Question about phase difference and free oscillate frequency in PLL

[Borber]

Re: question about PLL

Pin 10 on MC1648 must have capacitor to ground, you have not it on last schematics.
If I understand, when you apply 2V DC to varicaps the output frequency is varying for 7MHz. If so varicap voltage varies for about 20mV. The reason can be noise on power supply or simply hum voltage induced on varicaps. Try to applay DC voltage trough 1uH inductor instead of 100kohm resistor.

 

[asia]

Re: question about PLL

when the voltage to varactor is 1V ,the output frequency is 117MHz, the amplitude is 1.5V ,but when add 2.5V voltage to the varactor, the amplitude is 550mV(frequency is needed 155MHz), why it is so small? I think it cannot drive the nex stage circuit, is it ?

 

[eejli]

question about PLL

the gain is about 25mhz/v,could be normal. 550mv amplitude is hard to drive an mixer.

 

[throwaway18]

Re: question about PLL

I use the battery and orther DC source do the DC voltage

As Borber has pointed out the DC bias on the varactor must be high impedance at high frequencys.

 

[asia]

Re: question about PLL

ver strangely,yesterday all the things are normal, but today,when I add voltage to the PLL,the output of MC1648 and the OSCin,OSCout of MC145152 are all only be about 30mv to 40mV, I check the circuit carefully,the voltage add to the MC**** is 5V,and I cannot find the reason why?
Thanks