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Transistor Radio Series – The 7 MHz Scratch Synthesizer

Transistor Radio Series – The 7 MHz Scratch Synthesizer
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[Section Two — VCO]

Above — The VCO schematic. This VCO provides 3 output ports: main, offset mixer and a port to connect a frequency counter. A frequency counter proves essential, since this style of synthesizer uses a VXO with non-linear tuning; plus frequency overlap or gaps may occur across different divisions of the reference oscillator.
The oscillator and 1 buffer run on a AF low-pass filtered 8 volt DC supply. Any ripple on your oscillator’s DC supply can pump the VCO and cause noise modulation.

Two outputs from the PNP Colpitts oscillator get lightly AC-coupled to an NPN buffer that I forgot to label. Its 39 ohm collector resistor provides a low-amplitude signal which is further boosted by Q3 which drives a common base amp (Q4) to prevent any frequency counter’s digital noise from reaching the main oscillator.

The emitter of the unlabelled NPN buffer stage functions as a normal emitter follower. I ran 9 mA emitter current to preserve the main oscillator signal fidelity. This signal goes to an output port that connects to 1 of the offset mixer’s input ports.

Above

— The output of Q4 ( frequency counter port) terminated in a 50 Ω ‘scope input. When terminated with a high impedance it measured ~700mV Vpp. If you need higher peak to peak output voltage , run more current in the common base amp. Other ways to boost its output include dropping the 470 Ω DC voltage decoupling resistor to 150-220 Ω, and/or increasing the 470 pF coupling cap to 100 nF.



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The main output circuitry takes signal tapped from the tank inductor L1. I tapped about 2/3 down and could have tapped it lower. In honesty, I was too lazy to rewind the L1 toroid, so I employed a 2.7p coupling cap to provide some low-level signal to Q5 without having to run high current to prevent further distortion of the VCO signal. A FET source current of 8.45 mA preserved the VCO signal and provides good drive for the final feedback amp.

My initial feedback amp transistor choice was a 2N2222A. I ran a target 23-24 mA to help achieve a stage output impedance of 50 Ω and raised the *51 Ω emitter resistor shown to 75 Ω to get this. Without a heat sink, the 2N2222a ran at 39-40 degrees C. Rather than make a heat sink, I stuck in a 2SC1971 and changed the emitter resistor to 51 Ω as shown in the schematic. I targeted ~23 mA emitter current.

This BJT runs at 28.7 degrees C and won’t run away. The 2SC1971 I used was probably 1 of those cheap bootleg or counterfeit transistors as it cost < 40 cents with free shipping on eBay a few years ago.

I find these “probably bootleg” transistors work OK for HF projects. I’ve got some original,very old Mitsubishi 2SC1971s in my parts collection reserved for VHF amps + drivers and they cost a fortune now.

Above — A DSO capture of the unfiltered output from the 2SC1971 feedback amplifier (Q6) during some early experiments with attenuation on its output.

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