Very interesting, Fred. It makes sense, though, that plate modulated
class-C amplifiers aren't terribly linear. On the other hand, that kind
of distortion probably doesn't affect WWV's mission.
I wonder if the AM broadcast industry achieves lower distortion.
Feedback comes to mind.
73,
Scott K9MA
On 12/23/2016 15:53, Fred Jensen wrote:
Not CW, and this probably won't work in 4X-land, but if you want to
see the difference a Class-C amp makes, set your P3 span to about 6
KHz, and tune in WWV on its various frequencies. During the tone-on
minutes, the 2.5 and 20 MHz signals will look as you expect an AM
signal with a tone modulating it ... two sidebands on either side of
the carrier. If you look very close, you can see the 100 Hz
sub-carrier, and if you narrow things down enough, you can see the
sidebands generated around it by the IRIG-H time code. These two
transmitters are low level modulated with linear amplification.
The 5, 10, and 15 MHz transmitters are plate-modulated Class-C and
look markedly different. The two sidebands are there, but so are
their 2nd and 3rd harmonics [sometimes when WWV is really strong, I
can make out the 4th harmonic]. The 100 Hz sub-carrier and the IRIG-H
sidebands are there as you would expect, but they also surround the
audio sidebands and harmonics.
WWV builds one baseband with all the tones, ticks, burps, codes, and
voice and it modulates all 5 transmitters. The difference between the
TX is really striking and a bit disappointing. I've always assumed
WWV was the "gold-standard."
Fred K6DGW
Sparks NV DM09dn
Washoe County
--Northern California Contest Club
--CU in the Cal QSO Party
--7-8 Oct 2017
On 12/22/2016 10:22 PM, Vic Rosenthal wrote:
I didn't exactly do this, but I did investigate the effects of
distortion on CW signals. I disconnected my P3 from the K3 and set
the IF to 7 MHz. so it would act like a standalone spectrum analyzer.
I was interested in whether a class-C amplifier I had built would
sharpen the keying enough to create noticeable clicks. The amplifier
used minimal fixed bias to allow a small amount of resting plate
current in transmit mode, with full class-C operating bias provided
by a grid resistor.
I noted a broadening of the base of the signal on the P3, indicating
that the signal was not as clean as the K3 alone. But listening with
another receiver on either side of the S9+30 dB signal didn't reveal
audible clicks. I also asked other operators to listen and there were
no reports of clicks.
I am sure that attempting to use that amplifier (a pair of 813s) on
SSB would have gotten me ridden out of town on a rail!
Vic 4X6GP
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