CC1120: CC1120 + CC1190

At first look the schematic looks ok.

If you use the PA table stated in www.ti.com/.../swra393c.pdf or SmartRF Studio, could you post a plot showing the spectrum for some of the output power levels?

I can also do a quick layout review if you send the gerber files (if you don't want them public on the forum, send me a friend request)

It looks like the ref on the spectrum is set a bit low.

What is the spacing between the peeks?

Could you do the same but with the frequency set 500 MHz higher or lower (should be fine for the SAW filter). I suspect that these are spurs folded around xfref or something similar.

This is not something I have seen when I have measured on CC1190 so either you have measurement issues or schematic/ layout/ BOM issues.

What do you mean with 500 MHz higher or lower? The SAW-filter has a usable passband of 2 MHz.

Move the center frequency as much as possible (limited by the SAW filter) up or down and see which spurs are a function of the center frequency and which ones are variable.

Ok, thank you for the support. I will do some tests and keep you informed.

Regards,

Per Magnusson

Hi,
I found that if I remove the SAW-filter the disturbances dissapear. It seems that the filter can't handle the power from the CC120. I am using the QUALCOMM filter B39871B3725U410. Any idea why?

Regards,
Per

It should not be any issues here.

- Are you able to test with a 100 pF cap between the IPC and the SAW filter?
- Is it possible to take a look at the layout?

Hi, sorry but what do you mean with IPC? I included the layout in one of my earlier posts. See M4066003.zip in my post from Oct 10, 2018 11:05 AM

/Per

IPC: Integrated Passive Component.

See www.ti.com/.../swra407.pdf and the layout section. You have placed the IPC a fair distance from the CC112x and hence the matching gets wrong. Not sure why the SAW filter has an impact on this but the IPC has an impact. You also have a 15 pF cap on the schematic that is is included in the IPC.

Why have you placed footprints for 0 ohm between CC112x and the IPC?

Hi Per,

Some off topic: Take a look at this app note for SAW filter layout considerations: 

HC

Thank you,
I can see in the design note document that TER posted that the Discrete Reference Design is the best choice for performance. When we did the EMI scan at the external test lab we found that 2:nd harmonic was very close to the EN300220 limit. Do you think that the discrete soution is a better choice? I will of course create a lot of work to re-design the board.
/Per

I can also see in the design note that the JTI IPC has much better attenuation of the 2:nd harmonic (10dB) at 868 MHz than the Murata IPC (the one that I use). Are the figures reliable? See page 14 and 15 in SWRA407/DN039

Per M., to my understanding the SAW filter is mainly used to attenuate spurs generated by the CC1120. The spurs, being broadband low level signals, will be amplified by the PA since the PA acts as a broadband amplifier for low level signals. However, CC1120 will also have its own harmonics of course, and the SAW filter will attenuate these as well before being amplified again by the PA. The PA will generate its own harmonics and the pre-SAW filter has no impact on these. The question then is: Which device is the source of the high 2. harmonic? From the reasoning provided (given that it has no flaws) it could either be the PA or the CC1120. If it is the CC1120, the design note in my previous post could help by utilizing the correct VIA hole procedure for the SAW ground pins. This would give a better out-of-band attenuation and help decrease the 2. harmonic. This fix should be an easy layout ECO.

A simple test to check which is the source for the harmonic could be to place a series 0R resistor on the SAW footprint. If the 2. harmonic increases, this would indicate that the SAW actually had an impact on the 2. harmonic attenuation, and a "better" SAW would decrease it further. "better" being correct VIA use.

If the source for the harmonic is the PA, a pi-filter, or a single pole filter after the PA could help.

I think that it is the CC1120 that is the source because if Itake the development kit TrxEB with the CC112XEM and connect to my spectrum analyser I can see the same harmonics.

Per M., it could be that the Spectrum Analyzer is the source of the harmonics.

Ok, but what about trying to use the series resistor in the SAW footprint as I suggested?

The harmonic at 1.739 MHz is 2.5 dB higher without the SAW-filter. (Conducted measurement)

When I have done measured on CC1120 + CC1190 and looked at the output of the CC1120 the harmonics/ spurs are fairly low. I did a lot of measurements with and without SAW filter and it's only on close to max output power the SAW filter is needed and that was in the restricted band corner @863 MHz (if my memory serves me right about the frequency)

Could you measure the spectrum of the CC1120 part of your board (remove the SAW filter) and also the spectrum you measure on a CC1120 EM with the same setting, both on the chip and the spectrum.

You can risk getting a high 2. harmonic radiated if the notches in the design is not on the correct frequency. Based on experience conducted 2. harmonic has to be 10 20 dB below the limit to pass ETSI. What is the conducted 2. harmonic?

Ok, I will do some more measurements and come back to you with the readings, it will probably be on Monday. Regarding the BOOSTXL schematic and layout, are they correct and valid?

You could then try to scrape away (with knife) the top layer copper around each SAW filter ground pin, and assuring that each separated top-layer ground island has at least one ground-via left, and have the saw filter in place of course. Remember to scrape away the ground fill below the SAW as well.

I have a couple of more questions in the same topic.
1) The design note DN039 (SWRA407) says that a discrete design using multi-layer inductors has much better attenuation of the harmonics than other solutions. See page 13. Is this correct? If so, what is the reason?
2) When using CC1190, is RF shielding always needed to keep the radiated harmonics below the regulatory limits?

/Per

1) Q-factor of wire bound is one element. It doesn't matter for this design though since the SAW filter should reduce harmonics + spurs form the CC1120 part to very low levels.
2) Without the shield the radiated 3. harmonic will be higher. If you reduce the output power it's possible that it will pass but for max output power the shield is required.

A question about the output power. Has it been verified that the CC1190 can deliver +27 dBm at 868 MHz? 

Regards,
Per

See www.ti.com/.../swra492.pdf and www.ti.com/.../swra393c.pdf for what we have measured. It's possible to get close to +27 dBm but then you will most likely not pass the regulations with a good antenna.

I see that the text ideally should have been changed. This board is not available but the boosterpack is.

But is there a boosterpack for 868 MHz?

Both 868 and 915 according to www.ti.com/.../BOOSTXL-CC1125. Please the see the app note/ schematic for the changes required to use 868 vs 915.

As far as I can see it is only +15dbm output power from this pcb. Am I missing something here?

Sorry, linked to wrong boosterpack: www.ti.com/.../swra492.pdf

Sorry, but I still can't find a boosterpack for 868 with 27dbm output. Can you please refer to the ti article no?

The BOOSTXL-CC1120-90 is a BoosterPack for Applications from 902MHz to 928MHz.

From www.ti.com/.../swra492.pdf :

"The CC1120-CC1190 BoosterPack reference design includes schematic and gerber files [4]. The same
design can be used for both 915 MHz and 868 MHz applications with different bill of materials. The same
design can also be used for SigFox applications in 915 MHz frequency band with different bill of materials.
It is highly recommended to follow the reference design for optimum performance. The reference design
also includes bill of materials with manufacturers and part numbers."

Yes, I have read this but I assume that I then have to rebuild the boosterpack pcb, replacing a lot of components, some of them very difficult to replace such as the SAW-filter and the matching filter. I probably also have to remove the shielding cover to replace components that are mounted there. This is not an easy task to do without damaging the pcb.

Regards,
Per

With a hot plate for soldering this is fairly easy. And you basically only have to add 3 components and change the SAW filter.

Hi, What about components C13, R7, R8, R9?

Regards,
Per

Form the schematic: Use R7=0 ohms, R8=R9=DNP for Non-Sigfox Applications

C13 is unmodified.

Ok, thank you for the information.
This is actually exactly what I have done on my pcb but I am not getting +27 dBm when I measure conducted RF. Anyhow, is it so that if I buy the CC112x-CC1190 BoosterPack module and change/remove the different components I will be able to measure +27 dBm output power?

Regards,
Per