(Revised September 1980) CCWN 77:126 then is able to send in phase with the frames for a word or phrase. Transmitter Stabilitv The transmitter frequency must be sufficiently stable to stay well within the filter bandpass over the period of QSO. This is the most difficult parameter to achieve for practical ccw opera- tion. For a 0.1 second frame cw signal, the transmitter must be stable over the QSO to within io-7 or about I or 2 Hz for a 14 %lHz signal. High quality crystal oscillators have such a stability, except when the load placed upon them varies, or their supply voltage varies, such as when keying a transmitter. A typical crystal oscillator transmitter, when keyed, shifts 50 to 100 Hz during a dot or dash. For ordinary cw with a 500 Hz or 2100 crystal filter, this is not noticed. For a ccw transmitter, this would prevent reception because it is more than 10 times the bandpass and would be as detrimental as a 20 kHz shift would be for an ordinary signal, because most of the transmitted signal would be outside the bandpass to which the receiver is tuned. This condition produces an amusing situation. I have been able to copy weak ccw signals clearly through strong local cw signals very near the frequency. At times I have known a weak ccw signals was being transmitted to me, but upon checking the frequency by an ordinary filter, I heard strong QRM signals on the frequency and so strong there appeared no hope of hearing the ccw station. Switching to the ccw filter, I found nearly solid copy. The strong QRM sig- nals, from the point of view of ccw filter were widely swishing up and down the band as they are keved. When they crossed the ccw frequency, the time they were in the bandpass of the coherent filter was slight. The result was that they often had relatively little effect on the ccw signal. Don't let me mislead you, however. These interfering signals, through cross modulation, overloading of earlier stages of the receiver, and activation of the AGC, can and often do cause great problems. Basically, transmitter stability has been achieved by using high quality crystal oscillators which are not keyed and which are followed by several stages of amplifiers and buffers to nul- lify the loading effects of keying. Figure 3 shows a diagram of such a transmitter-exciter. The output of this exciter is about 0.1 watt and it has been used by itself with an antenna tuner and keyer in the final stage, as well as an exciter replacing a VFO. Tests have shown that after 30 minutes warm up, the oscillator is stable within a Hz while the exciter is keyed and remains so over a period of over an hour. The crystal tuning allows VFO operation over a range of about 20 Hz. The circuit is straight forward except that very little power is used from the crystal to facilitate stability, and two stages of isolation amplifiers are used to minimize the load on the oscillator circuit by later stages, particularly the one being keyed which would otherwise cause frequency variations. A second way of stabilizing a transmitter is to use phase lock loops to control the fre- quency of oscillators in a rig and use a highly stable oscillator as a reference for the PLL. McCaskey (1975) did this for a direct conversion receiver by using digital methods. To lock the receiver-transmitter oscillator to a desired frequency obtained by digital methods from a standard. Maynard (1978) has achieved this by using a 5.5 to 5.0 MHz synthesizer output and a 9 MHz frequency standard to control his HW-8 transmitter and receiver. I have done this by mixing the three frequencies (HFO, BFO, VFO) of a double conversion (SB-303, SB-401) tran- sceiver, and locking the result by controlling the VFO (Woodson, 1976). Figure 4 shows a simple circuit for locking the VFO of the SB-303 receiver by using the variable capacitance diode built into the VFO for frequency shift keying. A high impedance volt meter connected to point C can be used to monitor lock. In operation, the VFO is tuned slowly across the fre- quency of the standard. Lock occurs about 250 Hz above and remains to about 250 Hz below. Once locked, the xtal oscillator controls the receiver frequency and can be set more accurately than the VFO. -The xtal oscillator can be designed to be stable withing a Hz over operating