Re: [linrad] Calibrating analog hardware

["w3sz" <w3sz@xxxxxxxxxxxxxxx]

Hi, Joe,

I have used the pulser built from the schematic on Leif's website with good
result.

http://www.antennspecialisten.com/~sm5bsz/linuxdsp/pulgen/pulgen.htm is the
URL for those who haven't seen it.  It is tailored to provide significant
energy in the 2 meter band, though there is some fall-off from lower
frequencies.

Last year I got a 'last minute, don't want to take it home' great deal on a
Tektronix PG508 pulser at Dayton.  That works well also for this purpose,
and once I got it, it pretty much replaced the homebrew pulser.  But I tried
comparing them, and they do an equally good job at calibrating as far as I
can tell.

In terms of doing the step for calibrating to reduce aliasing [using a
series of stepped, discrete frequencies] I just use a PTS synthesizer.

I have done the calibrations both at the IF frequency and at the RF
frequency.  I haven't done any in probably a year, and so I am relying on my
aging memory and some notes here.  Leif may actually remember better some of
the problems I had.  Here is what I can recall/find in my notes:

Very early on [January 2001] I had a problem with the pulser not providing
enough signal at 144 MHz, related to my [poor] construction technique, but
this was resolved with a rebuild of the pulser.  Leif describes the pitfalls
of construction at the URL noted above.

Initially I had to increase the PRF of the pulser above the optimal range
[of 100 Hz or less], to 200 Hz or even higher, in order to get adequate S/N
to calibrate.  This can affect the accuracy of calibration.  Although if
your spectrum is already pretty flat and needs little correction, I don't
think its a big deal.  Leif can comment on this.

I also initially, when I was having this problem, did the calibrations at a
lower RF frequency [or even at the IF frequency], realizing that this was
not ideal.  But it was better than nothing, as it allowed me to experiment
with the smart blanker even though the pulser didn't put out enough signal
at 144 MHz.

Later after improving the hardware of both the pulser and the receiver, and
understanding better how to make my parameter choices within Linrad, this
was not a problem.  Thanks goes to Leif on all of these points.  He was very
patient with my questions and spent a lot of time going over the data I sent
him as I was very slowly learning.

In reviewing my notes from January-October 2001 I see that during that time
I started out running the homebrew pulser first through my DCI 144-146 MHz
bandpass filter, and then into the 144 MHz cavity preamp that was
immediately before the input to the TUF-1H mixer.  I found that the preamp
was saturating from the pulser signals and I needed to remove it from the
chain in order to be able to calibrate.  So I ran the pulser through the DCI
filter and directly into the RF input of the mixer.  This provided the S/N
benefit of having a filter in line and didn't clip the pulser peaks.  So
that is how I ended up doing all of my calibrations for 144 MHz:
Pulser==>DCI Filter==>input to mixer.

I hope that helps and isn't too redundant.  It is as accurate a recollection
of what I went thru as I can give.  Let me know if you need more detail...my
memory may improve as I rethink all of this.

I am leaving for Dayton in about 5 hours, and won't return until Sunday
evening.  I won't have email there, but I will have a 2 meter HT and we will
have an RV parked somewhere in the flea market with PA plates.  I will also
be at the weak signal banquet arranged by Tom Whitted on Friday night.

Have a great week all, and

73,

Roger Rehr
W3SZ







----- Original Message ----- 
From: "Joe Taylor" <joe@xxxxxxxxxxxxx>
To: <linrad@xxxxxxxxxxxxxxxxxxxxxx>
Sent: Wednesday, May 12, 2004 9:13 PM
Subject: [linrad] Calibrating analog hardware


> What have people found effective as a signal source when doing
> Linrad's procedure for calibration of analog hardware?  In
> particular, has a tuned amplifier for the RF (or IF) frequency in
> question been found necessary to get enough S/N?
>
> I have used a pulse generator that produces pulses about 25 ns
> wide -- so they have significant power up to 40 MHz or so -- but
> at a PRF of order 100 Hz the S/N in the calibration procedure is
> marginal.
> -- 73, Joe, K1JT
>




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