Hi Minto,
Am 07.08.2011 22:57, schrieb Minto Witteveen:
Hi Stefan,
DCF39 is more than 40 dB above the noise, but the
problem is that my FT897 only seems to have 40 dB dynamic range without
the AGC. (range between the receiver noise floor, and max LF output
before the signal gets distorted).
The noise floor without an antenna connected? Then you would need a
other RX or a narrow band (high Q coils) band pass filter. If it is the
noise with an antenna connected, try to find local noise sources!
Actually i cannot imagine 40 dB dynamic range so i think the antenna is
connected..
With the AGC it is much more
of course, but then I cannot use the LF output to measure the dB’s
between the band noise and DCF39. I will need to calibrate my S-meter
sometimes… But based on what I know about the S-meter, I guestimate
that DCF39 is more than 60 dB above the noise floor (but probably not
by much).
Wrt to the Speclab settings: I have everything
still on the default settings (just installed it, selected QRSS3 and a
suitable frequency range). But the FFT tab says:
Effect of FFT settings with fs= 11.0250
kHz:
Width of one FFT-bin: 336.456 mHz
Equiv. noise bandwidth: 504.684 mHz
Max freq range: 0.00000 Hz .. 5.51250 kHz
FFT window time: 2.972 s
Overlap from scroll interval: 50.0 %
336 mHz is OK too. I prefer 448 but it's a personal taste. I like sharp
limited signs, not the fuzzy ones ;-)
If you have suggestions for improved settings I am very
interested. I definitely need to read up on the math behind the FFT
settings (and possibly RTFM hi)
Oh, i've forgotten about all the maths i learned about FFT too. It's 10
years ago and i never needed it again. I'm an appliance operator, you
know ;-)
An improvement, especially if you are bothered by DCF-39 will be the
noise blanker! I'll send you a USR file that you can load in SL in a
personal mail.
By the way, the noise (man-made QRM) is now much lower
than it was this afternoon.
So this tells you you have to do some tests where the local noise floor
comes from! Probably you can gain 20 dB S/N by that!
73, GL, Stefan
I saw – but could not hear - your CQ an hour ago, see attachment. Using the TS130
so the receiver is still a bit off.
Pity I can’t TX … L
Regards,
Minto pa3bca
Sent: Sunday, August 07, 2011 18:37
Subject: Re: LF: Re: CW sked on 137 friday morning?
Hello
Minto,
Nice to read about your progress!
Am 07.08.2011 18:06, schrieb Minto Witteveen:
Hi Stefan,
I noticed I wrote ALC, but obviously this must be AGC….
I installed Speclab, and did some tests with the 879. With the ACG off
I adjusted the gain just to the threshold of (audible) distortion of
the DCF39 signal (500 Hz filter) and read-out the level in Speclab.
Then I dialed to 137.700 and read out the (average) noise level. This
turned out to be a 40 dB lower, however I could hear NO bandnoise at
this setting of the manual AGC… only the receiver noise.
So DCF-39 would just be 40 dB above the noise? This is much to low. At
least 30 dB would be missed i estimate.
But what was the FFT setting "width of one FFT bin" in the FFT register
card?
From this I inferred two things:
A. DCF39 is more than 40 dB above the noise (but how much I cannot
measure with this method)
B. The 879 is a really lousy receiver (But I already knew that)
But: there is also good news! I did see (but not hear) your CQ at 17:40
HR CET, see attached screenshot. My first RX on 137!
Congrats to your first LF reception! And it shows me that my
calls are not only for my pleasure :-) So here we may better estimate
how many dBs are missed. Looks like you choosed a relative broad BW for
the FFT settings. 488 (some use 366 mHz) mHz would be a better choice.
And i assume you don't use a noise blanker so far. This will improve
thing too!
So at least the receiver + software works.
In the screenshot you can see your first CQ recorded with the FT897. At
17:32 I switched to the TS130. At first glance the signal looks better
on the TS130. Also notice that this trace is a bit off-frequency and
drifts visibly. The 130 has an analog VFO, and the frequency readout is
in 100 Hz steps. The drift might also be attributable to the SA612
mixer I use (now with a 10 MHz xtal)
This proves that at least QRSS3 QSO’s will be possible, so I have no
excuse not finishing my PA and variometer.
Yes! :-)
How do you go on improving the RX system? Ah, i have an idea: Display
the whole band, as broad as the SSB filter permits. Use a slow
scrolling spectrogram and make your local noise visible. Normally local
QRM sources disappear and appear from time to time. And they could be
band limited.
You should see the band noise ideally and its minimum just after the
local sunrise and a few hours later... If that all is on an equal
level, it must be covered by local noise!
Try to run the PA0RDT on batteries and your TRX and PC as well
(netbook). Do not change the antenna position and see how and if the
noise is changing and so on... :-)
73, Stefan/DK7FC
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