LMX2581EVM reference spurii

Cosmo,

Let me add, that this is with the 100 MHz XO and 20 MHz phase detector frequency.   So Fosc spur is at 100 MHz offset and Fpd spur is at 20 MHz offset.  I think I got the 100 MHz Fosc spur mixed up with the phase detector spur.

Now if you raised the phase detector frequency from 20 MHz to 100 MHz, then definitely this spur will increase on the order of 4 dB and the integer boundary spur will increase on the order of 20*log(100MHz/20MHz) = 14 dB.

I also have more data on the Fpd spur.  This time, the input is a 400 MHz signal generator, so we know that it is Fpd, notFosc.

Regards,
Dean

Dear Dean,

The problem is not the fosc spur at 100MHz, but its 2nd harmonic at 200MHz. As I said originally, the fosc spur and its 3rd harmonic reduce as the R divider is increased. The 2nd harmonic is unchanged with frequency setting or Fpd. It is present when the output buffers are off, so it is definitely not a Fpd spur from the PLL. It is direct feedthrough, possibly from the fosc doubler, which I surmise is always in circuit, even when it is not used. The 200MHz spur is at a constant -46dBm, so assuming a wanted signal at +6dBm, this gives a S/N of 52dBc, which is not so good. The problem really gets worse when the wanted frequency is also close to 200MHz, as in this case the spur becomes a close in spur.

 It is possible that the 200MHz spur is common mode to the RF+ and RF- , in which case a balun should suppress it. I will try this.

regards

Cosmo Little

Cosmo,

I have an evaluation board without the XO and drove it with a 100 MHz LVPECL signal.   The phase detector frequency did not have much impact on the spur at 100 or 200 MHz ouput and with the doubler disengaged, I measured about -67 dBc.  With the doubler engaged, this degraded to about -63 dBc.  This was the 200 MHz offset.

My carrier was about +7 dBm and I did see this spur move with the carrier.

My carrier was 2700 Hz.   I tred both RFoutA and RFoutB and got similar results.  RFoutA on my board had a 51 ohm pull-up and RFoutB had an 18 nH pull-up.  My thought was that 18 nH is only 11 ohms at 100 Mhz while the 51 ohm resistor was more, so this should have impacted the spur significantly if it was coming through my power supply, but it didint.  I tried 20 MHz, 100 MHz, and 200 MHz phase detector frequencies. 

However, on your board, the spur is much higher and also you claim that the dBm level does not change with carrier, where it did more so on mine.  This makes me think that your spur might be coming through a different mechanism that is more dominant that what is on my board.  I am thinking power supply.  If so, the balun should help.    It would also seem that you might see some impact in tinkering with the pull-up component or tinkering with bypassing for the supply at this point.

Regards,

Dean

Dear Dean,

   I have measured the phase of the 200MHz spur at the RFA+ and RFA-  using a vector voltmeter, and it seems the spur is a common mode signal. hopefully a balun will greatly improve matters. Will post again when I have tried it

regards

Cosmo Little

Dear Dean,

 I have finally received the balun samples, and can now report excellent results, with greatly improved power output and flatness with frequency. The balun is a Minicircuits TCM2-43X, with presents a differential impedance of 100ohms to the RFB outputs.

  The following are power output figures measured with a power meter directly connected to the output connector:

1/- VCO Direct  buffer B power setting 30         power setting 40

1900MHz   13.8dBm                                                         15.3                             

2000MHz    13.6dBm                                                         15.1

3000MHz     10.6                                                                11.7

3750MHz         6.6                                                               7.9

2/- Divider=2, power setting 30                            power setting 40

950MHz          11.6dBm                                                      13.2

1500MHz          12.4                                                             14.1

1875MHz            13.8                                                           15.0

3/- Divider=16 power setting 30                             power setting 40

118MHz             9.7                                                               11.4

156MHz               9.8                                                               11.5

234MHz              9.9                                                               11.6

4/- Divider=38 power setting 30                             power setting 40

50MHz                  9.4                                                                11.1

The DC block is only 100pF, which probably accounts for the reduction in power at lower frequencies.

The Fin spur at 200MHz is now at -57dBm with the buffers off. This is a great improvement. Possibly with individual power supply pin decoupling this will improve further. My own test PCB will include this.

The use of a balun is recommended if one requires a wide output frequency range.

Finally, congratulations on a quite remarkable chip

regards

Cosmo Little

Cosmo,

Perhaps the balun is helping because the supply has a small amount of ripple at 200 MHz that is going direct to the output and the balun cancels it out.  Anyways, I am happy that the balun has brought this good improvement.

Regards,

Dean