Colpitts

My version is a 40 Meter CW/DSB (Double-Side-Band) Transeciver. It uses 22 each 2N2222's and will run from 12 Volts. The Receiver is a direct-conversion design with a switchable input attenuator and RF preamp, diode balanced mixer (diodes were allowed), a switchable low-pass-band-pass passive filter, and audio amplifier. (I may replace the audio final with the version presented by K8IQY, which I like much better). The Local Oscillator also serves as the VFO during transmit. The oscillator is a Colpitts design with varactor tuning, and two-stage buffer. Tuning is provided by two front-panel pots, Main and Bandspread, and an offset is provided during transmit.

This circuit provides an FM modulated signal with an output power of around 500mW. The input Mic preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset. The oscillator is a colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF capacitors and the inductor. ( Click here for Colpitt Oscillator Resonant Frequency Equation.) Frequency is around 100Mhz with values shown.

This is an image Schematic. No Description available.

Yes, this is the same circuit that was presented last time, but I’ve re-drawn it to show the Colpitts oscillator more clearly. "L" is approximately 1.5 uH; 19 turns in a T50-6 core (yellow). The value of C6 is found experimentally. I used 69 pF (a 47 pF and a 22 pF in parallel). More about this during check-out.

This is an image Schematic. No Description available.

This circuit provides an FM modulated signal with an output power of around 500mW. The input Mic preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset. The oscillator is a colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF capacitors and the inductor. ( Click here for Colpitt Oscillator Resonant Frequency Equation.) Frequency is around 100Mhz with values shown.

Modern set-top DBS TV tuners require high performance, broadband voltage control oscillator (VCO) designs at a competitive cost.To meet these goals, design engineers are challenged to create high performance, low-cost VCOs.

There are a lot of oscillator circuits today, but for our purpose only LC tank harmonic oscillators are usable because we need a good frequency stability. Other oscillators have too poor phase noise at these frequencies and are also not usable. Two structures have been studied here.

The oscillation frequency of the voltage controlled oscillator is controlled by the output of the PLL synthesizer and does the stable oscillation of 119.3 MHz.

Here are four simple projects, all of which could be built in a couple of hours or less. None of them is very original but all of them should work first time if built with care. Hopefully they all have practical applications for the QRPer.

A squegging oscillator is an elegant two-for-one solution to the problem. Figure 1 shows how simple it can be. If you have studied for your Extra Class ticket, you'll remember that a Hartley oscillator has a tapped inductor that provides feedback to sustain oscillation. (And if you took your test 25 years ago as I did, you'll remember having to do freehand drawings of Hartley and Colpitts oscillators as part of the test!) L1 and L2 form the tapped inductor in this case. Both are wound on the same form for tight coupling, indicated by coupling factor (K) of 0.98 in the schematic.

This circuit provides an FM modulated signal with an output power of around 500mW. The input Mike pre-amp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset. The oscillator is a Colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF capacitors and the inductor.

It uses 22 each 2N2222's and will run from 12 Volts. The Receiver is a direct-conversion design with a switchable input attenuator and RF preamp, diode balanced mixer (diodes were allowed), a switchable low-pass-band-pass passive filter, and audio amplifier. (I may replace the audio final with the version presented by K8IQY, which I like much better). The Local Oscillator also serves as the VFO during transmit. The oscillator is a Colpitts design with varactor tuning, and two-stage buffer. Tuning is provided by two front-panel pots, Main and Bandspread, and an offset is provided during transmit.

Figure 1, below, shows the 10.58 to 10.74 MHz VFO oscillator circuit. Yes, this is the same circuit that was presented last time, but I’ve re-drawn it to show the Colpitts oscillator more clearly. "L" is approximately 1.5 uH; 19 turns in a T50-6 core (yellow). The value of C6 is found experimentally. I used 69 pF (a 47 pF and a 22 pF in parallel). More about this during check-out.

Its frequency depends on the capacitance of the vary cap diode. The center frequency is changed by varying the biasing voltage of the vary cap through the 47K pot. You can use a 75cm telescopic antenna or simply a length of hook-up wire. Mine worked fine with a 6cm hook-up wire and gave a range of 100m with a good FM receiver.

Simplified circuit diagram of the two-stage Colpitts oscillator. In comparison with the classical single-transistor Colpitts oscillator [1,2,5] the two-stage modification includes an extra transistor Q2 and an extra capacitor C3. We note that both transistors, Q1 and Q2 in respect of AC signals are in a common-base configuration.