A Permeability Tuned Oscillator
VFOs still have a place in transceivers given their typically low
current requirements and relative simplicity. The PTO VFO described
here has been built using readily available hardware to provide a
conventional VFO tuning mechanism at very low cost.
amateur radio transmitters and receivers often requires the
construction of some type of variable frequency oscillator (VFO). This
allows the transmitter or receiver to be tuned across the frequency
band of interest. Most VFOs tune across a specific band, for example
from 3.5 to 4.0 MHz for the 80m amateur band, or from 14.0 – 14.35 MHz
for the 20m band.
Most analog VFOs are tuned with a variable capacitor. These days, it's
more common to find a digital VFO, such as one using a phase locked
loop (PLL) or a direct digital synthesis (DDS) oscillator chip. These
often use a microprocessor and a rotary encoder for tuning.
Analog VFOs have the significant advantages of simplicity. They may
also have lower phase noise and current consumption. However, analog
VFOs can suffer from poor stability caused by temperature changes,
supply voltage variations and poor construction.
Analog VFOs also often require the addition of some form of reduction
drive for tuning the variable capacitor. This mechanism allows the user
to tune across the 100 to 500kHz of the VFO’s tuning range with a
tuning rate of 5 to 20 kHz per turn. Since variable capacitors require
a 180 degree rotation to go from maximum to minimum capacitance (or
vice-versa), a mechanical reduction drive of up to 100:1 can be
required. Such mechanical drives are becoming more and more difficult
to find, and their size and weight can be a problem.
Permeability Tuned Oscillators
alternative approach is the permeability tuned oscillator (PTO) where
the frequency is adjusted by varying an inductor rather than with the
traditional variable capacitor. The inductance in these VFOs is usually
changed by rotating a ferrite or brass core into the centre of the
It’s relatively easy to cover the required frequency range with 20 or
more turns of a typical threaded metal bolt. However, the mechanical
arrangements of many such homebrew PTOs often leave much to be desired.
As these VFOs are tuned, the tuning knob typically simultaneously
rotates out from the panel of the equipment. This can result in the
tuning knob ending up anywhere from 30 to 50 mm out from the front
panel. Not only does this look odd, but any vibration can result in
undesirable frequency shifts. It also leaves the tuning control exposed
to accidental damage.
Why Make a PTO?
I required a VFO for a 20m transceiver. The transceiver’s IF was about
8.6 MHz so I wanted the VFO to tune from about 5.4 to 5.8 MHz. An
analog VFO would allow me to minimise current drain. However, I did not
have a suitable variable capacitor for the VFO nor a reduction drive.
That’s when I started to review the work done by others on PTO-type
Other PTO designs I discovered seemed to generally fall into two
categories – The majority of designs that used the simple “tuning knob
on a stick” approach where the tuning control was wound out from the
panel, and the second category, with only a few examples documented,
where considerable time and effort had been spent on the mechanical
design of the PTO in order to provide more conventional tuning of the
PTO. In addition, a few of these latter designs also explored improved
tuning linearity. However, most examples I found seemed to require the
construction of fairly elaborate mechanisms often with specially made
The more I looked at these ideas, the more I believed I could achieve a
viable PTO with basic hardware i.e. Parts easily purchased from a
hardware store. There were a number of obstacles that I had to
overcome, but in the end, I think I managed to overcome all of them.
Anyway, this design described here is the result.
Mechanics of the PTO Tuning Mechanism
to this design is the mechanical arrangement which allows the PTO to be
tuned in the conventional manner i.e. without the tuning knob rotating
into or out from the front panel. You can see the mechanism in the
picture at the top of the webpage. It depends on a long threaded shaft
which is free to rotate in place thanks to the careful use of some
locking nuts, and a parallel fixed shaft, in this case a short length
of bamboo (although metal would have been better!).
Rotating the tuning knob rotates the main threaded shaft. It is fixed
in position between the front and back panel, but is free to rotate. A
set of lock-nuts at the back-panel end of the shaft allows smooth
rotation. This threaded shaft also drives a nut which is attached via a
solder lug to the tuning slug. This slug enters or exits the main VFO
inductor and varies the inductance.
The tuning slug has a second solder lug on it which slides along a
second parallel bamboo rod. Bamboo was used because I happened to find
that these were just the right diameter to fit the hole in the solder
lug, and, of course, since it was cut down from a large packet of
kitchen skewers used for barbequed meat in the kitchen, it was cheap
and readily available. Since this rod is mounted parallel to the tuning
shaft, it prevents the tuning slug from rotating and ensures it slides
smoothly in and out of the main inductor.
Ideally, there should be an additional spring fitted between the solder
lug on the bamboo rod and the front panel to maintain a little tension
on the tuning mechanism. This would improve the feel of the tuning
mechanism as well as probably reducing any frequency shift due to
vibration. However, I’ve yet to find a suitable spring.
The PTO VFO “chassis” is made from three pieces of blank PCB, one
forming the base plate and the other two forming the back and front
panels. A pair of thick brass washers are soldered onto the front and
rear panels. These have the same diameter as the hole for the tuning
shaft. They provide additional support for the tuning shaft.
Because I couldn’t find a threaded shaft long enough in shops where I
live, the tuning shaft was built from one 50mm long 6mm diameter bolt
and a matching 30mm long 6mm diameter bolt. Both bolts were
“cheesehead” slotted zinc-plated bolts which were readily available
from several of the local hardware stores. 6mm diameter nuts and bolts
were used because the diameter of the unthreaded portion of these bolts
perfectly fitted into the shaft hole on the tuning knob I used.
The first 50mm long bolt is inserted into the PTO chassis through the
back panel and two nuts are threaded on to form the rear mount. The
first nut is threaded on such that the tuning shaft can just (and only
just) rotate. The second nut then locks the first nut in place. It also
acts as a limit for the tuning slug, preventing it from coming
completely out of the PTO inductor. I could have added a further nut
onto this shaft if the second (lock) nut had not been in a suitable
location to provide this function by itself.
The slotted head of the second 30mm bolt was then cut off with a
hacksaw. This allows the unthreaded shaft to fit into the tuning knob.
A nut was partially threaded onto the end of the first 50mm long bolt
and then the cut-down 30mm bolt was inserted through the front panel
hole and threaded into the other half of the end-nut. This had to be
attempted perhaps five or six times until the two shafts were
absolutely axially aligned and tight.
An additional nut could have first been wound onto the 50mm bolt
to provide a further mechanical tuning limiter, to limit the extent to
which the slug enters the inductor, but again, this was unnecessary in
my case. I allowed the slug to fully enter the inductor until limited
by the (pink plastic) inductor former.
I would have liked to have added a pair of nuts on the inside of the
front panel like those of the back panel, but the hardware I had
available did not allow this. As a result, I could not completely
remove the free play in the tuning shaft although it is quite
acceptable as it is.
The inductor former was mounted using epoxy glue onto the front panel.
The tuning slug was fully wound into the coil while the glue was drying
and the former was carefully aligned and clamped to ensure it was
parallel with the tuning shaft and bamboo support shaft.
PTO Coil Former
I would have liked to use a temperature stable inductor former, such as
a ceramic, PTFE or even phenolic type. However, none of these were
available so I just tried a plastic tube made by cutting down a
felt-tip pen just to see if the mechanical arrangement would work. As
it turned out, the temperature performance of the felt-tip plastic
turned out to be fairly good, so the bright red former ended up in the
Permeability VFO Inductor Tuning Slug
initial tests with all sorts of metal hardware quickly showed that the
variable inductor used in the main PTO tuned circuit had to be tuned
using a brass core. Tests using other materials failed to give
satisfactory results. Unfortunately, I was unable to find any brass
bolts (I would have preferred to use a 6mm diameter brass bolt with a
matching nut) so I was forced to improvise. Searching around the
hardware stores, I discovered some 240V mains plugs rated at 15A (They
look like they are good for more like 30A!). A similar plug is shown in
the photo below.
Figure 1 : Heavy duty 240VAC mains plug
“pins” on the connector were beautifully finished solid brass slugs.
These were each mounted in the plug with a 3mm bolt.
The plug was
surprisingly cheap, and very easy to disassemble. One of the thinner
(6mm?) diameter pins was found to be ideal for the task. The 3mm bolt
is used in the plug to hold it in place, threaded into an axial hole in
the end of the pin, and in this PTO, this is used to hold the two
solder lugs in place.
Winding the PTO Inductor
Some testing with a simple close-wound coil and the brass slug on my
bench resulted in a non-linear tuning rate for the PTO. I found that
the tuning rate was too slow at one end of the tuning range but much
too fast at the other.
Looking at another design (See reference 2), I noticed it used an
inductor which had an initial close-wound section, and then a series of
wider spaced turns with slightly increasing spacing until the final few
turns of the coil which were once more close-wound. This can be seen in
the photo opposite.
began to understand the very clever solution this designer had
identified and which I’ve subsequently come across again in several
other PTO designs. The initial close-wound section of the PTO coil sets
the (rough) minimum inductance of the tuning coil. The next section of
the coil then uses a variable pitch to give linear tuning across a
specific frequency band. The final few close-wound turns are likely to
provide a good mechanical finish for the inductor.
I decided to try something similar. The result, after several attempts, can be seen in the photo below.
Figure 2 : The PTO oscillator inductor uses variable spaced turns
The tuning rate was much improved over the original close wound coil, and gave an acceptable result for my 20m transceiver.
The PTO Oscillator
noted earlier, this PTO had to cover a relatively wide range, from 5.4
to 5.8 MHz. Since I wanted to maximize the range from the variable
inductor, I wanted an oscillator design that avoided, as far as
possible, using any additional capacitors in the tuned circuit. This
ruled out the Colpitts and Clapp oscillators. Similarly, for
simplicity, I wanted to avoid any tapped inductor. That ruled out the
A configuration that I’ve used before is the Franklin oscillator
which is an approach I've used before and documented here on my website. This design relies nearly completely on
just the tuned circuit inductor and capacitor. I adapted an existing
design from PY2DYW which allows the PTO inductor to be grounded, useful
in the mechanical arrangement I used, and which also included a buffer
amplifier. This isolates the oscillator from the mixer load and
provides an output level of about 2Vpp for use with diode balanced
The oscillator schematic is shown below.
Figure 3 : The prototype PTO oscillator used low cost NPN transistors
close look at the prototype photos will show that I didn’t bother to
fit the trimmer capacitor, C5, which is shown in the schematic. I found
I did not need to add this because my coil and capacitor combination
gave almost exactly the tuning range I was wanting.
simple chassis was temporarily covered with a tin-plate shield. This
will be replaced later with a more robust cover made from blank PCB.
The PTO output was terminated on a low cost RCS socket mounted on the
rear panel. DC connections were provided via a 1000pF feed-through
capacitor (for +8VDC) and a grounded solder lug.
The tuning coil was not wound particularly accurately, as you can see
from the photo above, but the tuning rate was none-the-less quite
acceptable. In total, 22 turns of the tuning knob were required to
cover 500kHz, from 5.35 to 5.85 MHz. At the lower (CW) end of the PTO
tuning range, the tuning rate was around 10 kHz per turn. Across the
subsequent SSB section of the 20m band, the tuning rate steadily
increased to 20 – 30 kHz/turn. This tuning rate felt very comfortable
when using the PTO with the receiver in the 20m transceiver.
The PTO oscillator draws about 20mA from the regulated 5V rail.
Stability was reasonable, but not adequate for long term use. I tried
two capacitors for C4 (150pF) in the VFO. The first was a ceramic type,
the second a polystyrene type. I measured the VFO frequency drift for
about 90 minutes in each case.
4 : The prototype PTO oscillator was tested using ceramic and
polystyrene fixed capacitors to determine the basic drift
characteristics for the PTO VFO
After about 45 minutes following power-up for each capacitor type, and
in the reasonably stable workshop environment with the VFO covered with
a thin tin-plate cover and without any other insulation, both versions
became quite stable. Neither drift curve is great, but these plots show
that, with the right combination of capacitors, say a 100pF poly and a
47pF ceramic, the different drift characteristics of all the components
would probably roughly cancel, and the resulting VFO would be quite
I may add a little “huff and puff” stability circuit to improve this
still further at a later stage. Space has been provided for this small
additional board in the PTO chassis.
Here is a list of some previously published PTO designs some of which provided valuable insights for this PTO design.
1. Walter Horn, I1MK , “A High-Precision Permeability-Tuned VFO”, QST July 1964
2. A commercial PTO which was used in the ITT Marine R700M HF receiver is described here
3. KD1JV has published a number of designs using simple PTOs on his website. One simple example is described here for his MMR-40 transceiver
4. WA6OTP describes his very well known PTO design here
5. A low cost 40m receiver with an equally simple PTO using a “drinking straw” coil former is described here
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