A diode detector would if the hum signal was present. But if no mixing
occurs in the transmitter, the low frequency (hum) signal is eliminated in
the transmitters output circuit as the audio frequency never got mixed up to
rf.
However in the case of the linear amp plate circuit ripple voltage there is
some modulation happening due to the varying amplitude of the plate voltage
causing the wanted signal to vary slightly in amplitude but it will be no
where near what it would be if the amplifier was operating class C or other
non linear mode.
Think about the carriers in the AM days on the bands. If two carriers come
into a diode detector spaced a 1 KHz apart you would hear a 1 KHz beat note
between them. The modulation got done in the detector. Switching over to a
product detector the 1 KHz beat would disappear or be attenuated a great
amount. The product detector only detects the product (multiplication) of
the BFO and the incoming signal. It will not detect the sum and difference
(the mix between them) of the two carriers.
The product detector operates as a switch that is turned on and off by the
BFO signal which allows signals to be mixed only with it. The incoming
carriers are two low in level to cause any switching action of the product
detector so there is no modulation of one carrier by the other to cause a
beat between the two. In a diode detector all signals are strong enough to
cause mixing between any other signal making it to the detector so you get
everything mixing with everything and giving all kinds of mixes.
Another way to look at receiver mixers, as a receiver receives more than one
signal off the air the off the air signals do not mix together and generate
yet more unwanted signals or the sum and difference between them. That is if
the receiver mixer can handle the levels of those signals coming in (it
remains linear). The local oscillator is strong enough in the mixer to cause
it to do the switching (multiplication) action and not the off air signals.
If the off air signals get to strong they will start to cause the mixer to
switch and become non linear and act as the local oscillator which creates
unwanted products, often called intermod.
The same thing happens in the linear SSB amplifier. The SSB signal consists
of multiple frequencies (from the audio) going into it to be amplified. If
some of those signals drive the amplifier too hard (into compression) the
amplifier starts to act as a mixer and while amplifying the signals it also
turns into a mixer and multiplies those various signals together to create
yet more signals as we commonly know as intermod signals. Intermod is the
process of mixing two or more signals together to create other signals,
unwanted in this case. Being close together frequency wise the new products
will fall close to our wanted signals also.
We most often think of modulation and mixing as sums and differences of the
signals that result in the new signal but It is really not sum and
difference but multiplication action that happens. It just so happens that
the end result of the products work out to the same result as the sum and
difference.
But for modulation to occur you can not sum signals together, they must be
multiplied.
73
Gary K4FMX
_____
From: TexasRF@aol.com [mailto:TexasRF@aol.com]
Sent: Thursday, October 03, 2013 4:07 PM
To: garyschafer@comcast.net; ww1c@outlook.com; amps@contesting.com
Subject: Re: [Amps] Plate modulation from power supply ripple?
Seems that the receiver mixers and demodulator would provide this
mixing/multiplicaton action and introduce apparent hum modulation of the
signal.
This is an interesting aspect of rf design that I have not previously
considered.
73,
Gerald K5GW
In a message dated 10/3/2013 10:49:25 A.M. Pacific Daylight Time,
garyschafer@comcast.net writes:
AM modulation is a multiplication (mixing) process. Without a switch
(amplifier driven into cutoff) there is no multiplication of the signals.
The variation in plate voltage due to power supply ripple will be seen in
the output as addition and subtraction of the main signal and ripple voltage
but at a very low level if the amplifier is linear. No side bands are
created as they are in an AM signal.
73
Gary K4FMX
_____
From: TexasRF@aol.com [mailto:TexasRF@aol.com]
Sent: Thursday, October 03, 2013 10:44 AM
To: ww1c@outlook.com; garyschafer@comcast.net; amps@contesting.com
Subject: Re: [Amps] Plate modulation from power supply ripple?
Hi All, even if the plate current did not change, wouldn't the changing
plate voltage result in changing power input and corresponding changes in
power output?
That sounds like modulation as well.
Taking this to an extreme, removing the filter C altogether, any hum
modulation then? How much?
73,
Gerald K5GW
In a message dated 10/3/2013 8:35:17 A.M. Pacific Daylight Time,
ww1c@outlook.com writes:
Hi Gary, All
I was thinking about your statement and looking a typical tube anode
characteristics (curves).If I read this correctly, change in plate voltage
does cause a change in plate current.Is this not plate modulation? I am not
sure if the operating class changes this.Perhaps I need to simulate this
too...
Thanks
MarkkuWW1C
> From: garyschafer@comcast.net
> To: ww1c@outlook.com; amps@contesting.com
> Subject: RE: [Amps] Plate modulation from power supply ripple?
> Date: Wed, 2 Oct 2013 12:36:06 -0500
>
> You don't hear the ripple because it does not modulate the linear
amplifier.
> If you were to run the amplifier in a non-linear mode such as class C then
> it would plate modulate it.
>
> 73
> Gary K4FMX
>
> > -----Original Message-----
> > From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Markku
> > Oksanen
> > Sent: Wednesday, October 02, 2013 12:40 AM
> > To: amps@contesting.com
> > Subject: [Amps] Plate modulation from power supply ripple?
> >
> >
> > All
> > I was simulating (with LTspice) a three phase power supply where the
> > secondary is in star configuration with full wave rectification. You can
> > get also a second voltage from this by connecting to the center of the
> > star, this voltage is half of the full wave rectified voltage and is
> > only half wave rectified (per phase), 3 pulse in stead of 6 pulse for
> > the higher voltage.
> > It turns out that even relatively high value (tens of uF) filter
> > capacitors would leave some 10% of ripple on the lower voltage. Now the
> > question is: How much plate modulation this make? The ripple frequency
> > is 150 Hz and to me this looks like your regular high level, plate
> > modulated AM transmitter. Why do we not hear this on a typical signal
> > if it is there?
> >
> > MarkkuWW1C
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
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