Problem with loop filter in a frequency synthesizer build using BH1415

bh1415

Hi guys,

I am trying to build a frequency synthesizer using BH1415. The VCO and Phase Detector seem to work well. But when I add the loop filter, the loop cannot be locked. The schematic is shown in the picture.



My question is:

1, Can anyone tell me is there any fatal mistake in the circuit?
2, How can I build a good filter?
3, How to simulate the performance of a PLL?

shematics diagram with bh1415

What do you experience?
Is it hunting or pegged to one extreme?
Are you sure you are programming the chip correctly and setting up the phase detector bit options right?

bjt filter

Seems that your active loop-filter has no gain in this configuration.


check this post:

active pi loop filter

beaver said:

Hi guys,
I am trying to build a frequency synthesizer using BH1415. The VCO and Phase Detector seem to work well. But when I add the loop filter, the loop cannot be locked. The schematic is shown in the picture.



My question is:

1, Can anyone tell me is there any fatal mistake in the circuit?
2, How can I build a good filter?
3, How to simulate the performance of a PLL?

Click to expand...

Your loop filter has no dc path to the VCO. Thus, the whole circuit cannot be stable !

bh1415 chip

To ALL

The circuit is driving me crazy! I've spent many days on it and it still does not work. While some guys only use 2 BJTs to build the filter and it works quite well.


To E-design

It is hunting. I am sure the chip is programmed correctly.

To vfone

My simulation is different from yours.

To LvW

The output of the filter is almost a DC voltage. Why is it necessary to have a DC path?

recomendation about filter capacitor

beaver said:

To ALL
The circuit is driving me crazy! I've spent many days on it and it still does not work. While some guys only use 2 BJTs to build the filter and it works quite well.
............
.............
To LvW

The output of the filter is almost a DC voltage. Why is it necessary to have a DC path?

Click to expand...

1.) Where have you seen a loop filter with 2 BJT´s ? It´s rather uncommon.
Normally, a passive filter is used or an active filter with one opamp or both in combination. But take care of the second pole to be outside the unity-gain-bandwidth.
My recommendation: start with a simple PI filter .

2.) A dc negative loop gain is required for ALL stable feedback systems.
What do you mean with "almost a dc voltage" ? (is it possible to be "almost pregnant?) . A capacitor (C5) cannot transfer any dc voltage ! As mentioned, the circuit is unable to work !!

Re: Loop Filter; BH1415

To Lvw:

haha.......by saying "almost a dc voltage", I mean the signal is a dc voltage with small amount of ac component.

Anyway, the filter with BJTs works well.

And you can find similar capacitors in many loop filters, thus I don't think there is any problem with the capacitor.

So, I am still confused.

Re: Loop Filter; BH1415

beaver said:

To Lvw:
haha.......by saying "almost a dc voltage", I mean the signal is a dc voltage with small amount of ac component.
Anyway, the filter with BJTs works well.
And you can find similar capacitors in many loop filters, thus I don't think there is any problem with the capacitor.
So, I am still confused.

Click to expand...

OK, when you think a capacitor can couple a dc voltage to the VCO, good luck !
(That is the Beaver-effect).
Another request: Would you be so kind and show me (or give me a reference) where I can see a "similar capacitor" between the loop filter and the VCO ? Thanks.
LvW

Re: Loop Filter; BH1415

Hello Beaver,

Just as a general comment: When confused, try to follow the advise of wiser / older people. Please try to understand what they are saying and also consider the posibility that you can be wrong.
Here, a simple way to ascertain who may be older/wiser than you is to see how many posts have a person, and how many people have him/her helped.

Best regards!

Re: Loop Filter; BH1415

I am trying to follow the advise of wiser people, such as LvW, but the shematic with a loop filter build by BJTs that I've enclosed in my previous reply confirmed me that even if there are capacitors between the loop filter and the VCO, the loop can work. Can anybody explain it to me? Thank you very much!

EDIT AFTER RETHINKING:
Also, I don't know which C5 is the capacitor LvW referred to. There is no C5 between the loop filter and the VCO. Is it C1?

Re: Loop Filter; BH1415

This should work for you

Re: Loop Filter; BH1415

beaver said:

I am trying to follow the advise of wiser people, such as LvW, but the shematic with a loop filter build by BJTs that I've enclosed in my previous reply confirmed me that even if there are capacitors between the loop filter and the VCO, the loop can work. Can anybody explain it to me? Thank you very much!

EDIT AFTER RETHINKING:
Also, I don't know which C5 is the capacitor LvW referred to. There is no C5 between the loop filter and the VCO. Is it C1?

Click to expand...

1.) What is the reason you are "confirmed that ......the loop can work" ?
2.) In your first diagram C5 is the capacitor between the filter and the VCO input.
Did you forget your first schematic ? (In your last diagram it is C17).
3.) General comment: A PLL is a rather complicated and strongly non-linear system with stability properties which require a good understanding of control theory rules.
4.) Therefore my recommendation: At first, try to understand the fundamental tasks of a capacitor and its behaviour with respect to dc and ac signals before designing PLLs.
________________
Good luck and regards

Re: Loop Filter; BH1415

Hi LvW,

Thank you for your reply.

1.) I am confirmed because I have build the circuit in a breadboard, and I saw that the output of the PLL is very stable. These days I've spent LOTS of time on the op-amp filter. I did my calculation carefully. However, it didn't work. When I saw the stable output contributed by the BJT filter, I was deeply impressed.

2.) In my understanding, the diode varactor D1, together with C4 and L1 is part of the VCO. And between the VCO and the loop filter, there is R5. If I am wrong, let me know.

3.) 4.) Thank you for your comment and recommendation.

Best Regards!

Added after 4 hours 48 minutes:

NEW EXPERIMENTAL RESULT

To all

After a comparison of the OP-AMP filter, E-design's filter, and the BJT filter, I removed the first order of the filter, i.e. R3 and C2, from the circuit (it's very similar to the one posted by E-design), and it worked!... But I don't know the reason.

To E-design

Thank you for your schematic!