receiver to hear.

The part I like about analog is simple common parts can be combined
to get a good result.  

The old and very faithful TO5 2n2219 makes a very good oscillator as well as its
smaller TO18 cousin the 2n2222A  and for most vendors the die mounted in the can is
the exact same part.  Handy to know.  Also a large die device spreads the current 
and reduces self heating as well.

Another good oscillator is the Hartley with a Seiler theme.  That is tapping the base(gate)
down from the top of the coil either capacitively or a physical tap on the coil.  That increases
the coil Q improving the close in noise sidebands. 

Look at the circuit for the FT707 or FT200 (aka Tempo one) older radios.  Simple but properly 
compensated it will stay put.  My Tempo one has that, transistor VFO and tubes all around it
for warmup drift.   Once at a stable temperature its good.  One such VFO from a junker
measure about 40hz drift at room temp and about 30-40hz/degreeC as it was when removed
a tweak to it brought that down to about half that.  Thats a 40 years old unit!   Look at how they 
did it as its documented  (look on the Foxtango site).


12605 2016-04-05 11:03:00 N0UU Re: VFO Ovens for a Stable VFO
OK, I'm going to jump in here with a bit of a different twist:

I work mostly playing in contests and DXing. No long term signal
transmissions or reception.

Waaay back in the 70s I built a Martain style rig for the lower bands: A
tight RF front end, a RAY mixer, diplexer, followed by a VMP4 MOSFET, and
a bunch of Plessey 1600 chips. Oscillator: A BC-221 Frequency meter
tweaked for each band. You could buy them new back then for $10 without
the calibration books. I just used a very early frequency counter to tell
where I was. The BC-221 put out way more power than I needed for the
mixer so taking Hayward's advice I put in a pad for each. Added some
isolation for the mixer. I need to dig it out and check for modern

Anyway, why all of the fuss about frequency stability? Chadwick's paper
on How much Dynamic Range we need is in a similar line of thought.
Stability is nice, but I'm betting that most of the stuff we're building
is Good Enough for daily use. Flame away.

12606 2016-04-05 11:43:07 Ed - K9EW Re: VFO Ovens for a Stable VFO
Hello Hans,

I'd forgotten about all the great projects and write-ups that are on your website.  I'll have to go back and look at them all again!

ed - k9ew

12607 2016-04-05 13:21:21 Nick Kennedy Re: VFO Ovens for a Stable VFO
​For my purposes, I don't mind riding the dial a little, making periodic corrections. I just don't want the guy on the other end to hear a significant change in my frequency as I'm making a five minute transmission.  Unless I'm using my VF-1.  Then it's part of the charm.


Nick, WA5BDU​

12610 2016-04-06 15:59:09 swift_glen Re: VFO Ovens for a Stable VFO
I've played with this tapped-down oscillator, which has potential of low noise. Harold Johnson also used this arrangement in his helical resonator oscillator. He notes a "gotcha" with this circuit that I also encountered with its LC equivalent...
When you tap down the base, the top portion of the coil isolates the "C" (resonating capacitor) from the base and emitter. This creates a VHF or UHF oscillator. It can actually oscillate with chaos at both HF & VHF. With a frequency counter attached, the HF frequency seems unstable because of VHF ringing.
The usual cure: a 47 ohm series resistor or a ferrite bead
12611 2016-04-06 19:57:03 N0UU Re: Neat board available
Richardson has a demo board available for $5.


If you can't find a use for this you shouldn't be on this list!!!!

12612 2016-04-07 10:23:19 Jim Strohm Re: Neat board available

Last week I ordered two of those boards.  Richardson charged me $15.95 for UPS ground shipping on a 2-lb package, which yesterday arrived partially crushed, with the contents ominously rattling around inside.

What are the odds that when I open the box, I find two naked, unprotected boards and three of those air-filled pillow packs?


12613 2016-04-07 15:26:22 Bill Carver Re: Neat board available
I think I ordered same day you did (3/29/2016) and mine arrived Monday. Each in individual Analog Devices boxes inside another box.

12616 2016-04-07 18:54:28 Jim Strohm Re: Neat board available
I opened mine and found my two boards in individual Analog boxes with the usual Analog marketing paper ... no software.  The Analog packaging is worth every penny of five bucks -- IIRC I saw somewhere that somebody said that some vendor wanted like $600 each for these boards.  I haven't explored further.

Meanwhile, I'm deeply disappointed with Richardson's shipping costs and policies, and I am a millimeter away from putting them back on my permanent "do not use" vendor list.

More news once I play with these.


12619 2016-04-08 08:58:02 kb1gmx Re: Neat board available
Another power detector that's useful is the Minicircuits ZN47-40 (-40 to +20dbm range).
Its designed for 10 -8000mhz and at 90$ not what I'd call cheap but like all test fixtures 
and test gear with care its a forever thing.

DEMI (Down east Microwave) also sells a version of the W1GHZ power detector
and its DEM APBM  10khz to 10GHZ at 75$.   

I don't consider them a cheap vendor but having done business with them for years at 
the professional level they are very good to work with.  You can deal directly with them.

There are a number of other power detector chips from other vendors that are useful.

One I do suggest as its cheap and easy to make is the DL5NEG diode detector.
Even made on G10 fiberglass its good to near 1ghz and down to -40dbm.   A simple 
hand scratched board a few cheap chip resistors and caps and a 1n5711 (a readily 
available substitute leaded diode) will do nicely.

12874 2016-05-22 11:00:16 ac2gl Re: Neat board available
Yes I purchased one. Don't be misled by the $5. They get about $18 for shipping, so the final price is quite a bit higher. And they don't allow bulk purchases.   73 Dave AC2GL
12891 2016-06-01 19:36:12 Lawrence Stoskopf Re: VFO Ovens for a Stable VFO
OK, let's go waaaaay back. In the 70s I was building a rig for 20M and down. The Martian bit with no preamp. Straight to a 23 dBM mixer. Hayward suggested that my LO be pretty high power followed with an attenuators to provide a match to the mixer and set the drive level. Back then you could get BC-221 freq meters without the book for $10 or so. Ended up with one for each band. Drift?  I'm contesting. Who cares about drift or exact frequency. Put one of the early counters on it and worked really well. Same for using one for a test oscillator......follow the counter. N0UU

Sent from my iPhone

12892 2016-06-02 06:39:46 John Lawson Re: VFO Ovens for a Stable VFO
Hi Bob, if you build the 8640 Jr make sure that you use a very high quality variable capacitor for tuning. Otherwise stability will suffer. John K5IRK

Sent from my iPad

12897 2016-06-02 11:19:20 bob_ledoux Re: VFO Ovens for a Stable VFO
I think my ARC-5 variable cap will qualify

12900 2016-06-02 14:23:15 John Lawson Re: VFO Ovens for a Stable VFO
That is the type of cap I was referring to in my earlier response. I have used it a number of times and it worked out great, John K5IRK

Sent from my iPad