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HF/VHF/UHF
TEST OSCILLATOR
This test
oscillator covers 16 MHz to 300 MHz in six bands, using varactor
tuning. It has a high level output to drive a frequency meter and
two sine wave outputs, one of which is 50 Ohms.
This article was published in Croatian in infoelektronika
magazine in June, 2012 (cooperatively)
Its built into a plastic
project box with an aluminum cover (on the bottom). The controls
are as follows: Large golden knob is coarse tuning, small black
knob with a blue index stripe is fine tuning, the green LED is
the power on indicator, the slide switch is the on/off switch.
The small black knob with the orange index stripe is the band
select knob. The two RCA connectors with red insulators are the
sine wave outputs. The RCA connector on the right side of the
box is the frequency meter output. The cord carrying the 12VDC
input power exits the box to the right. There are no scale
markings or band markings, since the oscillator is intended to
be used with a frequency meter. Turning a pot or switch
clockwise increases the frequency.
The time came, when I needed a simple test oscillator that covers
a wide range of frequencies. My main interest at the time is the
100 MHz to 300 MHz region, but since it wasn't much trouble to
extend the coverage to under 20 MHz, I did so.
The
Circuit
The circuit is a modified Colpitts oscillator, tuned with MV209
varactor diodes. The resonating inductor and the drain choke are
selected by a rotary switch. 1N5711 Schottky diode, D,
clamps the maximum positive voltage on the gate of oscillator FET,
Q1 to just above ground to reduce the distortion in the wave form
on the gate of Q1. Buffer Q2 provides about 1.8 volts peak-to-peak
at 20 MHz to drive a frequency meter.
A rotary switch selects the
appropriate resonating inductor and drain inductor for each
band.
Since the P-P voltage on the gate of
Q1 is too large to be buffered by a 2N5485 without sever
distortion -see the pictures further down this page), the signal
is reduced by a capactitive voltage divider formed by 1 picofarad
capacitor C11 and the input capacitance of Q3. This results in
about 650 millivolts on the gate of Q3, and about the same on the
source.
To make a 50 Ohm output, so the generator can drive a 50 Ohm
cable, I measured the output impedance of Q3 while using a 220 Ohm
source resistor. This was done by measuring the Peak-to-peak
output voltage without a substantial load, at 20 MHz, then loading
the source with an additional 470 Ohm resistor, and calculating
the effective output resistance of the source follower based on
the change in amplitude. This value was found to be about 139
Ohms. This is much simpler and more direct than trying to
calculate the output impedance as a function of data sheet values,
since the key parameters such as IDon vary too widely to allow
design without trimming. The output impedance is about 50 Ohms at
20 MHz, but this design approach does not separate out the
resistive from the reactive part of the impedance, and I expect
the impedance to change as a function of frequency.
Using a program that seeks numerical solutions to formulae, I
found the values of resistors for R11 and R12 that simultaneously
satisfy the requirement that the total resistance be equal to 220
Ohms and the resistance from the the connection between R11 and
R12 to ground, including the effects of the source follower, is
equal to 50 Ohms. The values, R11 = 150 Ohms and R12 = 68 Ohms are
the closest 5% resistor values.
The +24 volt power supply was made separately, and is described
separately on this site. You can find it with this link:
5 Volt to 24 Volt DC to DC
Converter.
Construction
The oscillator subassembly was
built, dead bug style, or "Ugly Bug" style as a friend calls it,
on a piece of copper clad fiberglass board, that was in turn
mounted to the bottom of a two section six pole rotary switch.
The board is held to the switch with solder on the two solder
tabs on the switch.
Construction started by drilling
holes in the copper to correspond to the connections on the dual
6 pole rotary switch. The oscillator was built dead bug style.
Solder on the solder tabs for on the switch can be seen at the 5
o'clock and 11 o'clock positions.
The resonating inductor for the 190
to 310 MHz band was made with a strip of copper sheet. This
copper strap, along with the switch contacts, was not low enough
inductance to get the oscillator above 300 MHz, so a piece of
#22 wire was placed in parallel with it. Notice the ring of wire
supported by capacitors. This is the +5 volt rail. All of the
drain inductors were soldered to the +5 volt rain and to their
respective contacts on the switch.
This is the finished oscillator with
all of the inductors attached. This was before the 50 Ohm buffer
was added. Notice the piece of #22 wire in parallel to the 190 MHz
to 300 MHz strap to further lower the inductance, which is
indicated by the yellow arrow in the photograph. With this style
of construction, it making major changes in the circuit
would be time consuming. Some attractive aspects of dead bug
construction is that it is compact and does not require a printed
circuit board.
The oscillator subassembly, mounted
on the rotary switch, the +24 volt bias supply, and all of the
other components mounted in the box. The light colored phenolic
board serves as both a mechanical adapter to hold the +24 volt
supply to the plastic box, and also holds the +5 volt regulator,
the varistor, and the decoupling component. The aluminum cover
is connected to the oscillator subassembly ground plane with a
pair of gray wires. A short length of RG-174/U is used to
connect the 50 Ohm signal to the RCA connector. Given the short
length, I suspect that this could have been twisted hook-up wire
without a noticeable affect on performance. The counter output
and the 150 Ohm, 640 millivolt (unterminated voltage)
output are connected to their respective RCA connectors with
twisted AWG 26 hook-up wire.
It may have been a little nicer to have put this into a fully
shielded box, but I am quite happy with the performance in a
plastic box. I think the careful, compact construction over the
groundplane provided by the copper clad board worked well in my
limted use of the test oscillator so far.
The hand-wound coils were wound on a drill bit (A trick
mentioned by legendary Harry Lythall). One trick to making
nicely formed coils is to stretch the wire a little bit before
starting the winding, and to keep tension on the wire through
the winding process. This keeps the wire from unwinding. Once
you have the coils wound, you will have to fiddle with them a
little bit in order to get the frequency range you want. The
fiddling consists of stretching the coil or compressing the
turns. This part is art, and given the realities of
component variation, this is a necessity.
Performance
Its clear that the sine wave output is much cleaner than the
counter output. Listening to the signal on an FM radio receiver,
the oscillator sounds quite and "feels" stable, that is to say
there is little short term frequency drift. There is substantial
drift from day-to-day as indicated by the frequency meter. At
the moment, that's about as far as I have been able to get in
terms of evaluating the performance, since I don't curretnly
posess calibrated instrumentation that covers most of the
oscillator's range.
The counter output is a
harmonic rich 1.8 volt peak-to-peak signal when operating at 20
MHz.
The sine wave output is
about 640 millivolts peak-to-peak when operating at 20 MHz. The
50 Ohm output has a similar appearance, but is only 160
millivolts peak-to-peak when terminated in 50 Ohms and operating
at 20 Mhz.
.
.
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50 Ohm RF Source 50 Ohm Test Oscillator
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