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CK-3V FM Transmitter: |
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Components of a simple bandpass filter: |
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design and construction of radio frequency oscillators:
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Electronic Canary: This circuit is a modified hartley
oscillator with a couple of extra components included. The
transformer is a small audio transformer, type LT700. The
primary is center tapped with an impedance of 1Kohms at 1KHz .
The secondary has an impedance of 8 ohms. The inclusion of R1
and C1 give this oscillator its characteristic "chirp". As the
100u capacitor charges via the 4.7K resistor, R1 the bias for
the transistor is cut off. This causes the oscillation to stop,
the capacitor discharges through the base emitter circuit of the
transistor and oscillations start again. Altering these
components alters the frequency of the chirp. The chirp is also
voltage dependent. When the push button switch is operated the
100u capacitor is charged. When its released, the oscillation
decays and the chirp becomes faster. |
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I.F. Amplifier: The I.F. transformer primary has 18 turns,
the secondary winding has 4 turns. The capacitors across the IFT
primaries are 82pF. The input/output transformer has 12 turns,
tapped at 3 turns from ground. This transformer is wound on a
ferrite core. The mosfets are 3SK45's. The diodes in the product
detector are 1N34's. I use a six pole SSB filter from a scrap
CB. The centre frequency is 7.8MHz. The -6db bandwidth is about
2.5KHz. |
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Regarding Tapped Capacitor Impedance Transformation in LC
Bandpass Filters: We often use a pair of capacitors to
match impedances at the termination ends of LC bandpass
filters. The circuit consists of a shunt capacitor at the
termination followed by a series capacitor connecting to the
high Z end of a parallel tuned circuit. |
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Resonator BFO Circuit: This circuit was used to stop all
the BFO drift. The circuit is extremely stable. Turn the
receiver off, and then on at any time and temperature, the BFO
frequency is exactly the same. |
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series regulator with Q900 : This is a series regulator
with Q900 being the control element, Q901 a driver, and Q902 an
error amp. ZD900 forms the emitter reference voltage source.
Since the generated high voltage and other voltages are linked
by means of the magnetic field of T900, any change in H.V. will
be reflected back to all of the other voltages. |
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Sine Wave Oscillators: This oscillator gives a really
beautiful sine wave, and is an excellent choice for a precision
audio oscillator. Its characteristic feature is the RC network
consisting of R and C in series with a parallel combination of R
and C, as shown in the circuit diagram below. The resistors and
capacitors can be different in value, but it is much simpler to
take them equal, and nothing of value is lost. |
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VFO/Buffer: It's basically a standard Hartley oscillator,
followed by Roy Lewallen's buffer (page 14.20 of the 2001
Handbook). Output is +7 dBm into 50 Ohms. Don't be tempted to
add a gate diode, this circuit doesn't need it, and it will
degrade the phase noise performance, according to Ulrich Rohde.
It should be suitable for any frequency up to 10 MHz or more
(depending on how good you are at making drift-free oscillators)
and may be tuned with a suitable capacitor or varicap tuning
diode. |
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Was That Really A Wildlife Tag?: In wildlife management,
very few tracking transmitters send some form of "station
identification." Radio tags for birds and small mammals can't.
These little devices must put out a signal to a ground range of
a mile or so for up to a year or more, yet weight only a few
grams, including battery. To do that, most of them consist of
simple blocking or relaxation oscillators with only a few
discrete components. You may hear the term "squegging
oscillator" used to describe them. |
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What I had to do to stabilize a VFO…: |
