Hi Manfred,
I suspect that the mixing is happening in your spectrum analyzer mixer setup
or even getting in after that mixer. Maybe even into the DDS generator. The
strong signal from your transmitter is overloading it. Your transmitter
signal may be getting into that mixer by a path other than your antenna
pickup.
If you want a class A amplifier just turn down the bias a little on your amp
so it draws significant plate current and then keep the drive low enough to
not cause an increase in plate current.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
> Mornhinweg
> Sent: Saturday, August 08, 2015 12:08 PM
> To: Amplifier Mailing List
> Subject: [Amps] IMD and spurious measurements
>
> Hi all,
>
> I have been playing yesterday and today, measuring the signal purity of
> my
> station under many different conditions. The test equipment used is the
> one I
> described a few days ago: A DDS signal generator driving a mixer, which
> has a
> small piece of wire to pick up enough signal, and feeds into the
> computer's
> sound card, where I use FFT software (most of the time, Spectrum Lab by
> DL4YHF).
>
> As a signal generator I'm using a small battery-powered MP3 player, with
> a short
> piece of cable connecting its headphone output to the radio's mic input,
> via a
> suitable voltage divider. I put several test signal files into the
> player, the
> most useful of which is a dual tone signal with one tone at 734Hz and
> the other
> at 1734HZ. So I get a spacing of 1kHz, but to tones are non-harmonic, so
> they
> don't hide some kinds of spurs that harmonic tones would swamp. The test
> signal
> files were generated in software too.
>
> While I was characterizing the IMD performance of my TS-450 transceiver,
> before
> even switching on the amplifier, I found something that looked strange
> at first:
> In addition to the two tones, and the IMD products, I found additional
> spurs at
> exactly the carrier frequency, 1kHz above the carrier, and 2kHz above
> the
> carrier. The one 1kHz above the carrier was the strongest, only 16dB
> below the
> main tones! The one at 2kHz was much weaker, and the one at the carrier
> frequency was even weaker.
>
> To make a long detective story short: The "spur" at the carrier
> frequency was
> actually the carrier, suppressed about 50dB from the max output level.
> Not
> brilliant, but within specs. And the other two spurs were generated in
> this way:
> The two test tones, spaced 1kHz, generate a SSB envelope that is a
> severely
> distorted 1kHz signal. The current consumption of the radio is largely a
> function of the output envelope, so that the current from the power
> supply to
> the radio had a strong 1kHz component on it, plus its harmonics. This
> signal was
> feeding into the audio stages of the transmitter, and getting added to
> the two
> test tones! So its 1kHz and 2kHz components appeared at the radio's
> output,
> while the 3kHz and higher components were suppressed below detection
> level by
> the SSB filter.
>
> Most of this unwanted signal was getting in through my external DSP
> filter.
> That's an MFJ-784B, which is powered by the same power supply feeding
> the radio,
> and connects to the radio's speaker output, and to an external speaker.
> What
> happened here is a classical ground loop: Some of the 1kHz-modulated
> supply
> current to the radio was flowing over the ground wire from the power
> supply to
> the DSP filter, then on through the audio cable's shield, directly into
> the
> ground foil of the radio's IF board. Due to imperfect conductivity of
> that
> ground foil, a significant 1kHz audio voltage built up there and fed
> into the
> mic amp circuitry, which is on the same board.
>
> At least that's what I thought at first.
>
> When disconnecting the DSP filter from the power supply, the 1kHz and
> 2kHz
> components in the output get about 20dB weaker, but don't disappear. The
> same
> happens when unplugging the audio cable between the units. Obviously
> there is
> some coupling internal to the radio, making audio components in the
> supply
> current appear added to the modulation signal, although the main spur
> caused by
> this internal coupling is about 50dB down from the carrier, and within
> the SSB
> channel width, so it's not an issue.
>
> But I want to keep using the DSP filter.
>
> I tried using the front panel headphone output instead of the speaker
> output, to
> drive the DSP. It's only slightly better. Installing a short and thick
> bonding
> wire between the DSP's box and the radio's, produced only a slight
> improvement.
> Opening the ground return of the audio cable barely improved the
> situation, and
> made the speaker emit a strong 1kHz tone while transmitting. Cutting the
> negative side of the direct wire between the power supply and the DSP,
> so the
> DSP's negative return is only through the radio, didn't cause any change
> at all
> - the offending signal comes in over the positive supply wire and
> through filter
> caps just as readily as over the negative side!
>
> Running the DSP from a separate, ungrounded power supply works fine, but
> I don't
> like that. I hate wall warts! So it looks like I will have to find and
> install a
> nice audio transformer between the radio and the DSP. Or just live with
> the
> problem, anyway no fellow ham has ever reported it! ;-)
>
> Then I switched on the amplifier (NCL-2000), and played a little with
> it.
> Despite running the amplifier at very low idling current, much lower
> than the
> recommended one, the IMD performance is slightly better than when
> running
> without the amp. The reason: When driving the amp, I run the radio at
> only 30W
> output, and at that level the radio is extremely clean (IMD3 around 60dB
> down),
> and the amp adds a normal but not excessive amount of IMD. Instead when
> running
> without the amplifier, I usually run the radio at the 100W level, and
> there it's
> very dirty, with the IMD3 down only 24dB from each tone! And the higher
> IMD
> products, while successively lower, are still significant even up to the
> 9th and
> 11th!
>
> But now comes the most interesting discovery of the day, and the reason
> why I'm
> writing all this: Until here I was doing all tests into a dummy load,
> with my
> mixer's pickup antenna running along the coax to the load. I wanted to
> make sure
> that I'm getting no RF from the antenna into any place where it doesn't
> belong,
> so I switched to the antenna, found a clear spot in the upper portion of
> 40
> meters, nestled amidst lots of strong broadcast signals, and made a test
> transmission. Suprise! My signal had more spurs installed on it, than a
> Christmas tree has stuff attached! And some of that stuff was even
> moving...
>
> It turns out that the whole mess of strong signals that is in the air
> gets fed
> from the antenna into the final stage, mixes with my transmitted signal,
> causing
> thousands of strong spurs, and all these get re-radiated!
>
> That's life, folks. Transmitter final stages are very big and powerful
> mixer
> stages, connected to antennas through only broadband filters. Except
> while
> driving a final stage deep into saturation, the trash generated by them
> from
> mixing external signals onto new frequencies is FAR stronger than their
> internally sourced IMD! It makes one wonder how much sense it makes to
> strive
> for -40dB IMD products in a lab's perfect dummy-loaded environment, if
> this
> won't cure the -20dB spurs caused by external signals as soon as a real
> antenna
> is connected!
>
> Comments, anyone? Do we have a case here for class A final stages, which
> should
> be a lot less prone to mixing external signals? Unfortunately I don't
> have any
> class-A radio at hand, to test this. It would be interesting if somebody
> who has
> one, could do the test, and compare how much external signal mixing
> happens in
> class A as compared to class B.
>
> Manfred
>
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
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