Hi all,

I'm Paolo from Italy, this is my 1st post. I'm trying to align the IF
filters of the R&S EK-07. From a general standpoint, I would like to
understand what the recommended procedure should be. I just followed
the
manual:

- a sweep at the antenna input, centered at the tuning freq. I
generate
this sweep from the combination 141T/8443A
- the 300kHz IF output and the scan ramp output of the sweep at the
scope in XY mode
- auto AGC

The filter has six fixed positions, the narrowest is 300Hz (crystal
filter) and the widest is 12kHz. The - only - tube is biased by AGC.
I
observe large modifications of the symmetries depending on the
combination sweep rate/AGC time constant: if there's enough time for
the
gain to get stable, I get more symmetric curves. There are three
positions for the time constant: 0.1, 1 and 10 seconds. Attack and
decay
times are unknown. I'm missing something: what setting should I choose
to perform the alignment?

Thank you and best regards
Paolo

paolo67 wrote:
gain to get stable, I get more symmetric curves. There are three
positions for the time constant: 0.1, 1 and 10 seconds. Attack and
decay
times are unknown. I'm missing something: what setting should I choose
to perform the alignment?

Turn off the AGC. Adjust the gain so that it doesn't clip. Don't use
AGC during the alignment procedure.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Tue, 13 Jan 2009 09:04:00 -0800, paolo67 wrote:

Hi all,

I'm Paolo from Italy, this is my 1st post. I'm trying to align the IF
filters of the R&S EK-07. From a general standpoint, I would like to
understand what the recommended procedure should be. I just followed the
manual:

- a sweep at the antenna input, centered at the tuning freq. I generate
this sweep from the combination 141T/8443A - the 300kHz IF output and
the scan ramp output of the sweep at the scope in XY mode
- auto AGC

The filter has six fixed positions, the narrowest is 300Hz (crystal
filter) and the widest is 12kHz. The - only - tube is biased by AGC. I
observe large modifications of the symmetries depending on the
combination sweep rate/AGC time constant: if there's enough time for the
gain to get stable, I get more symmetric curves. There are three
positions for the time constant: 0.1, 1 and 10 seconds. Attack and decay
times are unknown. I'm missing something: what setting should I choose
to perform the alignment?

Thank you and best regards
Paolo

Where are you picking off the signal strength from the IF? The really
"right" way to do this is to measure the IF signal strength with the AGC
_off_ and the sweep rate slow enough for the IF filters to settle (which
you can check by lowering the sweep rate until things stop changing). If
the radio doesn't have a diode (like the AGC detector or the AM detector)
that you can use for measuring signal strength, make up something
yourself.

I could see someone taking a short cut and having you measure the signal
strength by monitoring the AGC voltage. I certainly don't recommend
this, but if that's what the radio manufacturer wants to do you may
possibly want to do it before going back and doing it right. In this
case, before you go back and re do it right, you'd want the AGC set to as
fast as it would go, and sweep slowly enough so the AGC settles. And
redo the measurement right (did I say you should redo the measurement
right?).

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Thank you for your answers.

OK so AGC must be off. What made me confused is the fact that the
manufacturer explicitly indicates that this alignment must be done
with AGC auto.
With AGC off, the gain can be increased up to a point where the curve
gets clipped and wider, this is the point then. A scan width of 1sec
seems to be the limit for an optimal representation.

The receiver has a proper IF output, it is just before the AM detector
diode but not immediately after the filter, in between there is a
three stages, fixed bandwidth, high gain IF amplifier.

With AGC off the output level now varies depending on which IF filter
is selected: the manual says that the input level should be regulated
for 0dB at IF output, but this is assuming AGC on. Although I always
try for maximum gain, each filter seems to produce its own maximum
output level, and differences are as large as 15dB+ with no obvious
logic, so no the wider the filter the higher the level. Does this make
sense? How can I detect which is the "reference" level to look for or,
alternatively, what is an "acceptable" difference among the filters?

Thank you and best regards
Paolo

On Tue, 13 Jan 2009 12:49:08 -0800, paolo67 wrote:

Thank you for your answers.

OK so AGC must be off. What made me confused is the fact that the
manufacturer explicitly indicates that this alignment must be done with
AGC auto.
With AGC off, the gain can be increased up to a point where the curve
gets clipped and wider, this is the point then. A scan width of 1sec
seems to be the limit for an optimal representation.

The receiver has a proper IF output, it is just before the AM detector
diode but not immediately after the filter, in between there is a three
stages, fixed bandwidth, high gain IF amplifier.

With AGC off the output level now varies depending on which IF filter is
selected: the manual says that the input level should be regulated for
0dB at IF output, but this is assuming AGC on. Although I always try for
maximum gain, each filter seems to produce its own maximum output level,
and differences are as large as 15dB+ with no obvious logic, so no the
wider the filter the higher the level. Does this make sense? How can I
detect which is the "reference" level to look for or, alternatively,
what is an "acceptable" difference among the filters?

Thank you and best regards
Paolo

There are a lot of factors that go into designing a filter, and for an IF
filter the insertion loss is one of the easiest to overcome -- so I'm not
surprised that they should vary. An acceptable level of attenuation
through an IF filter is anything that gives you adequate signal on the
other side, so you could have a filter that absorbs quite a bit and is
still OK to use.

If by a "1 second scan width" you mean that the total scan time is one
second, then you could _maybe_ get away with using a 10-second AGC. _I_
don't like it, but the manual is aimed at busy techs, so maybe it makes
sense...

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Actually there is a row in the manual that says that after the
alignment the gain of the filter stage must be 30dB consistently on
all the positions, 40dB with zero bias. It is written in German and I
missed that row before. I will try to figure out what this means in
terms of IF output level.

1sec comes from 0.1sec per division by ten divisions. If I'm getting
it right, I should try to keep the gain as constant as possible for
the purpose of the alignment, this is the key point I missed before.

The fact is that, at the moment, with either AGC off or AGC on, the
filters are quite far from spec, sometimes the center freq is off,
sometimes the top is not flat and/or the width is twice or three times
than expected. There are ten trimmers per filter, sixty total.

Tomorrow, I will record the current conditions and try to make some
progress.

Thank you again and best regards
Paolo

"paolo67" wrote in message
...
Actually there is a row in the manual that says that after the
alignment the gain of the filter stage must be 30dB consistently on
all the positions, 40dB with zero bias. It is written in German and I
missed that row before. I will try to figure out what this means in
terms of IF output level.

1sec comes from 0.1sec per division by ten divisions. If I'm getting
it right, I should try to keep the gain as constant as possible for
the purpose of the alignment, this is the key point I missed before.

The fact is that, at the moment, with either AGC off or AGC on, the
filters are quite far from spec, sometimes the center freq is off,
sometimes the top is not flat and/or the width is twice or three times
than expected. There are ten trimmers per filter, sixty total.

Tomorrow, I will record the current conditions and try to make some
progress.

Wow, sounds like trying to do an old tube type TV IF alignment.
One can't just start tweaking each trimmer for maximum smoke,
of course. You've got to decide just which skirt of the filter's bandpass
shape each trimmer is supposed to be working on. If the manufacturer
doesn't tell exactly where each trimmer is supposed to be working,
it might be well to make a little chart of where you think each trimmer
should be working, and keep good notes of where you are going.
If someone hasn't already "aligned" it by "ear", you can usually
detect where each trimmer is working by twiddling it just a very little,
back and forth, while carefully watching the shape on the scope.
They normally will not drift off so far by themselves that you can't see
and watch just what you are doing, and where each trimmer was working.
Just keep cool, and don't make any giant moves until you can see
where you are going with each step along the way.
Also, keep in mind that the manufacturer's filter bandpass specs
were often pretty optimistic!
Old Chief Lynn, W7LTQ

"Lynn" wrote in message
...

"paolo67" wrote in message
...
Actually there is a row in the manual that says that after the
alignment the gain of the filter stage must be 30dB consistently on
all the positions, 40dB with zero bias. It is written in German and I
missed that row before. I will try to figure out what this means in
terms of IF output level.

1sec comes from 0.1sec per division by ten divisions. If I'm getting
it right, I should try to keep the gain as constant as possible for
the purpose of the alignment, this is the key point I missed before.

The fact is that, at the moment, with either AGC off or AGC on, the
filters are quite far from spec, sometimes the center freq is off,
sometimes the top is not flat and/or the width is twice or three times
than expected. There are ten trimmers per filter, sixty total.

Tomorrow, I will record the current conditions and try to make some
progress.

Wow, sounds like trying to do an old tube type TV IF alignment.
One can't just start tweaking each trimmer for maximum smoke,
of course. You've got to decide just which skirt of the filter's bandpass
shape each trimmer is supposed to be working on. If the manufacturer
doesn't tell exactly where each trimmer is supposed to be working,
it might be well to make a little chart of where you think each trimmer
should be working, and keep good notes of where you are going.
If someone hasn't already "aligned" it by "ear", you can usually
detect where each trimmer is working by twiddling it just a very little,
back and forth, while carefully watching the shape on the scope.
They normally will not drift off so far by themselves that you can't see
and watch just what you are doing, and where each trimmer was working.
Just keep cool, and don't make any giant moves until you can see
where you are going with each step along the way.
Also, keep in mind that the manufacturer's filter bandpass specs
were often pretty optimistic!
Old Chief Lynn, W7LTQ

Typically, Low IF shouldn't be aligned unless someone tweaked it or you had
to fix something. First sweep the Crystal filters and find the center of
those before going on to the tunable filter. These are not always perfect
or centered and you may find want to adjust the LO or tunable filters
slightly off to compensate. I like the idea of rocking an adjustment to see
what happens, as this can reveal where the problem is without upsetting the
whole alignment.

This receiver has a tube based PLL for the LO. This was completely off
on all bands due to the fact that all the cores of the Ls loosened,
they were kept firm with some sort of glue across the thread that
pulverized over the years. The vibrations of the drum did the rest.
After the alignment, it is now rock stable, better than the 8640!
However, I did not consider the IF filters when I did this, I tuned
the LO for *exactly* 3.3MHz IF that then become 300kHz after another
mixing with a fixed 3MHz quartz oscillator. It is not a good idea to
tweak this oscillator too, as it also generates the harmonics used by
the PLL. This is actually 3 to 5 Hertz off.

I am *very* worried of upsetting the filters. I like the idea of
identifying which trimmer changes which part of the curve before
attempting the real job and preparing a reference chart. I am
convinced that some previous owner tried the alignment before and not
necessarily did a terrible job. The manual just has charts with the
expected shapes of all six, the number of peaks, at which freq they
should be, the maximum ripple. The representation on the scope is
linear, I suspect it will not be accurate enough. Today I tried with
the digital scope, set the persistence to 10sec so that I can see a
curve produced by ten sweeps, this seems to be an improvement. Also,
-3dB=0.707x linear, the digital scope has cursors that also help.

Another thing I wanted to ask and you already mentioned: which filter
to start with? The narrowest, this one is easier to center at exactly
the right freq. You make a difference between "crystal" and "tunable",
however this thing must be quite odd, the crystal filter is also
tunable, has its own ten trimmers as the non-crystal types.

Thank you again for your help
Best regards
Paolo

"paolo67" wrote in message
...
This receiver has a tube based PLL for the LO. This was completely off
on all bands due to the fact that all the cores of the Ls loosened,
they were kept firm with some sort of glue across the thread that
pulverized over the years. The vibrations of the drum did the rest.
After the alignment, it is now rock stable, better than the 8640!
However, I did not consider the IF filters when I did this, I tuned
the LO for *exactly* 3.3MHz IF that then become 300kHz after another
mixing with a fixed 3MHz quartz oscillator. It is not a good idea to
tweak this oscillator too, as it also generates the harmonics used by
the PLL. This is actually 3 to 5 Hertz off.

I am *very* worried of upsetting the filters. I like the idea of
identifying which trimmer changes which part of the curve before
attempting the real job and preparing a reference chart. I am
convinced that some previous owner tried the alignment before and not
necessarily did a terrible job. The manual just has charts with the
expected shapes of all six, the number of peaks, at which freq they
should be, the maximum ripple. The representation on the scope is
linear, I suspect it will not be accurate enough. Today I tried with
the digital scope, set the persistence to 10sec so that I can see a
curve produced by ten sweeps, this seems to be an improvement. Also,
-3dB=0.707x linear, the digital scope has cursors that also help.

Another thing I wanted to ask and you already mentioned: which filter
to start with? The narrowest, this one is easier to center at exactly
the right freq. You make a difference between "crystal" and "tunable",
however this thing must be quite odd, the crystal filter is also
tunable, has its own ten trimmers as the non-crystal types.

Thank you again for your help
Best regards
Paolo

One of the biggest issues of a radio with variable selectivity and passband
tuning is figuring what is being tuned and how the IF and BFO might be
shifting and what oscillators are being shifted in and out for various
modes, then determine what filters are the least adjustable and see what
shape you can get from them. Typically the monolithic filters, be they
crystal, mechanical or ceramic can only be adjusted for input and output
matching.

PLL cores are adjusted so that the VCO can be reliably tuned across it's
range. The steering voltage is usually specified either at some frequency
or at the extremes.

Offset oscillators for various modes also need to be accounted for.

Transceivers are even more critical because you will wind up poor fidelity
and/or spurious. Loading changes can make a shift between transmit and
receive too. I can't figure out how the High Fidelity SSB guys deal with
phase noise and carrier rejections as they approach response below 200 Hz.
Especially the ones who do a base boost to try to get fidelity out of the
cheap 500-4000 Hz speakers in mobiles, but sound like a craphouse studio on
a good quality station speaker.

3-5 Hz is pretty good but it just makes it more noticeable when the hum on
the incoming signal is out of phase with the hum modulation in the receiver