Stacked Torroid VFO Experiments 2008
Introduction
I have noted a newer trend in VFO design is to stack 2 powdered iron torroid
inductors. For a given inductance, this allows the builder to use less windings and larger gauge wire
than would be
possible for a single torroidal inductor. In a VFO, the goal of these builders
possibly might be to reduce heating effects, increase unloaded Q, or perhaps to reduce core
magnet flux density. For me the goal was far simpler, I just wanted to make
compact, larger value inductors for VFOs at 3 MHz and less.
This page contains some basic experiments with stacked torroidal inductors and
includes a prototype VFO I built for 3 MHz. In addition, during these experiments,
low-budget lighting techniques to enhance my close-up photography were developed and
a stab at basic html and JavaScript programming was undertaken.
In the photograph above are 3 rapidly wound FT50-43 ferrite torroids. This was my genesis experiment. I measured the inductance of the 2 six turn inductors and then the stacked one. Their values were 19 and 20 uH for the singles and 43 uH for the stacked torroidal inductor. Permeability variation, wire spacing, meter tolerance and other factors need to be considered, but the basic ideas is stacking increases inductance per turns and facilitates the use of thicker gauge wire if so desired.
In the photograph to the left is the T68-6 hamburger. The two T68-6 cores were
epoxy glued together and compressed lightly in a vice for several hours.
In VFOs, low noise and minimal frequency drift are
essential. With respect to temperature-related drift, the builder must consider the thermal stability of the
circuit components, RF shielding and the mechanical rigidity of the PC board,
anchors and chassis. To address inductor thermal stability, #6 material is
chosen by most builders as it has the lowest temperature coefficient of the
commonly available powdered iron torroids.
To my surprise, I learned from Wes, W7ZOI that I had an incorrect understanding of the wire gauge versus temperature stability
relationship. I used to think that heavier gauge wire had greater frequency
stability than smaller gauge wire wound on a given torroid. As it turns out,
smaller wire is often better for thermal stability. The smaller wire will get right on the core during winding.
Stiffer, larger gauge wire is more difficult to wind so that the wire is right against the core
and so air gaps are created that will expand and contract during temperature
changes. Smaller gauge wire will have a reduced Q, but it won't be as
significantly lower as you might guess. The thermal stability characteristics of large wire is surprising, however, it can be mitigated somewhat by annealing the wire with temperature cycling or by dunking
it in boiling water. Roy, W7EL first reported annealing coils in 1980 and this has been confirmed
during experiments by builders using
temperature controlled ovens. One of the initial tests I performed for this web page was to see if boiling the stacked coil affected the epoxy glue. The glue was not effected by annealing the stacked coil with
5 or even 10 minutes of boiling in water.
A Good VFO Topology
Shown above is the final VFO, buffer and frequency controlling scheme. The
tuning range of this VFO was from 2.998 to 3.027 MHz. The VFO
is a tapped Hartley which lacks the usual gate-clamping diode. The main
oscillator and buffer are based upon articles written by Jacob Makhinson, N6NWP
which were published in QST for Dec 1996 and Communications Quarterly for Spring
1999. This is a stellar VFO in turns of spectral purity because it was designed
by N6NWP to be so. Spectral purity refers mostly to reduced phase noise; short-term, rapid and random fluctuations in the phase of the output sine wave which in turn,
contributes to receiver noise. Although I bread boarded a couple of common-base BJT
oscillators and tested them, I went back to this design and recommend you
at least consider it for your next VFO project. You do not need to use a stacked inductor.
VR1 is a L78L05 in a TO-92
package. Using a simple zener diode as the voltage regulator might also be a
good choice. I have read that zener diodes below 5.6 volts have a negative
temperature coefficient and zener voltages above 5.6 have a positive temperature
coefficient. Accordingly, the optimal zener voltage for a minimum temperature
coefficient is probably between 4.7 and 5.6 volts. Zener diodes tend to be noisy
devices, but a bypass capacitor may mitigate this problem. The L78L05 is a lower
noise device. I have observed better temperature stability using the L78L05 device rather than 5.1 - 5.6 volt zener diodes in my VFO experiments, but by no means can I
scientifically quantify
this. Very low temperature coefficient voltage regulation devices are available,
but the cost versus benefit is highly questionable in a simple VFO project.
Most builders would tune this VFO with a good quality air-variable capacitor either directly or
capacitively connected to the hot end (top) of L1 and be done with it. Depending
on the capacitor, this
would also provide a good tuning range if desired. Finding cheap and/or good air-variable capacitors is becoming increasingly difficult. In keeping with the "popcorn" nature of this web site I wanted to investigate if
just varactors could be used to tune this VFO.
I tried connecting varactors to the hot end and also at different tap points
along L1. The result was increased noise and decreased frequency stability. The
best varactor connection point for noise and frequency stability was to connect
them to the lower tap per the original N6NWP VFO schematic. The
original schematics called for a inductor with a inductive reactance of about
150 ohms or so at the desired frequency The problem with this is that the tuning
range of the varactors was limited to as low as 4 KHz. If I connected the
varactors to the hot end of L1 via a small value coupling capacitor such as 68
pF, the tuning range was also low. While this low frequency swing is perfect for
use in a VFO frequency drift correction circuit, it is not practical for tuning a VFO
for use in a receiver.
This is where the stacked torroidal coil comes in. With a
stacked core inductor, one can easily build VFOs for 3 MHz or lower with an
inductor XL of 300-400
ohms or more. With a higher inductance (and thus lower C), the potential frequency swing you can obtain is
better suited for tapped L1 connected varactors. Note that 2 back-to-back varactors were
used to optimize Q and phase noise; you the builder could elect to just use one
varactor to increase tuning range with a compromise in the for mentioned
parameters.
I chose an inductor consisting of 36 turns of #22 magnet wire on 2 stacked T68-6 torroids.
This turns number was chosen arbitrarily - I knew I could get 36 turns of #22
wire on
the stacked torroids. The inductor was boiled for 5 minutes after the taps had been created
and the enamel was stripped off the wire. The
formula I used for the taps is as follows: Divide the total turns by
7.25 to get the first tap and by 1.45 to get the second tap. Note that zero temperature coefficient C0G ceramic caps were used
as indicated in the schematic to reduce frequency variations from changes in ambient temperature. I
only use newer multilayer, monolithic C0G ceramic caps
I purchase from Digi-Key. I have found NPO and C0G
to be similar, but a salient discovery was that some older and surplus NPO/C0G capacitors
currently being sold are terrible
for drift in VFOs. Once the basic circuit was built, I temperature compensated
the tendency of this VFO to slowly drift up in frequency by replacing a 100 pF
C0G capacitor with a 100 pF polystyrene cap.
Temperature compensation is almost always
required. The best way to test for drift is a frequency counter, but if you do not have one, you can use a receiver set in the SSB/CW mode. I use both. Consider keeping a small
assortment of newer low
to midrange value RF temperature compensation caps in stock. On capacitors other than NPO (which uses the letter O instead of the normally stated ppm
value), the temperature coefficient is listed as P for positive and N for negative, followed by a 3-digit value in ppm/°C.
For example, N220 is - 200 ppm/°C. and P100 is +100 ppm/°C.
If your VFO is drifting upward (more commonly seen on my bench) you need to insert
one or more N or negative coefficient capacitor(s).
If your VFO is drifting downward, then use positive coefficient value(s).
On hand, I have polystyrene, Philips and Murata 5% N750 ceramic disk capacitors
and some high Q N750 trimmer caps for negative coefficient parts. For positive coefficient
compensation, I would personally try to using a silver mica cap or 2 as I don't really have any
true positive coefficient caps in stock.
Most builders minimally use 4 or more C0G/NPO capacitors to reduce heating
effects and to average out temperature coefficient variances
plus compensation cap(s) for resonating their VFO. I did not follow
this rule-of-thumb in these experiments.
The VFO is lightly coupled to a JFET source follower which is biased with the current source comprised of Q3 and Q4.
RFC1 was wound on an FT37-61 ferrite although a small RF choke could have been used. Anecdotally, I found this home
made RFC
may provide temperature compensation, although I cannot verify this. Small RF chokes are getting expensive and I tend to wind my own as I have 100s of ferrite cores in my parts stock.
To the left is the next inductor I wound. This coil has 50 turns of #26 wire and is approximately 19 uH. The taps were at 7 and 34 of the total turns. It was annealed and soldered into the same circuit as the previous inductor. I went with a larger inductance, as I was disappointed with the small 37 KHz tuning range of the first VFO circuit. I was hoping that by using a a larger L and thus requiring less total C to resonate at 3 MHz, my tuned frequency range would increase for the same varactor pair. Note that, I have used two different varactors. The combination of these 2 varactors gave the best frequency swing and lowest frequency drift. Experiment with all aspects of your VFO! My collection of varactors is considerably limited. Some varactors, such as the MV2109 pair I tried, caused the oscillator to stop and/or behave in a weird fashion during tuning. A BB104 worked well, but had even less of a tuning range than the pair of varactors shown in the schematic.
To the right is
the VFO(b) schematic containing the above 50 turn inductor and the necessary
capacitors to resonate it around 3 MHz.
It tuned from 2.996 to 3.047 MHz; still a little disappointing. I was hoping for
at least 100 KHz of frequency swing. Such is the life of the amateur
experimenter! I had to use 2 polystyrene capacitors for temperature
compensation this time and the circuit was not quite as thermally stable as the
original.
To build this VFO for other frequencies, choose an XL (such as 150 ohms) for the inductor
at your frequency of choice. A
simple program to do this basic, but boring arithmetic is located on the bottom of this web page. For
the 5 pF coupling capacitor, I put two 10 pF caps in series. For the L1 and JFET
gate coupling cap, you can use 10 pF for frequencies at or greater than 2 MHz.
For frequencies of 10 MHz and greater, consider using a 2 pF coupling cap to
keep loading on the inductor light. Capacitors to resonate your inductor are
typically just found experimentally using a frequency counter. If you are using
tuning diodes, consider not connecting up
your varactor pair until you have a functioning, relatively stable VFO running.
Tuning resolution is not great with a 100K pot. A large size knob can help, but
the more you spend, the better tuning becomes! A big 2 watt Allan Bradley pot with a big knob worked well on a receiver
I built a few years ago, but a 10 turn-pot proved better. Ten-turn pots are expensive and it may be best to bite the bullet and
just purchase a air-variable capacitor with built-in reduction. This also
eliminates the varactor pair and the 1000 uH RF choke. The choice of how to tune your VFO
is yours to make.
Above are photos of my frequency counter measurements of the VFO(b) at start up, 1, 5, 10, 40, 60 and 80 minutes. When you consider that my circuit was lying uncovered on a bench, the inductor was not anchored by anything but its wire, the furnace was cutting in and out and that it had survived 4 days of bread boarding and had many long-leaded components (for re-use), this is actually pretty decent. I listened to the VFO on a nearby receiver while using its 500 Hertz crystal CW filter and I did not have to make any receiver tuning adjustments after the 10 minute warm up period. To me, the ultimate way to test VFO frequency stability is by listening to it in a receiver. Your ears more sensitively perceive frequency changes than your eyes do when staring at a frequency counter sampling every 1-2 seconds or so.
RF Amp and Low Pass Filter
To the left is the RF amp I built for the 3 MHz VFO. The VFO and buffer output
is too low to properly drive a 50 ohm load, so an amplifier is required. The
output power is 7 dBm, which was chosen to drive a diode ring mixer. A 50
ohm pi LP filter follows the RF amp. The 3.88 MHz cutoff frequency was
chosen so that standard value capacitors could be used. A VFO output LPF is
strongly recommended.
In more comprehensive receiver designs,
consider using a better LPF such as a half-wave type.
In the photo above is the VFO output into my oscilloscope. Note the camera reflection.
Miscellaneous Images and Chatter
Above. The single-sided copper circuit board ready for bread boarding. Copper was removed to create a B+ strip, 5 volt section, a pad for the inductor resonating caps and a bare area for the inductor to lie on. Currently, for build and keep VFOs, I anchor my coil with 3 small zap straps around the inductor and through holes drilled in the circuit board. The circuit board is secured into its chassis with 4 thick bolts.
Shown above is the VFO bread board looking quite messy from all the various components present and also those which had been soldered in and later removed. This board was used over 4 days and 3 different VFO topologies were built and tested. By far, the tuned Hartley presented on this web page was the winner. A red LED can be seen and flashed when the circuit was powered up to warn me. It is easy to forget that you have power on when experimenting otherwise.
Above are 2 high quality air-variable capacitors. Either of these would be great for tuning a VFO. Most builders use a main tuning cap plus a second, smaller air variable trimmer capacitor to set the band edge. In the experimental VFOs, I did not use a trimmer cap and tuned them close to 3.0 MHz with appropriate type fixed value capacitors and luck.
The above image illustrates how I currently light my photographic subjects. In this case, I am photographing my radio station keyer with a background consisting of recycled, shiny, white paper. Two full spectrum Ott-Lites were used and I find they provide adequate brightness and good depth and color for close up shots. These were purchased last year when a local store had them on sale at 40% off their regular price. I use a modest, lower Megapixel camera set for Macro photography. For every picture that is kept, 10 are discarded. The usual problems are motion artifact and lighting issues.
Applications
Calculate Inductive Reactance (XL) from Frequency and Inductance Calculate Inductance from XL and Frequency Tapped Hartley VFO Inductor Tap Points Calculator
These are just written in html - I also wrote JavaScript versions,
but these seem to work okay.
Conclusion
I am hopeful that this web page was informative and interesting. Good luck with your own VFO experiments!
