Question about phase difference and free oscillate frequency in PLL

[asia]

I use MC1648,MC145152,MC12016 and OP LM358 do a 155MHz PLL
but I have a lot questions don not understand.
1. when the phase differentence is the same, then the output of LM358 is zero, and the varacto is forward basing ,it cann't oscillate, I do not know the reason
2. I do not kown the meaning of free oscillate frequency, some one told me it is the frequency that when the voltage in the negetiative of varactor is zero, but if that ,the oscillate cann't oscillationg at all, the varacto is forward basing

can smoe one explain them two me Thanks a lot
of course ,please point out the wrongs of my circuit
Thanks!!!!!!!!!!!!!!!!!

 

[Borber]

schematics pll mc145152

You have not specified Vcc. At 5V the BIAS, pin 10, has about 1.5V. If output voltage of LM358 goes below 1V the varicap diode is forward biased and stops oscillations. So output voltage must be allways higher than 2V. You will have to modify your circuit.

 

[asia]

pll con lm358

Thanks for your reply. pelase check the circuits in the word are ok? or only one?
but in the datasheet of MC1648 ,the circuit is ok in my first design
Thanks a lot

 

[Borber]

fv1043 varicap diode

Both circuits are OK generally but you must provide that output voltage from LM358 is allways higher than 2V.

 

[asia]

pll with mc1648

but in my opinion, when the phase are same, the output of LM358 is zero,am I right? I cannot understand this problem
Thanks

 

[Borber]

use varicap to pull crystal oscillator

At this PLL there is allways a error signal. It not need to be zero. You may externally set varicap voltage at 3V and adjust wanted frequency 150MHz with inductor on MC1648. When you reconnect PLL loop the varicap voltage will be close to 3V when locked.
You may modify oscillator in this way:

 

[asia]

pull crystal oscillator with varicap

thanks very much
but I do not understand you meaning, when it locks, the out volatge of LM358 is 3V,why?can you explain to me? I thinks the output of the filter formed by LM358
is 0V or 5V(all the VCC are 5V),is it right?

by the way, do you have the data about it,can give me some?Thanks!!

 

[Borber]

7mhz pll

You should read data sheet for MC145152 about φV and φR. When PLL is locked error pulses almost disappear and these outputs are practically disconnected from loop filter around LM358. Capacitors in filter are charged and they keep error voltage until oscillator frequency is changed. Practically this situation is never acheieved because there is allways a certan amount of frequency error.
Loop filter integrates φV and φR pulses and it's output is DC. How big is this DC depends on how much is oscillator detuned.

 

[asia]

Re: question about PLL

for the design of my, the external voltage is 3V too when I tune the inductance to realize the frequency that I need(for example ,155MHZ)? or other value ?
or it is not very strictly a unchargeable value?

 

[Borber]

Re: question about PLL

Output voltage swing of LM358 is 0-3.5V. Your oscillator should be set for 155MHz at 1.75V. This is half of the output voltage swing. Apply 1.75V from external source and set oscillator frequency as close as possible to 155MHz. Use diode connection as I proposed. After you reconnect PLL it should be locked.
This voltage 1.75V is not strictly necessary. It may differ but it is the best if it is in the middle of regulating voltage range.

 

[asia]

Re: question about PLL

Thanks for your answers, I understand it now
and I have another question
the output of crystal oscillator connected to the MC1415152 is changed from 10.21MHZ to 10.31MHz, and I want the 10.24MHz, is it not very well?need change a steady crystal oscillator ?

 

[Borber]

Re: question about PLL

If you measure frequency of crystal oscillator you should do it on pin 26, it is OSCOUT pin. Frequency should be close to 10.24MHz. If you experienced deviation from 10.21 to 10.31MHz then something is wrong with crystal. You can not pull crystal frequency so much. Maybe only +-1kHz from it's nominal value.

 

[asia]

Re: question about PLL

Borber:
May I ask you another question?For this PLL,we need to calcluate the Kvco=2ΠΔfvco/ΔVvco
but when I add the volatge(for example 2V) to the varactor, the output frequency of MC1648 is changing at all times, and I cannot read it rightly, a middle value is ok?or I wrong?
Thanks!!

 

[Borber]

Re: question about PLL

If you applied 2V DC externally the frequency of oscillator should be stable as 1e-5.

 

[asia]

question about PLL

but when the applied 2V DC externally the frequency of oscillator charged range is 7Mhz, it is too large, and I cannot read it correctly,do you kown the reason??

 

[Borber]

Re: question about PLL

Wondering of 7MHz is too much. Is your DC clear from noise?
What varicap diode did you used? Can you post schematics of oscillator tank circuit?
VCO sensitivity can be calculated from Cj/V curve of varicap diode.

 

[asia]

Re: question about PLL

may be the reason is that the DC noise is too much
I check it later

 

[Borber]

Re: question about PLL

Bottom varicap in last circuit has no purpouse. It is AC shortcircuited. You must place 100kohm resistor between output of LM358 and point where diode cathodes are connected together.

 

[eejli]

Re: question about PLL

free runing measing the vco is not in the pll loop, test the vco frequency when the control voltage is 0.

 

[asia]

Re: question about PLL

I am very depressed, I have designed this simple circuit for a long time and there is no any evolve. need help!!!
I use the battery and orther DC source do the DC voltage, the frequency change range is too large,about 7MHz, and I do not kown the reason ,the varactor is FV1043
waht is the reason?
Thanks!!!!!!!!!!!!!!!!!!!!!!!!