The Tempo One has a unique way of generating a CW signal. For that reason, it is also susceptible to some unique faults. The image below is a capture of a nanoSaver scan of the 9 MHz, 2.4 kHz wide crystal filter used in the Tempo One.
Marker 1 is 9000.000 kHz
Markers 2 and 3 are the standard frequencies of the two carrier crystals, 8998.500 kHz and 9001.500 kHz.
Markers 4 and 5 are at the measured 6 dB bandwidth of the filter, 8998.770 and 9001.140 kHz, respectively.
Markers 6 and 7 are the updated carrier crystals frequencies based on being 300 Hz away from the measured 6 dB bandwidth frequencies, 8998.470 and 9001.440 kHz.
Marker 8 is 200 Hz lower than the updated upper carrier frequency of marker 7 ( 9001.440 kHz ).
Marker 9 is 200 Hz lower than the standard upper carrier frequency at marker 2 ( 9001.500 kHz )
Based on the 6 dB frequencies, the current center frequency of the filter is ( 8998.770 + 9001.140 ) / 2 = 8999.955 kHz, a small shift down of 45 Hz. This small shift will have some serious consequences for this type of carrier generation however.
The CW carrier generation method employed by Yaesu in the Tempo One uses a 5 pF capacitor switched into the 9001.500 kHz crystal oscillator to lower that oscillators frequency by 200 Hz.
According to the Tempo One instruction manual page 6, “On AM & CW, crystal 9001.5 is used and its frequency is lowered 200 cps by diode switch D104, 1S1007, bringing carrier into filter passband.”. This is not true. A filter with a center frequency of 9 MHz and a 2.4 kHz 6 dB bandwidth, the upper 6 dB frequency would be 9 MHz + 1.2 kHz = 9001.2 kHz. The carrier crystal at 9001.5 kHz is 300 Hz above that 6 dB frequency. A lowering of the carrier frequency to 9001.3 kHz leaves the carrier frequency 100 Hz above and outside the filter passband.
As shown in the scan below, if the upper carrier frequency is set to 9001.5 kHz, the loss at the down shifted frequency shown at marker 9, is 14.27 dB below that at the center of the passband. If a new upper carrier frequency is determined by adding 300 Hz to the measured upper 6 dB frequency at 9001.140 kHz it would equal the 9001.440 kHz of marker 7. A 200 Hz downward shift of marker 7 leaves marker 6 at 9001.240 kHz. The loss at 9001.240 kHz relative to the passband center is 10.62 dB. This is an improvement of 3.65 dB.
That 3.65 dB can make all the difference between having adequate drive on all bands and not.
