Solidstate Crystal Oscillator
Brief History of radio crystals in Amateur Radio
The earliest radio transmitters used free running oscillators to determine the output frequency and were very unstable. Once commercial radio came into existence in the 1920's a stable method of frequency determination was needed and crystals fit the bill. World War II created a tremendous demand for channelized radios and crystals were produced by the millions. Amateur radio equipment of the same period and later into the late 1900's often used many crystals in both receivers and transmitters and in fact the Novice license required a crystal controlled transmitter. Since that time all of the crystal manufacturers we once knew, International, Texas, Billey, etc have either gone out of business or ceased to make crystals. Today most commercial equipment uses a solid-state method of developing frequency. It is virtually impossible to order specific frequency crystals and when you can they are very expensive. So a method is needed for the user to replace defective crystals in electronic equipment or find one for a specific frequency for that old transmitter. This article describes one method to do that. There are others and this may not be the best for all applications but it is presented to show how it can be done. For more info on the early crystal industry see the links at the bottom of this page.
I have many crystals in my junk box but often not the one I need. The popular 75 meter AM frequencies of 3837 KHz and 3885 KHz. are crystals that are in demand and hard to find. An alternative is the solid-state programmable crystals. I chose the Micrel (microchip.com) DSC8001 programmable chip which is about $3 programmed to any frequency from 1 to 150 MHz. The chip runs on 3.3 volts or less and produces a clean square wave output.
While these are surface mount chips There are only four leads to connect and the largest size option chip is easy to mount upside down and solder (carefully) directly to the pads using small size wire.
The output of the DSC8001 in my design drives a single FET (2N7000) with output filtering to produce a nice sine wave of about 12V P-P (no load) This output would directly drive the VFO or crystal input of a transmitter or be used as a marker oscillator to tune your receiver to a channel. Additionally the unit could be used as a QRP transmitter as it keys very nicely through a side panel key jack. This would also be used to turn the oscillator off during receive via a contact closure from the transmitter.
(Click any photo to see a larger view)
As you can see the frequency is accurate, certainly accurate enough for AM or CW use with a boat anchor. One thing that you cannot do with a solid-state crystal that you can with a physical crystal is net or pull the frequency. Whatever you order, that is what it is.
The key jack controls the output and without a plug install the output is on. Inserting a key gives excellent CW and it could be used a a code practice oscillator to a nearby receiver or as a QRP transmitter connected to an antenna. The secondary of the output transformer is has about 8 turns to give 12V P-P output but for lower impedance into an antenna you would probably want to lower the number of turns. See the schematic link below.
A programmer for the DSC8001 and many other Microchip parts is available for about $150. Blank chips are about half the cost of buying pre-programmed chips so you would need to buy a fairly large number to justify buying your own programmer.
Schematic and Calculators
Crystal History Links
This page last updated 2/22/2020
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