Using a MAX2870 Frequency Synthesizer / Signal Generator

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

shabaz — Fri, 23 Feb 2024 00:23:44 GMT

Reading up a bit more, both the MAX2870 and the ADF4001 have a configurable lock-detect (LD) output pin. which can be set in two modes; digital, and analog-sourced (the output is digital in both cases, but the analog lock detect is supposedly more accurate). I wanted to have an LED that would indicate both out-of-lock, and unconfigured state (i.e. board powered up but no SPI config done), which is hard to do without using the analog detected version, which outputs narrow pulses when locked.

I figure it is a nice opportunity to learn how to use interrupts and try to use the CH32V003, as a 'monitor' chip. It will be programmed to detect pulses, and if it finds them, then the LED will be turned off. If the pulses disappear, then the LED blinks fast as a warning that something is wrong. This will be optional functionality, since it is only monitoring and not controlling anything other than the LEDs.

I've tested this source code (just with pulses from a sig-gen, I've no idea how narrow the pulses will be in real life, but if it doesn't work, then I can always switch it to the (perhaps less accurate) digital lock detect with just a code change.

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RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

shabaz — Thu, 22 Feb 2024 00:48:40 GMT

The proposed block diag.. it grew to three PLLs! This is the flow:

* On-board 10 MHz oscillator module, plus the capability to attach an external reference

* First PLL is used to generate a 27 MHz clock.

* Second PLL is a Si5351 combined PLL+VCO which can be configured with decent granularity to anything up to around 150.000MHz. This PLL is optional, it can be omitted and bypassed if desired.

* The output from the Si5351 (or the 27 MHz clock if the Si5351 is bypassed) clocks the MAX2870. The advantage of the Si5351 stage is that since it can be adjusted, that means we can achieve extremely high granularity (in the order of Hz or less).

Although the design is now three times the size! I think it's worth it, since it allows very high granularity, suitable for any radio application, plus it can be locked to an external reference.

Regarding the PLL loop filter for the ADF4001, I wasn't looking forward to trying to figure it out, but I used the ADIsimPLL software, it is very neat, it generates the circuit plus provides lots of useful info. Really impressed. This was its output:

Putting it all together, here's the circuit. Very cramped! I might have to move to additional sheets, but I wanted to keep it to a single page if possible.

I think everything is there, apart from the connector to be able to attach to a microcontroller. Anyway, I'll leave it in this state for a week or so, in case there are any errors or suggestions.

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

shabaz — Wed, 21 Feb 2024 17:18:53 GMT

Been refreshing myself on PLL intricacies by reading up and writing some notes.

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

shabaz — Wed, 21 Feb 2024 02:56:17 GMT

I figured it is possible to do much better than that design, with some teamwork..

It would be awesome to have 10 GHz or so (there is a ham band of interest there!), but the freq synth chips for that are too pricey. People could pull off a harmonic for that so it's maybe not a huge issue.

I'm thinking it would be good to have a board with 5V input and the control signals, so that users can add the microcontroller of their choice. If the connector is in a well defined position, it could be a plug-on microcontroller board with pin headers.

It looks like others felt a transformer on the output would be good too, I took their suggestion of the MiniCircuits transformer, it's a bit pricey (£10) but only one could be populated. I still need to sort out the oscillator (I'm thinking there should be an option for external reference too; it would be easy to do this with logic gates or an RF switch). Any other ideas welcome (and if anyone is interested in assembling it, it would be good to know. Most of the discrete are 0402-sized, and the MAX chip is 0.5mm-pitch QFN, but all this is totally doable).

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

shabaz — Tue, 20 Feb 2024 20:31:05 GMT

This is from the MAX2870 reference design (which is 4-layer). Only layers 1 and 2 are shown here (purple is top layer copper, and blue is layer 2). They have split the ground plane on these layers, I don't know why. The ground plane is whole on layer 3, apart from a single trace shown in orange below.

The highlighted components are the loop filter parts. The green trace is on the bottom layer.

The green layer is the bottom layer, it has various traces including the various VCC pins power distribution.

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

DAB — Tue, 20 Feb 2024 19:48:46 GMT

Very good review.

Thanks Shabaz.

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

shabaz — Mon, 19 Feb 2024 22:32:58 GMT

Forgot to add this to the main content:

I tried the MXO 4 'scope as a spectrum analyzer, and it's brilliant at the frequencies at which it can work at. The result is as good as the spectrum analyzer (except that the SA has more configuration options since it is dedicated for that task! so it's not a fully like-for-like comparison, and one would definitely miss those capabilities so it can't fully replace a SA). The update rate however was 50-100 times faster than the real SA.

The MXO 5 probably will work quite decently as an occasional 2 GHz spectrum analyzer, again not for complete SA replacement. I wonder if in later generations, the MXO could be sold as a complete spectrum analyzer (maybe that's still 5-10 years away until technology reaches that!).

RE: Using a MAX2870 Frequency Synthesizer / Signal Generator

genebren — Mon, 19 Feb 2024 22:08:42 GMT

Thanks for sharing! Very informative and interesting. If I every need to play around in these frequency ranges, I be sure to follow your recommendations.