# Is an external oscillator for MAX2769 GPS receiver needed?

I am designing an RF frontend for GPS using MAX2769 chip. However, due to lack of theoretical and practical background in signal processing, I am unsure how to interpret the documentation when it comes to the usage of an external oscillator.

The docs say in the Crystal Oscillator section:

The MAX2769 includes an on-chip crystal oscillator. A parallel mode crystal is required when the crystal oscillator is being used.

1. If an internal ostcillator is included, why would I need an external one?
2. Can I not use the internal crystal oscillator (to avoid needing an external one) but still achieve the desired functionality or receiving I and Q data?
3. Do I need an external oscillator to keep the internal one in check because of its lower quality (PLL)?
4. Does XTAL (pin name) stand for external?
• The datasheet wording is a bit confusing, it's be better to refer to the chip as having an on-board crystal exciter, as all it can do is get a quartz crystal resonating as opposed to actually oscillating on its own. (it's probably just something resembling an amplifier) – Sam Jan 12 '17 at 22:38

Maxims 2769 does not include a crystal. It does include an oscillator that is able to excite an external crystal. Load capacitance for an external crystal can be tuned with configuration registers to slightly pull it to counteract aging.

An active external oscillator can also be used, but pulling is not supported in this setup.

The wording in the datasheet is somewhat sloppy about this. If the unit would include a crystal, you can be sure MAXIM would advertise the fact more clearly and also give performance characteristics for it.

Choose a (low-end) TCXO for GPS, a cheap uC crystal will boost phase noise (might work, but will degrade link budget).

1. Can I not use the internal crystal oscillator (to avoid needing an external one) but still achieve the desired functionality or receiving I and Q data?

You will need something that oscillates to generate a baseband clock. So, no, you won't get around using an external oscillator; the fact that you can use a relatively cheap crystal oscillator instead of an active oscillator is a feature.

1. If an internal ostcillator is included, why would I need an external one?

Because that's how the device was designed. The internal oscillator is used to excite the external one, which in generates a stable clock.

1. Do I need an external oscillator to keep the internal one in check because of its lower quality (PLL)?

well, the PLL links the different frequencies, but there's more than one PLL in the chip, so I can't answer this question.

1. Does XTAL (pin name) stand for external?

it stands for crystal.

See the pin table on page 10 of the datasheet.

Seriously, this is a relatively complex chip, and making a GPS receiver might really not be the project of choice for a signal processing and RF beginner. Maybe you'd want to start with understanding how the synthesizer inside the device works if looked at in isolation.

• might really not be the project of choice for a signal processing and RF beginner, it was not my choice -- yet another "team" project at university. I have almost memorized the pin table (and the whole doc), but without understanding how the device works the pinout itself is not very useful. Luckily, I have access to multiple implementations I can use for reference, but would love to actually understand what's going on. I'll wait a bit more for others before picking the final answer, nonetheless this was very helpful. Thanks! – Alfis Jan 12 '17 at 20:08
• @Alfis I think that is an excellent team project; it's only a problem if your team is a "team". I think it's a very very useful skill to be able to cooperate in understanding a complex problem. You should really meet up with the others in person and start discussing the device until you understand what each part in the block diagram does, and how they work (roughly). Start by asking yourselves "why would I need this device? Why doesn't <simpler alternative> cut it here?". That's complex enough, and everyone gets some time to read the datasheet (are there app notes) and then you discuss. … – Marcus Müller Jan 12 '17 at 20:14
• … That way, everyone is forced to partake in the project (lest they want to feel dumb during discussion. Then, after you know why use the MAX2769 , you can tackle the "how does it do that". Just go through every element in the block diagram and make sure that everyone has the same level of understanding. Some might have more problems with a basic thing (like the mixers) than others, and then one must explain for the others. In the end, identify the elements none of you fully understand. Then, assign research to the lesser competent, and development to those who had more experience. – Marcus Müller Jan 12 '17 at 20:17
• That is exactly how it works in theory. In practice, from my bacc. studies and halfway through two masters, never. Literally not even a single time (out of ~15 "team" projects). – Alfis Jan 12 '17 at 20:28
• wow, you guys do a lot of projects. That's pretty cool (I guess, in theory, at least)! – Marcus Müller Jan 12 '17 at 20:30
1. There is no internal XTAL or Ref oscillator only external. this is necessary to locate the accurate GPS carrier frequency.
2. The internal VCO has high f error and phase noise and only reduced when using the external XTAL ref. It is used to synthesize other frequencies as part of the PLL and it's output is as stable as the XTAL and then very stable when locked to the Rx signal.

3. The external XTAL must be parallel type with required load caps (1 big and 1 small one twice the load in pF from Xtal datasheet ) then the 5 bit XTALCAP register can tune the XTAL as a VCXO with internal capacitance and presumably a coupled reversed diode acting as a series Varicap from a DC bias using a 5 bit DAC which then is used to fine tune the external ref. freq. This Xtal goes to gnd rather than feedback but internal feedback provides the gain to oscillate shown as the sine oscillator symbol in block diagram.