FREQUENCY ACCURACY AND STABILITY


Many LF applications, especially the very slowest narrow-band modes, require a high level of frequency stability, and with it the ability to set frequency precisely. This Exciter meets these requirements easily.

The stability, precision and signal purity of the Exciter is excellent. Even using a simple crystal reference, carrier frequency resolution is 0.1Hz or better (the FSK step size resolution is the same), and after a 30 minute warmup, drift at 135kHz carrier frequency is 0.1Hz or better. With a 10 - 16MHz TCXO reference, the drift is further reduced. 12.800 MHz is an excellent choice for the TCXO, because:

The operating frequency range of the Exciter is from 0.1 Hz to at least 300kHz, depending on the reference frequency used. There is no keying chirp, no amplitude variation or bounce, even on frequency change, and the harmonic content of the sinewave output is low, typically -60dBc. Spurious outputs are below -40dBc throughout the operating range. Phase noise and amplitude noise on keying is dependent on the keying method used.

The 24-bit version has typically 0.085Hz step resolution. The 32-bit version (D6B) has 0.033 mHz resolution (i.e. 2.4 ppb at 136kHz), and is used mainly for GPS locking. This version should only be used with a high quality TCXO, an OCXO or better reference.

Since external control involves stopping the transmission for the duration of the command, there will be some noise, but in practice it has little effect. Obviously it would not be appropriate to attempt to achieve a fast sweep generator by external control, which is why the sweep generator is built in. There is about 2µs phase discontinuity and no amplitude discontinuity at each step in sweep mode.

Many LF exciters exhibit phase discontinuity on keying (dashes and other elements which occupy a duration greater than one dot timer interval have 'beads' on them). This is not the case with this Exciter, as every element is perfectly timed as a single entity. This is important at slower speeds, since very long bit times with no timer phase discontinuity occupy the minimum bandwidth possible.

The Exciter can be used with transmitters that require 5V (CMOS/TTL) drive. In this situation simply omit the D-A converter and power amplifier. The output from PB7 (pin 19) can be used directly to drive the transmitter, and other outputs ignored. When examined on an oscilloscope, this output appears to have jitter. This is however at the clock (sampling) rate of 1.4 MHz and does not appear on the transmitted signal, as it is strongly attenuated by the transmitter output circuitry and the antenna tuner.


Copyright Murray Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.