Did I write ‘manual AGC’ ?
Obviously I meant to write manual (IF) gain control.
Minto pa3bca
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censeo Carthaginem delendam esse
Sent: Monday, August 08, 2011 00:20
Subject: Re: CW sked on 137 friday morning?
Hi Stefan,
Let me try to explain a bit better
what I have done. When I disable the AGC of the FT897 I need to dial-down the
manual AGC in order to avoid overloading the IF AND the AF sections when tuned
to DCF39. (If I don’t DCF39 sounds
horribly distorted). With this setting I measure the LF output of the 879 with
Speclab.
With the same manual AGC setting I tune to say 137.700 and measure
the LF output again with Speclab. But because the manual AGC is dialed down
significantly the band noise ‘drops through’ the noise floor of the FT897. So
this method is unusable to determine the strenght of DCF39 w.r.t. to the
noise.
If I dial up the AGC (or set it on
auto again) the noise floor increases with about 10 dB when I connect the
antenna, I think this should be enough..
By the way, this moment I can not
only see your CQ with Speclab, but with headphones I can even hear
you.
Regards,
Minto
pa3bca
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censeo Carthaginem delendam esse
Sent: Sunday, August 07, 2011 23:29
Subject: Re: LF: Re: CW sked on 137 friday
morning?
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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