Measuring filter shape factor
 

On Fri, 13 Jan 2006 04:47:07 GMT, Telamon
wrote:

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

Yep, that's a very good method, thanks! To refine this, I might use a
noise generator modulating a signal generator (instead of using a
station music). This should show the passband nicer and more steady, I
think.

I do have a signal generator, and I guess I could make a noise
generator using a Zener diode followed by an amplifier. Let's see...

Thanks again.

Michael

Measuring filter shape factor
 

On 12 Jan 2006 12:00:25 -0800, wrote:

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?

Well I agree, but the point it is that I would like to be able to
compare my results to the actual manufactures' specifications of the
shape factor for various receivers. So if everyone defines the shape
factor as BW(-6dB)/BW(-60dB), then somehow I need to be able to
measure this. But I think I now have enough input from this group to
be able to do this. Thank you very much for your help.

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

Measuring filter shape factor
 

On Fri, 13 Jan 2006 05:10:44 GMT, Telamon
wrote:

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

Many thanks to everyone who invested their time and effort to help me
with my filter shape measurement problem. Your advice has been very
valuable to me and I am now back on track as a result. I will report
what I'll find.

Michael

Measuring filter shape factor
 

I'm not sure why the XY display, but seeing 40db is about the limit.
You really need test equipment to get any meaningful results. Network
analysers that cover a few MHz aren't that expensive. You can also just
use a RF generator and power meter, which is a cheaper solution since
you don't need phase measurements to see attenuation.


Telamon wrote:
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

Measuring filter shape factor
 

wrote:
I'm not sure why the XY display, but seeing 40db is about the limit.
You really need test equipment to get any meaningful results. Network
analysers that cover a few MHz aren't that expensive. You can also
just use a RF generator and power meter, which is a cheaper solution
since you don't need phase measurements to see attenuation.



If you DON'T have a spectrum analyzer (most don't) or if you DON'T have
expensive test gear, a decent Oscope with x-y and a sweep generator can be
used as a poor man's spectrum analyzer. 50 years ago, there was no other
way to do it, in fact. If you look at, say, a Hammarlund receiver manual,
that's exactly the recommended way to adjust the IF filter response curve.
You don't have to go down to 60 db to see a bad shape factor.

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

Measuring filter shape factor
 

In article 43c76b90.3683890@news-server,
(Michael Thorpe) wrote:

On Fri, 13 Jan 2006 04:47:07 GMT, Telamon
wrote:

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

Yep, that's a very good method, thanks! To refine this, I might use a
noise generator modulating a signal generator (instead of using a
station music). This should show the passband nicer and more steady, I
think.

I do have a signal generator, and I guess I could make a noise
generator using a Zener diode followed by an amplifier. Let's see...

You are welcome. If the PC audio analyzer has a max hold function and
you modulate a an RF generator with a diode white noise source you will
have the high frequency roll off in seconds.

--
Telamon
Ventura, California

Measuring filter shape factor
 

"Once worked??" Many people have visited Germany, where Hilter once
waged world war. Thus anyone who has been to Germany is a Nazi.

You can go to opensecrets.org and trace money given directly by Jack
Abramoff. Hopefull this link will do the trick:

http://www.opensecrets.org/indivs/search.asp?NumOfThou=0&txtName=abramoff%2C+jack&tx tState=%28all+states%29&txtZip=&txtEmploy=&txtCand =&txt2006=Y&txt2004=Y&txt2002=Y&Order=N

I see Tom Delay, Ted Stevens, Elizabeth Dole, Arlen Spector, etc.


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

"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.

Measuring filter shape factor
 

Measuring filter shape factor
 

You need to show a direct connection. Getting money from Indian Tribes
is not illegal.

I gather you left your thumb up your ass rather than look at the Open
Secrets link. Figures.

clifto wrote:
wrote:
"Once worked??" Many people have visited Germany, where Hilter once
waged world war. Thus anyone who has been to Germany is a Nazi.

Yeah, exactly. I see you liberals don't like Kennedy's excuse any better than
we conservatives do.

You can go to opensecrets.org and trace money given directly by Jack
Abramoff. Hopefull this link will do the trick:

http://www.opensecrets.org/indivs/search.asp?NumOfThou=0&txtName=abramoff%2C+jack&tx tState=%28all+states%29&txtZip=&txtEmploy=&txtCand =&txt2006=Y&txt2004=Y&txt2002=Y&Order=N

I see Tom Delay, Ted Stevens, Elizabeth Dole, Arlen Spector, etc.

I already posted two liberal rags that named a lot of Democrats involved in
the scandal. You can name all the lying liberal rags that don't mention them
as many times as you want, and it won't change that 40 out of 45 Senate
Democrats received tainted money from Abramoff.

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