Hi Luis,
As you mentioned, monitoring the amplitude of a properly-functioning
RTTY transmitter is not very interesting. Since FSK is a
constant-amplitude modulation mode, all you see is a constant-amplitude
envelope on the screen. It gets more interesting if there is a problem
in the RTTY transmitter that causes the two tones to come out at
different amplitudes, which would be easy to see on the screen.
The traditional "plus sign" (crossed ellipses) display is normally used
in the FSK receiving demodulator as a tuning aid. As you tune the
receiver the ellipses change their angle. When you see the "plus sign"
then the receiver is tuned correctly.
You can also see if the transmitter has the wrong (or at least
different) frequency shift. In that case the ellipses don't make a
right angle with each other even when the receiver is tuned correctly.
That might be useful in a transmit monitor, but it can't be done using
an RF coupler because it only detects the amplitude, not the frequency.
A "trapezoid" display would be interesting, but it requires two RF
coupler/detectors, one at the input of the amplifier and one at the
output. Any non-linearity in the transmitter shows up as a
non-linearity in the trapezoid. An alternative that serves the same
purpose is a "triangle" display, in which the RF signal ramps up and
down from zero to PEP and back. You feed the mic input of the SSB
transmitter with a (say) 1500-Hz sine wave that is modulated with a
(say) 100 Hz triangle. The P3 TX monitor would then show any
non-linearity in a manner similar to a trapezoid.
Alan N1AL
On 09/13/2015 01:29 PM, Luis V. Romero wrote:
Hello Elecraft folk:
Just installed my new P3 TX Monitor board that I ordered at Dayton (!). I
have been waiting for this capability for the P3 since 2010, so as to enable
me to monitor transmitter waveforms accurately like I did in my Heath
SB-401/SB303 station with the SB610 in ancient times (the late '70's).
So now my O'Scope can go back to the bench where it belongs. Since I already
had an accurate dual meter peak reading Wattmeter (Autek WM-1), the bundled
SWR/Wattmeter capability is not important to me. A good, full featured
visual transmitted waveform monitor was what I was looking for.
However, a big feature is missing from the TX Monitor option: Accurate
monitoring of the direct FSK signal for RTTY. All that is visible on RTTY
transmissions is a solid envelope that fills the screen, just like sampling
RF using my bench 'scope.
True, RTTY is a full carrier FM signal, so the display is showing me the
"correct" interpretation for an envelope sampler, but I was hoping to at
least see the same display we see on RTTY receive: Two pulses with a valley
in between. If both pulses are equal, there is no passband ripple. If
there are single peaks and the fall to the valley is smooth and sinusoidal,
all is well and the transmitter is not being overdriven.
My preference, however, would be to see a traditional crossed ellipsis
display like on my old SB610 and the simulated one on software RTTY apps
(that are NOT real transmitted waveforms!), but I can live with the above.
Neither mode is available on the simple envelope sampler add on that
comprises the TX Monitor today.
I don't use AM here, but the a "Trapezoid" display would also be helpful to
set amplifier linearity and peak modulation.
These features would make a P3 with the TX Mon option a real, full featured,
transmission monitor, which was my assumption that it was to be. Right now,
it's a simple envelope sampler which, while helpful (and cute to show off to
guests), doesn't do what a true full featured transmission monitoring tool
*should* do, especially for RTTY and AM, and especially for its price. I can
do most of what it does with a chunk of wire and any o'scope.
Am I out of line with my expectation? Are these features in the works for a
future software update? If not, they should be!
Lu Romero - W4LT
K-Line
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