455 kHz Signal Generator

A stable 455 kHz signal generator is a useful piece of equipment for the radio hobbyist. It can be used for aligning the IF (intermediate frequency) section of a receiver, and it can be used as a BFO (beat frequency oscillator) for receiving CW and SSB signals.
The following circuit uses a 455 kHz ceramic resonator which gives good frequency stability, but still allows for fine frequency adjustment. I've built both a very simple low parts count version as shown in the schematic to the right, and a deluxe version. Both are inexpensive, and the circuits can be customized as necessary to add or delete features. The barebones version is just a 455 kHz square wave oscillator and nothing more.

The deluxe version produces a very clean sine wave with adjustable amplitude modulation, and has a buffered output with a set output impedance.

Here is the schematic of the deluxe version:

Larger Version Here

Parts List

U1 - 4069 or 74C04 CMOS hex inverter

U2 - 4011 or 74C00 CMOS quad NAND gate

X1 - 455 kHz ceramic resonator, ECS Inc. ZTB455E

X2 - 455 kHz ceramic filter, Toko AHCFM2-455CL

Other parts values are as indicated. Transistor type is not critical. Any general purpose type should work fine. I used 2N2222's

Circuit Notes

Inverters U1a, b & c form a low frequency oscillator used to modulate the 455 kHz carrier. With the component values shown here, the frequency can be varied over the range of about 250 Hz to 5 kHz.

Inverters U1d & e along with ceramic resonator X1 form a fairly stable (0.5%) 455 kHz oscillator.

NAND gate U2a is the modulator and provides a 100% square wave modulated signal. NAND gate U2b buffers the original unmodulated signal, and its propagation delay ensures that its output is in phase with the output of U2a. So, at point 'A', the % modulation pot produces a signal that varies from a completely unmodulated signal at the bottom end to a 100% modulated signal at the top end.

Transistor Q1 is an emitter follower buffer which feeds into ceramic filter X2. This filter removes the harmonics from the square wave signal producing a clean sinewave. The bandwidth of this filter is ± 4 kHz, so it lets the modulation signal sidebands through.

The output of the ceramic filter is fed into a final Q2 emitter follower buffer stage. The emitter resistance can be changed to suit the desired output impedance.

Power supply voltage is not critical. If you use standard CMOS IC's, a supply voltage anywhere between 5 and 15 volts should work fine, However, at higher supply voltages the power dissipation of the output transistor and its emitter resistor should be considered. Be aware also, that the 74HC and several other versions of CMOS logic cannot be operated above 5 volts. Check the maximum supply voltage specs for your logic chips before exceeding 5 volts.

If you want to eliminate both transistors, you can use the 6th inverter, U1f, in the 4069 IC biased into linear mode as shown in the Alternate Output Stage figure shown above. I've played with this a bit, but didn't try to refine it at all.

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This page last updated: January 3, 2016

Copyright 2010, 2015, Robert Weaver