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January 12th 06, 07:00 PM posted to rec.radio.shortwave

In article 43c63d58.6926500@news-server,
Michael Thorpe wrote:
On Wed, 11 Jan 2006 21:23:09 GMT, "Dale Parfitt"
wrote:

"Michael Thorpe" wrote in message
news:43c4e407.4277000@news-server...
Amongst all these political propaganda posts, I wonder if there is a
little room for a technical question related to radio please?

If so, here goes:

How does one measure the IF filter shape factor in a shortwave radio?

I am technically minded and do have a signal generator, etc., but I am
not sure of the exact procedure. Do I get the audio level at the peak
frequency, then detune both sides off the peak for -3dB of the
demodulated audio, note down the width between these points, then
detune to obtain for -50dB of the demodulated audio, get the width at
these points, and then the ratio of the two widths is the filter shape
factor?

I would appreciate a link to the exact procedure if there is one
(don't seem to be able to find this in Google) or an expert advice
please.

I would also appreciate advice as to what actual values of filter
shape are considered poor, good, excellent, etc...

Although I have never given it much thought, I don't believe one can fully
characterize a filter while it is installed in the receiver- certainly,
parameters such as insertion loss would not be easily measurable. Perhaps
programs like Spectran may give some rough insight.
My method is classic- the filter is removed, properly terminated and
connected as the DUT on a scalar network analyzer.
If you have a vector network analyzer available you could also observe group
delay- an important factor in narrow filters.
See Paul Kiciak's (N2PK) site for his clever VNA- ideally suited for
filters.

Thanks, Dale, for your input. I would have no problems measuring
filters when taken out of a receiver. But this is not always possible,
for example with modern DSP software-defined receivers. There you have
no choice, the filter is no longer just a component you can desolder,
so you do need to measure the entire receiver. Parameters such as
insertion loss is immaterial in such cases - all that matters is the
filter shape.

But when the whole system is put together, it's no longer "filter
shape factor", it's some other parameter, "adjacent signal rejection"
or something like that.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

January 12th 06, 07:24 PM posted to rec.radio.shortwave

January 12th 06, 07:39 PM posted to rec.radio.shortwave

January 12th 06, 08:00 PM posted to rec.radio.shortwave

The distortion analyzer has a tracking bandpass filter. I'm using the
HP8903E, but for this purpose, even the 334B would do the trick. You
can use the analyzer to display the signal level of the sine wave
inpedentent of noise to the degree the bandpass filter can remove it.
[The 8903E reads amplitude, distortion, and frequency.]

I never reached the -60db point, because the signal was lost in the
noise. But you have to ask yourself, if the signal is in the noise, you
do care about the rejection?

In the dark ages, I was designing switched capacitor filters for modem
products. When you sold the chip as a Bell standard filter, you had to
implement that exact filter. However, the reality was the noise floor
and THD totally negated the effectiveness of the filter. [Power supply
rejection is a factor too.] So yeah, I could attenuate the alternate
(out of band) channel, but the filter noise was more significant than
what I was attenuating. Once the product became integrated, nobody
could see the filter implementation. The amount of filtering was
reduced to a level where the alternate channel (out of band) signal
level was minimum by picking a compromise between the number of poles
of filtering and the noise level of the amplifiers. Less poles of
filtering meant fewer all passes were required.

Now I'm not happy the noise floor of the radio crapped out at around
-45db. I don't know if measuring at the line outputs would have been
better. I need to do the measurement again.

It is quite possible that you can use your sound card to measure the
amplitude of the signal independent of the noise. BTW, I should have
mentioned I turned on the 1khz modulation on the RF signal generator.

My goal was to test a crystal filter I picked up at the flea market and
installed in my radio. [A good deal at $10.] One thing I learned is the
coupling capacitors make a difference in terms of the signal level. I'd
have to open the radio to see what I used, but I recall if the caps
were too small, the signal level was attenuated.

Michael Thorpe wrote:
On 11 Jan 2006 14:14:01 -0800, wrote:

http://www.lazygranch.com/images/radio/cwfilter.gif
I did this test as follows:
1) radio in AM
2) AGC turned off
3) RF generator connector to antenna input
4) distortion analyzer connected to earphone output

Pick a frequency. I used 10Mhz. Set the RF generator and radio to that
frequency. Leave the radio dial alone. Sweep the RF generator manually
and measure the audio level from the distortion analyser. Plot the
results. This was a crystal filter, so you will need a RF generator
with 1Hz steps.

This is an in-situ measurement, i.e. this is not the same as just
testing the IF filter by itself. You need the distortion analyzer to
get the audio signal level independent from the noise.

Many thanks, this is interesting. What I am still unsure about is how
you determine the -6dB and -60dB points using the distortion analyzer:
When you say "measure the audio level", what exactly do you measure,
the audio signal fundamental (i.e. filtered at the modulation
frequency)?

So in fact you are only using the distortion analyzer to filter the
audio and to measure the level? I don't have a distortion analyzer,
but I guess I could simply use an oscilloscope or an audio spectrum
analyzer (PC sound card based).

Michael

January 12th 06, 08:02 PM posted to rec.radio.shortwave

How exactly does one see 60db down?

Brian Denley wrote:
Michael Thorpe wrote:
Amongst all these political propaganda posts, I wonder if there is a
little room for a technical question related to radio please?

If so, here goes:

How does one measure the IF filter shape factor in a shortwave radio?

I am technically minded and do have a signal generator, etc., but I am
not sure of the exact procedure. Do I get the audio level at the peak
frequency, then detune both sides off the peak for -3dB of the
demodulated audio, note down the width between these points, then
detune to obtain for -50dB of the demodulated audio, get the width at
these points, and then the ratio of the two widths is the filter shape
factor?

I would appreciate a link to the exact procedure if there is one
(don't seem to be able to find this in Google) or an expert advice
please.

I would also appreciate advice as to what actual values of filter
shape are considered poor, good, excellent, etc...

Thanks in advance,
Michael

Michael:
If you don't have a spectrum analyzer, you can use an oscope (setup for
x-y) while driving the filter (and x ) with a sweep function generator.
Remember that if you look at the IF output prior to the detector, you will
get a curve going positive and negative. If you look at the response after
the detector, it will look more like the IF filter shape that you expect.

--
Brian Denley
http://home.comcast.net/~b.denley/index.html

January 12th 06, 10:00 PM posted to rec.radio.shortwave

wrote:
clifto wrote:
David wrote:
On 11 Jan 2006 21:06:53 -0800, wrote:
Read Rich Lowrie's latest editorial at National Review Online.

He says the Abramoff scandal is 100% Republicans.

Then he's stupid, or ignorant, or lying. 40 out of 45 Senate Democrats got
money from Abramoff.

There is a difference between getting money from tribes, and getting
money from Abramoff.

You should switch from kool-aid to beer.

It took me all of 120 seconds to find this in the liberal Chicago Tribune:

http://www.chicagotribune.com/news/local/southsouthwest/chi-0601110210jan11,1,5780990.story?coll=chi-newslocalssouthwest-hed

"Kennedy on Tuesday also explained why Weller used $1,640 of his own money
to reimburse a lobbying firm where Abramoff once worked for a 1999 trip
to two Indian reservations in Louisiana and Mississippi.

"He visited those reservations at the request of former U.S. Rep. David
Funderburk (R-N.C.), and THOUGHT THE TRIBES HAD PAID FOR THE TRIP [emphasis
by clifto], Kennedy said. As soon as he confirmed otherwise, he repaid
the money from his own pocket, he said."

Here's one from liberal rag UnionLeader.com:

http://www.theunionleader.com/article.aspx?headline=Sullivan+defends+Democrats+k eeping+lobbyist%E2%80%99s+money&articleId=c0963d8c-692b-47e6-b316-f42c8053aeef

"Top Democrats received donations from scandal-plagued Washington
lobbyist Jack Abramoff?s Indian tribe clients for legitimate reasons
and are right to refuse to give up the money, the state?s Democratic
chairman says."

I bet if I took the time to search further, which I'm not going to bother
doing, I could find *tribal* money going to Democrats via Abramoff in mass
quantities.

Note that unlike most of the Republicans involved, Kennedy and Reid have
no intention of giving the money back or giving it to charity. They're
keeping it.

--
If John McCain gets the 2008 Republican Presidential nomination,
my vote for President will be a write-in for Jiang Zemin.

January 13th 06, 01:36 AM posted to rec.radio.shortwave

On Thu, 12 Jan 2006 16:00:51 -0600, clifto wrote:

wrote:
clifto wrote:
David wrote:
On 11 Jan 2006 21:06:53 -0800, wrote:
Read Rich Lowrie's latest editorial at National Review Online.

He says the Abramoff scandal is 100% Republicans.

Then he's stupid, or ignorant, or lying. 40 out of 45 Senate Democrats got
money from Abramoff.

There is a difference between getting money from tribes, and getting
money from Abramoff.

You should switch from kool-aid to beer.

It took me all of 120 seconds to find this in the liberal Chicago Tribune:

http://www.chicagotribune.com/news/local/southsouthwest/chi-0601110210jan11,1,5780990.story?coll=chi-newslocalssouthwest-hed

But the Indians didn't work for Abramoff.

January 13th 06, 03:58 AM posted to rec.radio.shortwave

wrote:
How exactly does one see 60db down?

Very carefully!

--
Brian Denley
http://home.comcast.net/~b.denley/index.html

January 13th 06, 04:47 AM posted to rec.radio.shortwave

In article 43c63f8a.7487750@news-server,
(Michael Thorpe) wrote:

On 11 Jan 2006 14:14:01 -0800, wrote:

http://www.lazygranch.com/images/radio/cwfilter.gif
I did this test as follows:
1) radio in AM
2) AGC turned off
3) RF generator connector to antenna input
4) distortion analyzer connected to earphone output

Pick a frequency. I used 10Mhz. Set the RF generator and radio to that
frequency. Leave the radio dial alone. Sweep the RF generator manually
and measure the audio level from the distortion analyser. Plot the
results. This was a crystal filter, so you will need a RF generator
with 1Hz steps.

This is an in-situ measurement, i.e. this is not the same as just
testing the IF filter by itself. You need the distortion analyzer to
get the audio signal level independent from the noise.

Many thanks, this is interesting. What I am still unsure about is how
you determine the -6dB and -60dB points using the distortion analyzer:
When you say "measure the audio level", what exactly do you measure,
the audio signal fundamental (i.e. filtered at the modulation
frequency)?

So in fact you are only using the distortion analyzer to filter the
audio and to measure the level? I don't have a distortion analyzer,
but I guess I could simply use an oscilloscope or an audio spectrum
analyzer (PC sound card based).

rkhalona gave you the right definition of shape factor and a web page
that explains it.

BW(-60dB)
SF =---------------
BW(-6dB)

http://www.radio-electronics.com/inf...ty/selectivity.
php

The audio passband is often but not always defined at the IF stage with
a IF passband filter. The -6dB and -60dB is referenced to the filter
center IF frequency of 0dB. The filter is usually symmetrical and the SF
number refers to how fast the slope changes. Steeper is better. The -6dB
down point is usually considered the bandwidth of the filter.

With what you have the easiest thing to do would be to use the PC audio
spectrum analyzer. On a strong signal playing music use a max hold
function on the analyzer software and the passband of the filter should
become apparent. Look at where the high end response rolls off 6dB down
relative to what looks like a flat audio spectrum from low to middle
audio frequencies. Where the high frequencies roll of 6dB down from the
flat audio response range will be 1/2 the filter bandwidth.

For this to work the station you have are receiving must be have a wider
modulation bandwidth then you are using for an audio bandwidth filter so
this may only work for narrow filters.

If this does not work well enough then you will need an RF generator
that you can modulate at a frequency higher than the largest filter you
want to measure and use the PC analyzer as described above.

--
Telamon
Ventura, California

January 13th 06, 05:10 AM posted to rec.radio.shortwave

In article ,
"Brian Denley" wrote:

wrote:
How exactly does one see 60db down?

Very carefully!

Things start getting pretty dark after 40dB down so be very careful!

--
Telamon
Ventura, California

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