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

"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 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

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.

"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