ZL2PD Hunts for Varicap Diodes
Republished from Jan/Feb 2007 issue of
“Break-In”, New Zealand’s amateur radio magazine, with the permission
of the editor.
most people who have electronics as a hobby, I’ve found some components
increasingly hard to find. I keep a list of the really hard to find
parts, adding to it from time to time, and removing the occasional item
when I finally track them down, or when I find a suitable substitute.
Most recently, I really needed to find some high capacitance varicap
My problem was simple. I was in the process of building an 80m (3.5 -
3.9 MHz) SSB transceiver using a Collins 455 kHz mechanical filter in
the IF chain. Such designs need some good front-end receiver
selectivity in the form of at least a couple of selective tuned
circuits. I’m also planning to build a simple transistor VFO for the
This design approach requires three tracking tuned circuits - two in
the preselector filter ahead of the mixer, and one in the oscillator.
The simple approach calls for a three gang variable capacitor. Thirty
years ago, you could visit a local parts supplier and purchase a
suitable part, probably along with a nice 10:1 reduction vernier drive.
More recently, say ten years ago, I would have simply bought a set of
suitable varicap diodes. A common choice for HF band designs was the
excellent Philips BB212. Three such high capacitance varicaps would be
ideal for my transceiver. Guess what component has been on my ‘rare and
endangered’ components list for more than three years?
(i.e. variable capacitance) diodes operate using a reverse bias voltage
applied to the P-N junction of the diode. As this reverse bias voltage
increases, the depletion layer at the P-N diode junction widens. With
normal operation of a silicon diode, it will take about 0.6V to force
electrons across this depletion layer, the reason for the this voltage
drop across a conducting diode. In the case of a reverse biased
varicap, since minimal current can flow in a good reverse biased diode,
the result is an insulating layer between two conducting silicon
layers. This layer widens with increasing reverse bias voltage. The
result is a device which can be used as a voltage variable capacitor.
Varicap diodes are also produced with a ‘hyperabrupt’ junction in which
the P-N junction is doped to enhance this effect. Capacitance can
readily range across a 2.5:1 ratio across the specified voltage range
in a standard varicap, and up to 10:1 (and often beyond) for
hyperabrupt devices. Typical VHF and UHF varicaps range from 1 to 50 pF
while HF varicaps range from 50 pF to 500 pF.
Two common circuit arrangements are used. Figure 1 (a) shows the
arrangement for a single varicap diode while Figure 1 (b) shows the
more common dual varicap diode configuration. This latter arrangement
has some practical benefits, ensuring RF voltages across the tuned
circuit will not cause the diodes to conduct, but it does reduce the
available capacitance by a factor of two. To reduce inductance effects
of device leads, and to reduce oscillator noise, some designs also
feature multiple paired varicaps in parallel.
Since bias currents are tiny, the resistor can be as high as 100k
without any problem, although RF chokes are also commonly used.
Figure 1: Single and double varicap tuned circuits
low capacitance varicaps for VHF and UHF circuits are still reasonably
easy to find, high capacitance varicaps for HF and AM band designs are
as rare as unicorn’s tusks. Those wonderful Philips BB212 dual varicap
diodes appear to have become nigh on unobtainable. A few can
occasionally be found, but at a price to make your head spin. Well,
mine, at least.
Other alternatives exist – Toko make some nice varicaps in the KV1500
series, On-Semi (previously known as Motorola) still make some of the
MV series, while Zetex also make some useful devices too. The problem
is, you’re not likely to find anything like these down at your local
parts supplier, assuming you’re lucky enough to still have such a
store! If you could buy them, most now seem to be made only in
eye-strainingly small surface mount device (SMD) packages. The biggest
problem? Unless you’re up to buying a few thousand at a time, you may
not be able to buy these anyway. I found I could order a few over the
internet, but the process is slow, and to get just a few parts, the
cost can be very high.
heard rumours of workable alternatives to these devices. Somewhere I
had read that zener diodes could be used in their place, but I couldn’t
find any information about typical performance. Others suggested that
high voltage rectifier diodes made great varicaps, while another source
I’ve even used rectifiers (like 1N4004 power diodes) in place of
varicaps in the past for the occasional circuit, but I suspected these
would not provide the wider capacitance range needed for my
transceiver. As it turned out, a few quick tests proved this to be the
gathered up as many different types of diodes as I could find from my
parts box so I could test how these might perform as varicaps. These
are shown in the photo at the top of this page. These include many of
the types of diodes which others suggest are useful as varicaps
- A selection of small and large LEDs
- A standard 1N4001 1A rectifier diode
- A large 4A rectifier diode
- The largest power diode I could find in a plastic
package (about 10 mm in diameter!)
- A variety of small 400 mW zener diodes, and
- A Motorola 1N759 12V 1W zener diode.
Incidentally, some care may be needed if you use LEDs as varicaps. Some
have reverse breakdown voltages as low as 5V. I’ve tested a number of
LEDs at with reverse bias voltages beyond 12V without apparent
difficulty, but longer term use might encounter reliability issues.
These diodes were all measured using the test rigs and methodology
described in more detail in the appendices at the end of the article.
Figure 2 shows the results. I’ve not
seen many graphs of capacitance for typical diodes used in this way, so
these may also prove useful to other homebrewers.
Figure 2: Diode performance for a variety of common diode types (Right
click the graph to view at full scale)
Because my SSB transceiver uses a 9V supply for most receiver and
transmitter stages, I was particularly interested in varicap
performance over the 1 – 9V range shown here. There is little point in
tuning varicaps below 1V. In many cases, the device’s Q, such as it is,
seems to fall below 2V, and any oscillator voltage across a tuned
circuit can rapidly drive a single varicap into forward conduction.
This completely ruins the reverse bias varicap effect.
Most of the these diodes fall into either a 10 – 20 pF or 15 – 30 pF
varicap class across the 1V to 9V reverse bias range. These will be ok
for most fine tuning applications or in huff-and-puff oscillators, but
they were useless for my transceiver. I needed much greater capacitance
By the way, the largest rectifier diodes had negligible capacitance
variation, and those results are not included in the graph. Clearly,
these diodes were completely inadequate for my higher capacitance
as despair was beginning to close in, I tested a Motorola zener diode I
happened to find lurking in the corner of my junkbox. In
sharp contrast to the other small zeners tested, that Motorola 1W zener
diode I found delivered a useful 40 to 100 pF range. I’ve plotted this
in Figure 3.
Figure 3: Capacitance performance of 1W zener diodes and the Motorola
(On-Semi) 1N759A 1W zener diode
This particular zener diode was not suitable for my requirements, nor
could I find any more of these in my workshop. It did show
some of the right features, but, more importantly, it
suggested where to focus my varicap hunt. It was like
seeing a murky photo of the outline of a rare mythological beast – A
sniff of a clue to justify a full scale hunt.
I headed off to the local parts store and bought a set of 1W zener
diodes, one of each voltage across the range available. A quiet spring
afternoon of measurement and analysis in the ZL2PD shack produced the
results I've also plotted in Figure 3 (above).
My first reaction was disappointment. I had thought that 1W diodes,
perhaps because of their larger silicon wafer structure, would deliver
the same response as the Motorola 1W zener. Sadly, while useful, not a
single 1W zener reached the halcyon heights of the 1N759A. Nor was
there any apparent relationship between their voltage rating and their
test results. But, on the positive side, the results were better than
the general mix of diodes I’d measured earlier.
OK. So, maybe if 1W zeners were better varicaps than 400mW zeners,
could 5W zeners be even better?
Samples of 15V, 24V and 75V 5W zeners were quickly obtained. By the
way, these parts are big. The 15V and 24V zeners measured almost 10mm
in length and over 3mm is diameter, while the 75V part, although
slightly shorter, has even heavier duty copper conductors, and a body
nearly 5mm in diameter. Serious zeners! Figure 4 shows the results.
Figure 4: Graph of 5W zener diode varicap performance
Testing these diodes also required a change of the test oscillator. The
full schematic details of these test oscillators can be found in the
appendix at the foot of this page.
5W zener diodes have capacitance, serious capacitance! Not only did the
75V zener feature a solid 190 – 310 pF range, a ratio of 1.6, but I was
stunned to find that the 15V 5W zener had an amazing 380 – 760 pF
range, or a useful ratio of 2:1. Results predictably follow voltage
ratings, even though the parts were from a mix of manufacturers. While
the smaller 1W zeners offered capacitance ratios of up to 3:1, equally
useful for other homebrew applications, these big zeners certainly
seemed to match my varicap needs.
At first glance, some may feel these very large capacitance values are
too high to be useful for anything beyond AM and low band HF use. In
fact, they open up a wide range of uses when combined with a lower
value series capacitor or used in centre-fed pairs across a tank
circuit. Both arrangements reduce the impact of RF voltage swings on
RF Performance Tests
issue was to determine if these diodes had adequate Q to support real
RF applications such as front end tuned circuit and oscillator tank
tuning. Given the noise floor in lower HF bands, the target of my
transceiver, I could accept modest Q values. While the test oscillators
(See Appendix B for the circuit details) gave me some confidence that
the diode performance was probably OK, I decided to try building a
simple toroid L/C tuned circuit at 3.5 MHz. This would also allow me to
adjust the voltage on the diodes in circuit while evaluating the Q
using a grid dip meter.
This worked perfectly. 45 turns on a T37-2 toroid gave me the required
8uH inductance to resonate with the 75V/5W zener on 80m, and tuning the
diode from 2.5 to 7V covered the 80m band perfectly. The coil/diode
seemed to deliver the typical selectivity similar to that of a
conventional single L/C tuned circuit.
A more precise test was required. For this next test, I built a
variable bandpass “ultra-spherical” filter. The simple version I tested
is shown in Figure 5. Developed by Wes Hayward W6ZOI, and first
published in "Ham Radio" magazine back in June 1984, this is a tunable
LC bandpass filter which provides up to 20dB of selectivity at the band
edges of 80m when tuned mid-band. Although Wes used a standard variable
capacitor, I set out to make and test a version using these diodes.
Figure 5: Variable bandpass filter
(Source: Wes Hayward W7ZOI - See text)
The results exceeded my expectations! The filter tuned from 3.5 – 3.9
MHz as bias was adjusted from 1V to 9V, and when tuned mid-band, to 3.7
MHz (about 2V bias), I measured the performance shown in Table 1. This
table also compares these results with those published in the original
article for the same filter using conventional capacitors and high-Q
Table 1: Varicap Tuned Ultraspherical LP/BP
The original W7ZOI filter was designed using inductors with Qu=250
while the T37-2 toroids I used typically reach Qs around half these
values. I expected to see increased losses and poorer band edge
results. While my filter had slightly reduced band edge attenuation,
the overall performance was quite acceptable. More importantly, the
test confirmed the viability of the zeners as varicaps.
tests indicate that 5W zeners can be used as high capacitance varicaps,
although the physical size of these devices could be a problem in some
cases. With care, this should not present a major limitation.
I should caution against assuming that any and every 5W zener diode
will give the results in line with those shown here. I’ve tested
several types, and they do seem to give similarly useful results.
However, as those television adverts say, “Individual results may vary”
and I would recommend testing any zener diodes before committing to
buying larger quantities.
Wes Hayward, W7ZOI, ‘The Peaked Lowpass: A Look at the Ultraspherical
Filter”, ‘Ham Radio’ magazine, June 1984, p 96 – 104. (See Figure
14 and 15)
Appendix: Measurement Method
simple FET HF test oscillator was built to cover 10 – 50 MHz (Figure
A.1). A set of fixed capacitors ranging from 4.7pF to 820pF were used
to produce a graph of frequency versus tuning capacitance, and
Microsoft Excel was used to plot the graph and to derive a simple
curve-fit equation. The plot for this second oscillator is shown
in Figure A.2.
The same method was adopted for a second
oscillator using larger values of capacitance covering 250 – 550 kHz.
Figure A.1: 10 - 50 MHz test oscillator
Figure A.2: Oscillator frequency vs tuned
circuit capacitance for the oscillator in Figure A.1
Figure A.3: 500 kHz test oscillator
Each varicap was measured using the HF oscillator, with the
oscillator’s frequency measured as the varicap bias was varied from 1V
to 9V. The higher capacitance 5W zener diodes were measured using the
second (500 kHz) oscillator. Measured frequencies were converted to
equivalent varicap capacitance using the curve-fit equation obtained
from the initial test, again using Excel.
Varicaps with sub-50pF capacitance were measured using the HF
oscillator and a parallel 220pF fixed capacitor (C1 in Figure A.1). A
similar graph of frequency versus tuning capacitance, and a curve-fit
equation, was used to more accurately derive these relatively small
Want to go back to the main page? Click