kind of crystal used in PLL ---- overtone or fundamental !!!

[abhijitrc]

Hi guys,

In PLL based synthesizers what kind of crystal generally we should use !!!--- fundamental or overtone ?? Which one is better for Phase noise term! Generally i have seen 10 MHz or 20 MHz crystals people are using can i use 88MHz overtone crystal in place of them?? what will be the advantage or disadvantages!!!

 

[jiripolivka]

It does not matter. The reference signal must be clean and stable for a PLO to operate. Use a well filtered power supply and avoid mechanical vibration for a clean PN spectrum.

 

[hosein_rabiee_g]

Hi.
nowadays Xtal freq doesn't make difference in phase noise. but sc-cut crystals have lower PN than others.

 

[abhijitrc]

In our receiver design we have synthesizer based LO and that is further multiplied by an active multiplier circuit to obtain the LO frequency at ku-band, to make the receiver compact we are planning to use a VCO that can operate at ku-band, my following doubts as well as query are as:

1. Whether a VCO operating at ku/K-band is available??

2. As VCO frequency increases, is it difficult in achieving good pulling and pushing??

3. In order to obtain good pulling and pushing a buffer is incorporated which requires high current, due to high current requirement for buffer, phase noise decreases due to increases sensitivity to power supply noise

4. How much Phase noise degrades ?

5. Which architecture is good?? Direct synthesizer or a synthesizer with an active multiplier???

6. For FM receiver how much Phase noise is a critical parameter??

Based on this fact whether a synthesizer operating with a VCO at ku-band is a good choice??

 

[hosein_rabiee_g]

For k/ku band vco, see hittite. direct synthesizers may have phase noise as good as multiplier. it depends on design.
pushing, pulling, phase noise and other parameters ratios are relative to freq and may be the percentage of them is equal.

 

[jiripolivka]

For k/ku band vco, see hittite. direct synthesizers may have phase noise as good as multiplier. it depends on design.
pushing, pulling, phase noise and other parameters ratios are relative to freq and may be the percentage of them is equal.

To generate stable LO signal at Ku-band and higher, there are several options:

-Use a lower-frequency VCO to synchronize it at a harmonic of your reference frequency, then use a multiplier to get to Ku-band. This is called PLO with multiplier

- Use a VCO directly at Ku-band (Hittite is one good choice, there are others, too). To synchronize it to your reference frequency you may need a frequency divider. Depends on PL loop. This is a "direct PLO"./

- If you need tunable LO source at Ku-band, you can select a synthesizer, again with your reference frequency. Synthesizers are like the above but frequency dividing and multiplying ratio is contollable. Best source for such synthesizers is "Luff Research" in the U.S.

Concerning the phase noise, the general rule is :

PN out = PN ref + 20 log{fout/fref} +3 dB

This is the scaling formula for PN spectrum closer that ~100 kHz from carrier. Synthesizers often have PN "wings", above 100 kHz from carrier their PN grows again. YIG tunable filters help in some cases.

First of all, find the best reference source as it determines the PN.

- - - Updated - - -

To add to your points 5 and 6: There are many concepts or architectures known, some good improvements appeear just now.
PN directly affects FM, mainly narrow-band. At Ku-band you are safe, also you can adjust the preemphasis and deemphasis as needed. Common FM like radio and TV are barely affected for a general customer.
Some improvement is achieved by QPSK and forward error correction in HiFi satellite links.