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 30 minute warmup drift at 180kHz 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:
- It provides an accurate 19,200 bps serial communications (RS232) rate
- It is suitable for JASON, which requires a clock frequency 12.7 - 12.9 MHz for correct carrier spacing
- 12.8 MHz is a frequency widely used in cellular phones, so easy to obtain cheaply
The operating frequency range is from 0.1 Hz to at least 300kHz, depending on the reference 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.
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 2us 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.