I had a problem that a nearby 50 kW BC station was picked up by my
vertical antenna, causing the diode TR switch in my K3 to conduct, and
squirting junk all over my receiver. It didn't occur to me that it might
also be messing up my transmitted signal!
Interestingly, I still heard it with a loop antenna connected to the
K3's RX antenna input, because the vertical was re-radiating all of the
crap that was being generated in the TR switch. So the loop picked it
up. It went away when I listened on the loop and unplugged the vertical.
The BC RF was also making my SWR meter act weirdly, so I built a 1.5 kW
highpass filter and put it right at the antenna. That keeps the BC RF
out of all of my equipment, including the amplifier.
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
On 8 Aug 2015 20:07, Manfred Mornhinweg wrote:
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
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http://ludens.cl
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