[Elecraft] IMD and CW

[Ron D'Eau Claire]

The fact that the RF amplifier is running Class C should not be an issue if
it is plate-modulated. Plate modulation is the "gold standard" because only
the RF carrier is being amplified in the Class C stage. Beyond that its
non-linearity is employed to mix the modulating signals with the RF carrier.
While the process is extremely linear, it takes a lot of audio level power -
at least 1/2 of the RF carrier power.  

Fred's observation makes me think WWV may be using some sort of "compromise"
system such as grid modulation rather than plate modulation. Much lower
audio (baseband) power required, but it is not as linear either. 

73, Ron AC7AC



-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of K9MA
Sent: Friday, December 23, 2016 2:39 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] IMD and CW

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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--
Scott  K9MA

k9ma at sdellington.us

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