[ARC5] Lopsided modulation

Sun Feb 25 16:57:40 EST 2018

    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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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL