[ARC5] Lopsided modulation

[Fuqua, Bill L]

  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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