ZL2PD Single Span HF Test Oscillator
This compact RF oscillator covers the entire
0.4 - 30 MHz range in a single sweep of the dial and has a terminated
50 ohm output of more than 300mV across the entire HF band.
signal generators use a series of ranges to cover the HF band. This
oscillator is a little different. It tunes the entire HF band from 400
kHz to above 30 MHz in a single range. It was
designed to test receiver front end designs and HF filters and be
compact enough to sit around my workshop bench. The
oscillator output level also allows it to be used as a temporary
oscillator for testing diode mixers, and it’s sine wave output
not possible to directly cover the entire HF band in one range with a
traditional LC oscillator. However, mixing an oscillator operating on a
higher frequency band with a lower frequency fixed oscillator, it is
possible to achieve the required range. This is shown in the design’s
schematic in Figure 1. A voltage controlled oscillator (VCO) operates
from 48 MHz to 85 MHz. The VCO output (100-150mVpp into 50 ohms) is
mixed with the output of a 48 MHz crystal oscillator in a diode mixer
to give the required output.
Figure 1: Schematic of the test oscillator
The varicap diode is the key to successful wide range VCOs, and the
device I used was one of four recovered from an old video recorder
tuner. Other wide range varicaps such as Motorola’s MV104 or a Philips
BB911 will also work well.
The 48 MHz crystal oscillator is typical of those found in equipment
such as printers, video cards and the like. These provide a 5V
square-wave TTL- compatible output level. I found two plastic
encapsulated 48 MHz oscillators in an old Epson printer. The output of
the crystal oscillator I used was unable to drive the diode mixer
directly, but the series combination of C5 and R3, a 1000pF ceramic
capacitor and a 100 ohm resistor, worked well. The square wave output
is also ideal for diode mixers.
The use of a 48MHz oscillator, and the resulting VCO range, was largely
based on the availability of suitable parts. If you are looking to
substitute parts and to modify the design to suit, the VCO frequency
must be high enough to permit the required 30 MHz range to be achieved
within a single span. It’s unlikely that any lower VCO frequency range
would be successful. Also, the crystal oscillator, which sets the lower
VCO frequency bound, must be far enough away from the upper output
frequency of 30MHz to allow the simple 3-pole low pass filter stage to
filter off any residual 48MHz oscillator signal as well as the sum
component of the mixer output. The prototype reached 35 MHz with an
output rolloff of about 3dB.
The output of the SRA-1 double balanced mixer (DBM), M1, provides both
difference (wanted) and sum (unwanted) products. A variety of
diode-type DBMs will work fine here, including one made from 1N4148
diodes and a couple of ferrite beads. The desired (difference) output
is selected using a 3-pole elliptical filter. This filter also places a
notch at 48MHz to minimize any oscillator feed-through.
The filtered output is then amplified by 20dB with an ERA-5
Mini-Circuits “MMIC” amplifier to give an output of 300 – 400 mVpp into
a 50 ohm termination. I used a surface mount version of the ERA-5
amplifier which is around half the size of a grain of rice. Careful
soldering is required.
When operating, the unit draws just over 100mA at 12V. Not really
suited to battery use, but then this oscillator is really intended for
the test bench. A well-regulated supply is required since this voltage
directly feeds the varicap.
Photo 2: The view inside the box illustrates the simple construction
method used with a section of folded tinplate from a tin can used to
form the walls of the box.
tuning of a VCO across a broad range of spectrum like this often
requires a multiturn precision variable wirewound resistor. With these
now costing as much as $US20, I looked for a cheaper alternative. The
solution lay in using a 15 turn preset resistor. These cost more like
$US2 each. A further advantage of this preset resistor was the improved
MHz per turn tuning rate.
To add a control knob, I used parts from an AM/FM radio volume control
potentiometer. Most of these volume potentiometers seem to have a thin
edge-adjusted knob which is screwed with a tiny
Philips screw onto a tiny brass rod. This, in turn, is
connected to the volume control vane (or wiper) inside the pot.
I broke one apart, salvaging just the brass rod, knob and miniature
screw. A couple of minutes with a file added a "flat screwdriver" end
to this little brass rod. This was inserted into the adjustment slot of
the multiturn preset resistor, which was also made of brass, and
carefully soldered in place. Works like a charm!
the circuit directly onto a small piece of blank PCB in just a few
hours. The diode mixer is mounted through this blank PCB, with its
various pins isolated from the ground plane formed by the blank PCB as
required. The 48 MHz oscillator (Epson SG-615) was mounted upside down
onto the PCB. Ferrite beads (shown as ‘FB’ on the schematic) are used
as RF chokes to feed DC to each stage.
The multiturn trimmer is glued onto a scrap of PCB to lift it slightly
higher off the PCB, and to allow the shaft of the tuning knob to rotate
freely. A slot was milled in the PCB for the knob to go through the
A box was fabricated from tin plate, cut into an 18mm wide strip and
soldered around the edge of the PCB. The front panel layout was
designed in CorelDraw, printed out, and covered with contact plastic to
make it more durable. This was then glued onto the front of the PCB
with contact adhesive.
8 turns 24SWG on 5mm slug-tuned Neosid former, tapped at 3 turns from
L2 8 turns 28SWG on Amidon T25-10 toroid
L3 7 turns 28SWG on Amidon T25-10 toroid
T1 10 bifilar turns 28SWG on Amidon T25-10 toroid
oscillator is easy and quick to build, and uses relatively few parts.
Many components can be substituted without any problems. To test this,
I built another version using an LM375 IC as the VCO (It’s an obsolete
National chip similar to Motorola’s legendary MC1648), a homemade DBM
made with 1N4148 diodes, and a
discrete 20 dB wideband amplifier. It gave similar results.
Stability is not equivalent to a crystal-locked or synthesised
oscillator, and tuning across specific bands is very fast, but it’s
fine for general test applications where this sort of tuning is often
preferable. If you want to tune to spot frequencies regularly, an extra
‘fine tune’ control could be added.
Although I’ve not done so, it’s also easy to add a PLL chip to lock the
VCO to selected 5kHz channels for better stability, or it's also
possible to add a ‘huff and puff’ stabilizer. The latter is certainly
less complex to build, and either approach would improve stability.
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