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PLL filter problem
Hello all,
I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) |
what frequency steps are you using???and what values do you have for the
loop filter?? "Damien Teney" wrote in message ... Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) |
what frequency steps are you using???and what values do you have for the
loop filter?? "Damien Teney" wrote in message ... Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) |
Damien Teney wrote:
Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) How clean is the power supply? Any noise, ripple or hum will appear in the output of the VCO. -- Michael A. Terrell Central Florida |
Damien Teney wrote:
Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) How clean is the power supply? Any noise, ripple or hum will appear in the output of the VCO. -- Michael A. Terrell Central Florida |
We need some more information.
-------------------- What is the tuning sensitivity of the VCO in MHz/volt ? What are the values for the divide by N ? What is your desired reference frequency ? =================== Even if you have calculated the loop filter correctly, there are many other ways to have a synthesizer full of undesired spurs.... For example, how well filtered is the DC voltage feeding the VCO ? Many VCOs will respond quite nicely to any sort of noise on the DC voltage and that'll show up as undesired noise on the VCO output. --------- Another question is how well buffered is the VCO output to the MC12017 pulse swallowing prescaler ?? Quite a few of the pulse swallowing prescaler chips slightly change their input impedance when going from one divide ratio to another one, and this may easily result in spurs on the VCO output signal. So, all in all, designing a quiet synthesizer can take some time and careful circuitry. Of course, one of the early things to solve is the loop filter components, and the first three questions will help in doing that calculation. I do not recall the website ofhand, but there is one that will calculate the loop filter given the proper information. Maybe one of the main manufacturer's webpages ?? I forget..... Jim Pennell N6BIU |
We need some more information.
-------------------- What is the tuning sensitivity of the VCO in MHz/volt ? What are the values for the divide by N ? What is your desired reference frequency ? =================== Even if you have calculated the loop filter correctly, there are many other ways to have a synthesizer full of undesired spurs.... For example, how well filtered is the DC voltage feeding the VCO ? Many VCOs will respond quite nicely to any sort of noise on the DC voltage and that'll show up as undesired noise on the VCO output. --------- Another question is how well buffered is the VCO output to the MC12017 pulse swallowing prescaler ?? Quite a few of the pulse swallowing prescaler chips slightly change their input impedance when going from one divide ratio to another one, and this may easily result in spurs on the VCO output signal. So, all in all, designing a quiet synthesizer can take some time and careful circuitry. Of course, one of the early things to solve is the loop filter components, and the first three questions will help in doing that calculation. I do not recall the website ofhand, but there is one that will calculate the loop filter given the proper information. Maybe one of the main manufacturer's webpages ?? I forget..... Jim Pennell N6BIU |
"Michael A. Terrell" ) writes:
Damien Teney wrote: Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) How clean is the power supply? Any noise, ripple or hum will appear in the output of the VCO. And of course, one reason three terminal regulators were invented was so small sections could have their own regulators. They are cheap enough that it's hardly worth not having a separate regulator for the VCO. And maybe even a separate regulator for the phase detector (and any amplifier) section. Michael VE2BVW |
"Michael A. Terrell" ) writes:
Damien Teney wrote: Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) How clean is the power supply? Any noise, ripple or hum will appear in the output of the VCO. And of course, one reason three terminal regulators were invented was so small sections could have their own regulators. They are cheap enough that it's hardly worth not having a separate regulator for the VCO. And maybe even a separate regulator for the phase detector (and any amplifier) section. Michael VE2BVW |
Damien Teney wrote:
Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) Some other points. Do you have a filter to remove the reference frequency from the loop. A notch filter will clean out a lot of noise. I had to straighten out and re-certify the design of a commercial synthesizer after the purchasing department bought different capacitors used in the VCO circuit, and in fixing that, I found several other problems. Does your power supply use a linear or a switching regulator? To achieve a very low phase noise we had to use a pair of low dropout linear regulators inside the shielded case. How do you bring signals in and out of the synthesizer case? feed-thru capacitors and coaxial connectors are a must. The Microdyne synthesizer I mentioned used a DB 25 to connect the power and logic inputs, and had a two rows of feed-thru capacitors behind the connector to filter all signals, including DC. Also, make a spare top cover with small holes to reach test points. Screw it down to the case, and make any adjustments. Working without a cover can change the frequency, and in some cases, take the VCO to the edge of its range where you will get a lot more noise. A big part of building a clean VCO is electronics, but a lot is mechanical, too. A solid case, good shielding, and board layout. Mount the PC board to the case with short spacers, preferably permanently attached to the case. For homebrew work you can use brass spacers and solder the them to the case after they are screwed in from the back. It will make a big difference. A clean ground system in the RF circuits is critical. We had to replace a couple cases where a PEM standoff didn't make good contact after it was pressed in, but moved the three circuit boards to a new case, and they passed on the first try. -- Michael A. Terrell Central Florida |
Damien Teney wrote:
Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) Some other points. Do you have a filter to remove the reference frequency from the loop. A notch filter will clean out a lot of noise. I had to straighten out and re-certify the design of a commercial synthesizer after the purchasing department bought different capacitors used in the VCO circuit, and in fixing that, I found several other problems. Does your power supply use a linear or a switching regulator? To achieve a very low phase noise we had to use a pair of low dropout linear regulators inside the shielded case. How do you bring signals in and out of the synthesizer case? feed-thru capacitors and coaxial connectors are a must. The Microdyne synthesizer I mentioned used a DB 25 to connect the power and logic inputs, and had a two rows of feed-thru capacitors behind the connector to filter all signals, including DC. Also, make a spare top cover with small holes to reach test points. Screw it down to the case, and make any adjustments. Working without a cover can change the frequency, and in some cases, take the VCO to the edge of its range where you will get a lot more noise. A big part of building a clean VCO is electronics, but a lot is mechanical, too. A solid case, good shielding, and board layout. Mount the PC board to the case with short spacers, preferably permanently attached to the case. For homebrew work you can use brass spacers and solder the them to the case after they are screwed in from the back. It will make a big difference. A clean ground system in the RF circuits is critical. We had to replace a couple cases where a PEM standoff didn't make good contact after it was pressed in, but moved the three circuit boards to a new case, and they passed on the first try. -- Michael A. Terrell Central Florida |
Michael Black wrote:
And of course, one reason three terminal regulators were invented was so small sections could have their own regulators. They are cheap enough that it's hardly worth not having a separate regulator for the VCO. And maybe even a separate regulator for the phase detector (and any amplifier) section. Absolutely. For example, if you need really clean 5V supplies in various places, a good technique is to run a pre-regulated "semi-clean" 12V rail around the system, and then regulate locally to 5V. Don't rely totally on the IC to clean-up the noise on the input rail. Remember that it's a feedback system, so there is always a high-frequency corner, above which the regulator can't keep up. Above that corner frequency, the regulator becomes increasingly transparent to high-frequency input noise and sharp spikes. If low output noise is important, include some good old-fashioned R-C filtering between the 12V line and the input to the regulator IC. Don't forget to leave more than the minimum required voltage at the input of the regulator... and that's not the average DC level - it's the voltage at the *bottom* of any noise spikes at the input, so you have to allow a little extra input voltage. Even so, it's still only R = V/I, like we used to do routinely for zener diodes (I'll swear that half the attraction of IC regulators is that they let people avoid that simple calculation). If you want to use two poles of R-C filtering, simply split the R into two parts. You're still faced with other sources of noise, for example noise originating in the regulator itself, and things like induced voltages and ground loops... but all those are actually separate problems. In different applications, R-C input filtering combined with a voltage clamp device is a good way to protect regulator ICs and their downstream components from the very sharp spikes induced by lightning. The pre-filtering reduces the peak amplitude of the incoming spike and also attenuates its high-frequency components, down to levels that the voltage clamp and the regulator IC can safely handle between them. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Michael Black wrote:
And of course, one reason three terminal regulators were invented was so small sections could have their own regulators. They are cheap enough that it's hardly worth not having a separate regulator for the VCO. And maybe even a separate regulator for the phase detector (and any amplifier) section. Absolutely. For example, if you need really clean 5V supplies in various places, a good technique is to run a pre-regulated "semi-clean" 12V rail around the system, and then regulate locally to 5V. Don't rely totally on the IC to clean-up the noise on the input rail. Remember that it's a feedback system, so there is always a high-frequency corner, above which the regulator can't keep up. Above that corner frequency, the regulator becomes increasingly transparent to high-frequency input noise and sharp spikes. If low output noise is important, include some good old-fashioned R-C filtering between the 12V line and the input to the regulator IC. Don't forget to leave more than the minimum required voltage at the input of the regulator... and that's not the average DC level - it's the voltage at the *bottom* of any noise spikes at the input, so you have to allow a little extra input voltage. Even so, it's still only R = V/I, like we used to do routinely for zener diodes (I'll swear that half the attraction of IC regulators is that they let people avoid that simple calculation). If you want to use two poles of R-C filtering, simply split the R into two parts. You're still faced with other sources of noise, for example noise originating in the regulator itself, and things like induced voltages and ground loops... but all those are actually separate problems. In different applications, R-C input filtering combined with a voltage clamp device is a good way to protect regulator ICs and their downstream components from the very sharp spikes induced by lightning. The pre-filtering reduces the peak amplitude of the incoming spike and also attenuates its high-frequency components, down to levels that the voltage clamp and the regulator IC can safely handle between them. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
"Michael Black" wrote in message ... "Michael A. Terrell" ) writes: Damien Teney wrote: Hello al I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) How clean is the power supply? Any noise, ripple or hum will appear in the output of the VCO. And of course, one reason three terminal regulators were invented was so small sections could have their own regulators. They are cheap enough that it's hardly worth not having a separate regulator for the VCO. And maybe even a separate regulator for the phase detector (and any amplifier) section. Michael VE2BVW In addition to proper loop filter parameters, separate supply regulation, etc., a VCO needs to be placed and usually shielded to keep outside noise from perturbing the control voltage ... you could be getting stray pickup of RF, digital noise, etc. onto the control voltage ... Carl - wk3c |
"Michael Black" wrote in message ... "Michael A. Terrell" ) writes: Damien Teney wrote: Hello al I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) How clean is the power supply? Any noise, ripple or hum will appear in the output of the VCO. And of course, one reason three terminal regulators were invented was so small sections could have their own regulators. They are cheap enough that it's hardly worth not having a separate regulator for the VCO. And maybe even a separate regulator for the phase detector (and any amplifier) section. Michael VE2BVW In addition to proper loop filter parameters, separate supply regulation, etc., a VCO needs to be placed and usually shielded to keep outside noise from perturbing the control voltage ... you could be getting stray pickup of RF, digital noise, etc. onto the control voltage ... Carl - wk3c |
Yes but the VCO, if I control it manually (with a potmeter instead of the
PLL), it is quite stable (enough for me ;-). So the main problem I have now would not be solved with a shield, I think. |
Yes but the VCO, if I control it manually (with a potmeter instead of the
PLL), it is quite stable (enough for me ;-). So the main problem I have now would not be solved with a shield, I think. |
and your filter values Damien?
i use those pll chips myself in various tx's..6.25khz to 100khz steps and have no problems so will dig out the values for 6.25khz for you tonight to compare.(using LF351/(6) (i think) as filter. also pll built on double sided pcb (1w exciter.) no seperate screening apart from vco coil,vco regulated with 78L08,( which also supplies the 145152) noise is not a problem.sounds like as you suspect really bad vco overshoot. "Jim Pennell" wrote in message nk.net... This link is to a webpage which shows quite a few PLL design program links. I am not sure which will best fit your application, but it is a place to start. Actually, many of them may be able to help sort things out. http://www.circuitsage.com/pll.html Jim Pennell N6BIU |
and your filter values Damien?
i use those pll chips myself in various tx's..6.25khz to 100khz steps and have no problems so will dig out the values for 6.25khz for you tonight to compare.(using LF351/(6) (i think) as filter. also pll built on double sided pcb (1w exciter.) no seperate screening apart from vco coil,vco regulated with 78L08,( which also supplies the 145152) noise is not a problem.sounds like as you suspect really bad vco overshoot. "Jim Pennell" wrote in message nk.net... This link is to a webpage which shows quite a few PLL design program links. I am not sure which will best fit your application, but it is a place to start. Actually, many of them may be able to help sort things out. http://www.circuitsage.com/pll.html Jim Pennell N6BIU |
Okay thank you. I don't know well what values you want; I took what was
advised in the motorola application note loop damping factor = 0,707 VCO gain = 25 132 741 rad/V VCO center frequency = 77 MHz PLL supply voltage = 5V etc. The values of the components of the filter are given in a previous post. "zindazenda" a écrit dans le message de news: ... and your filter values Damien? i use those pll chips myself in various tx's..6.25khz to 100khz steps and have no problems so will dig out the values for 6.25khz for you tonight to compare.(using LF351/(6) (i think) as filter. also pll built on double sided pcb (1w exciter.) no seperate screening apart from vco coil,vco regulated with 78L08,( which also supplies the 145152) noise is not a problem.sounds like as you suspect really bad vco overshoot. |
Okay thank you. I don't know well what values you want; I took what was
advised in the motorola application note loop damping factor = 0,707 VCO gain = 25 132 741 rad/V VCO center frequency = 77 MHz PLL supply voltage = 5V etc. The values of the components of the filter are given in a previous post. "zindazenda" a écrit dans le message de news: ... and your filter values Damien? i use those pll chips myself in various tx's..6.25khz to 100khz steps and have no problems so will dig out the values for 6.25khz for you tonight to compare.(using LF351/(6) (i think) as filter. also pll built on double sided pcb (1w exciter.) no seperate screening apart from vco coil,vco regulated with 78L08,( which also supplies the 145152) noise is not a problem.sounds like as you suspect really bad vco overshoot. |
Woohoo ! I finally found, I had used 92 ohms resistors instead of 92 Kohms
resistors... how inattentive I am sometimes ;-) But it isn't yet perfect, I've took a picture of the oscilloscope, showing the output signal that is fed into the VCO (http://www.mcarsweb.com/_divers/oscillo.jpg the vertical scale is enlarged). It is slightly varying, at a frequency of about 187 KHz. I don't know if I should change the filter values, or add a additional small filter, with a resistor and a capacitor between the output and the ground. What do you think about that ? Damien |
Woohoo ! I finally found, I had used 92 ohms resistors instead of 92 Kohms
resistors... how inattentive I am sometimes ;-) But it isn't yet perfect, I've took a picture of the oscilloscope, showing the output signal that is fed into the VCO (http://www.mcarsweb.com/_divers/oscillo.jpg the vertical scale is enlarged). It is slightly varying, at a frequency of about 187 KHz. I don't know if I should change the filter values, or add a additional small filter, with a resistor and a capacitor between the output and the ground. What do you think about that ? Damien |
Well, you're not locked. Did you measure the gain of theVCO? In other words,
did you disconnect the tuning diode from the PLL and then plot the VCO's output frequency versus tuning voltage? You need this data to accurately calculate the loop components. Another item to look for is the Vcc supplu rails to the PLL and VCO. These need to be clean. Did you do a good job of de-coupling the supply to the PLL? Regards, KD0QO "Damien Teney" wrote in message ... Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) |
Well, you're not locked. Did you measure the gain of theVCO? In other words,
did you disconnect the tuning diode from the PLL and then plot the VCO's output frequency versus tuning voltage? You need this data to accurately calculate the loop components. Another item to look for is the Vcc supplu rails to the PLL and VCO. These need to be clean. Did you do a good job of de-coupling the supply to the PLL? Regards, KD0QO "Damien Teney" wrote in message ... Hello all, I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately it doesn't work very well: the VCO output is on average equal the same as the frequency programmed in the PLL, but it is constantly varying a bit around it. I guess the problem comes from the output filter, but I've re-calculated it and it should be okay. Where could the problem come from ?? FYI, the PLL is build from a schematic of the Motorola application note 980 (http://www.mcarsweb.com/_divers/sche...cation-note_98 0.gif). Thank you in advance for your answers ;-) |
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