[ARC5] Lopsided modulation

Sun Feb 25 17:02:50 EST 2018

I never knew about Kahn's pudding patent. That's hilarious.

Thanks, Richard!

--Cheers,
Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)

On 2/25/2018 1:57 PM, Richard Knoppow wrote:
> The hum modulation in mine is quite audible. It may not be due to the 
> power transformer. I have other generators that I mostly use. I need 
> to work on the 1001A at some point, for one thing the nuts that hold 
> the slugs in place have come loose on the two lowest bands. A little 
> epoxy will fix that.
>    The formulas for calculating FM are in many books as are charts of 
> indices vs nulls. At some point if one reduces the index one has PM 
> although narrow FM and PM are not the same.
>     About sixty years ago a fellow named Leonard Kahn was selling 
> units to convert AM transmitters to SSB. He wrote several engineering 
> papers and held many patents. Essentially, any signal can be analyzed 
> as a combination of FM and AM. In the Kahn system the amplitude part 
> is fed into the modulator and the FM to the oscillator. If everything 
> is exactly right the output will be a replica of the input. At least 
> two local broadcast stations used his system to generate either SSB 
> with carrier (KBIG) or, in one case, residual sideband to squeeze the 
> signal so the carrier could be moved a little to avoid interference 
> with another station.
>   Kahn also promoted the use of synchronous detectors to detect double 
> sideband AM with or without carriers. This method can eliminate most 
> of the distortion from selective fading.
>   He also has a patent on making a "pudding" from mashed up dry 
> breakfast cereal. I think this was to show how easy it is to get a 
> patent.
> https://patents.google.com/patent/US3930051
>    For more see:
> https://patents.google.com/?inventor=Leonard+R+Kahn
>
> On 2/25/2018 1:36 PM, Fuqua, Bill L wrote:
>>    The first carrier null occurs at when the deviation is 2.4 times 
>> the audio frequency.
>>
>> This is wideband FM and also has multiple sidebands. But Narrow band 
>> FM with a modulation index of less than 1 the carrier is not greatly 
>> affected. I did not notice the actual sideband levels were mentioned. 
>> You should be able to calculate the modulation index. I will  have to 
>> think about the math but suspect the modulation index is less than 
>> 0.5. At that level of FM the carrier would be reduced by about 1dB.
>>
>>    The only solution in the transmitter is to add a buffer between 
>> the VFO and the amplifier.
>>
>> I would not worry about it. It did not seem become a problem until it 
>> was noticed. I suggest that others check their AM arc5 tx.
>>
>>     My GR1001A had some 120 Hz sidebands. The SB12a was the first SA 
>> that I had that had the resolution to see them.  Since the best 
>> resolution was 100 Hz I could not see 60 Hz sidebands if they were 
>> any to notice.
>>
>> 73
>>
>> Bill wa4lav
>>
>>
>>
>> -----------------------------------------------------------------
>> *From:* arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> 
>> on behalf of Richard Knoppow <1oldlens1 at ix.netcom.com>
>> *Sent:* Sunday, February 25, 2018 4:16 PM
>> *To:* arc5 at mailman.qth.net
>> *Subject:* Re: [ARC5] Lopsided modulation
>>       Is this a plate modulated rig, transformer makes it sound
>> like one. I still think you could hear it if the oscillator is
>> FMing. I think  you will be on the wrong track going after the
>> audio. You might also see if the B+ for the oscillator is varying
>> with the modulation. A scope is the test tool for this but a VOM
>> will show variation on a steady tone to the microphone. The
>> amount of FM will depend on the particular oscillator circuit.
>> Even though the final amp is running AB1 the plate current is
>> still varying with the modulation so it could affect the plate
>> supply. Another experiment would be to run the oscillator from an
>> independent supply.
>>       Also, you will not see the relative strength of the
>> sidebands on a scope anyway. A spectrum analyzer is best but you
>> can use a selective receiver, say one with a crystal filter in
>> it, by modulating with a steady tone. You can separate the
>> carrier and the two sidebands with that setup.
>>       BTW, someone posted that the carrier never went away in
>> either AM or FM, this is true of AM but not FM where the carrier
>> definitely nulls at certain combinations of modulating
>> frequencies and modulation indices. In fact, carrier nulling is a
>> method of calibrating the modulation index.
>>
>> On 2/25/2018 11:46 AM, AKLDGUY . wrote:
>>> Modulator is homebrew:
>>> * Dynamic mic salvaged from Kenwood TR-9130, with input circuitry 
>>> closely adhering to the Kenwood schematic -->
>>> * 3 stages of MPF-102 FET amplification to 50k pot for level setting 
>>> -->
>>> * LM-380 power amp driving the 8 ohm winding of a small audio line 
>>> transformer connected backwards (Jaycar MM-1900). The 20k and common 
>>> taps feed the modulator grids, with 5k tap used as center tap -->
>>> * pair of 2E26s in AB1, screens regulated at 150V, bias via center 
>>> tap from 3x9 volt batteries decoupled with 47 uF 35V electrolytic -->
>>> * Thordarson 75 watt modulation transformer T-11M75 set for 8k:8k 
>>> ratio, appears to be a NOS item.
>>>
>>> I have had a report that my audio is good, but want to fix the 
>>> unequal sideband level. Checking with CRO is not an option, a 
>>> multimeter is all I have.
>>>
>>> Neil ZL1ANM
>>
>> -- 
>> Richard Knoppow
>> 1oldlens1 at ix.netcom.com
>> WB6KBL
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