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Measuring filter shape factor
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 |
#2
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Measuring filter shape factor
Networksciences (original filter supplier for Drake) has a very
elaborate website on this subject. Try them first. |
#3
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Measuring filter shape factor
"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... Thanks in advance, Michael Hi Michael, 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. Dale W4OP |
#4
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Measuring filter shape factor
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... Thanks in advance, Michael Hi Michael, 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. Dale W4OP Look at the interstation atmospheric noise with a spectrum analyzer? |
#5
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Measuring filter shape factor
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. All that said, Bush should be impeached. 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 |
#7
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Measuring filter shape factor
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. All that said, Bush should be impeached. Can't you just use the random pink noise between stations, rather than a sweep generator? |
#8
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Measuring filter shape factor
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 You need to generate a swept frequency response of the IF filter Take a look at the following article on filter selectivity: http://www.radio-electronics.com/inf...electivity.php The shape factor is traditionally defined as BW(-60dB) SF =--------------- BW(-6dB) (see the third figure in the article illustrating a typical filter response and the meaning of the two bandwidths) For an ideal filter (see second figure), this bandwidth ratio would be equal to ONE (1.0). The closer the shape factor is to 1.0, the sharper the selectivity. For example, if you look at the specs for the Eton E1 on the following link (Monitoring Times review) http://www.monitoringtimes.com/html/...k-etone1xm.pdf the shape factor for the 7 KHz filter would be: SF = 12 KHz/7 KHz = 1.71 (not a very sharp filter, but not bad for a radio of this type) The other two filters have even worse shape factors. With DSP receivers, these filters can be made extremely tight and shape factors can be very close to 1.0. Hope this is helpful. Perhaps Pete can chime in with a detailed measurement procedure. RK |
#9
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Measuring filter shape factor
Sine-wave measurements are more precise. My test is very close to real
life since it is testing the entire signal path, i.e. from antenna input to audio output. I couldn't find the connector I made that taps the line output, so I used the earphone output. I have audio test gear that uses random noise with FFT analysis, in addition to a swept sine. You always get more accurate results with a swept sine. FFT analysis is only used when you need speed. As you will notice from the graphical output, the noise floor is quite high. I may do the test again BTW, you need a decent distortion analyser for David 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. All that said, Bush should be impeached. Can't you just use the random pink noise between stations, rather than a sweep generator? |
#10
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Measuring filter shape factor
At the time I did my test, I had a "hole" in my generators. The audio
gear went as high as 100k, and the RF gear went as low at 1Mhz, but the IF was 455Khz. Hence I tested the whole audio path. The impedances in the radio itself will effect the performance of the filter, so there is something to be said for in-situ measurements. Oh yeah, note that Abramoff didn't donate any money to the Democrats. rkhalona 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 You need to generate a swept frequency response of the IF filter Take a look at the following article on filter selectivity: http://www.radio-electronics.com/inf...electivity.php The shape factor is traditionally defined as BW(-60dB) SF =--------------- BW(-6dB) (see the third figure in the article illustrating a typical filter response and the meaning of the two bandwidths) For an ideal filter (see second figure), this bandwidth ratio would be equal to ONE (1.0). The closer the shape factor is to 1.0, the sharper the selectivity. For example, if you look at the specs for the Eton E1 on the following link (Monitoring Times review) http://www.monitoringtimes.com/html/...k-etone1xm.pdf the shape factor for the 7 KHz filter would be: SF = 12 KHz/7 KHz = 1.71 (not a very sharp filter, but not bad for a radio of this type) The other two filters have even worse shape factors. With DSP receivers, these filters can be made extremely tight and shape factors can be very close to 1.0. Hope this is helpful. Perhaps Pete can chime in with a detailed measurement procedure. RK |
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