Dear Sirs,

I have a query about the HMC832 when used with the output frequency divider.

The data sheet gives graphs of phase noise when used with various divide values of the output divider. This shows the expected reduction in phase noise of 6dB with each divide by 2. What is the effect of the output divider on the integer boundary spurs? For a perfect divider I would expect the spur level to be reduced by 6dB for each divide by 2, and the offset frequency of the spurs to be unchanged.

My question is relevant as I have been evaluating a competing device with a similar output divider. The use of the output divider introduces new integer boundary spurs at 1/N the spacing of the original spurs, where N is the divide value. These are at quite high levels.

An example: VCO on 2000.05MHz, divider not used, int boundary spurs at +/-50kHz, level -28dBc

VCO on 2000.05MHz, divide by 2, output at 1000.025MHz, int boundary spurs at +/-25kHz, level -17dBc

int boundary spurs at +/-50kHz, level -27dBc

Is this effect present in the HMC832?

I have not yet purchased the evaluation board, but intend to do so.

When operating the HMC832 in Fundamental mode (1.5 to 3GHz output) the typical Integer-N Boundary spur level is -55dBc (sometimes better, sometimes a little worse) when inside the loop bandwidth. This spur would never approach -28dBc on the HMC832.

Sometimes the Integer-N Boundary spur is accompanied by another spur at half the offset or multiples of the offset with amplitudes similar or lower amplitudes. For example, in the stated case above with an output of 2000.05MHz with a 50MHz comparison frequency, another spur may appear at 25k or 100kHz offset.

When operating with the internal VCO Output Divider engaged (Divider mode), the Integer-N Boundary spur amplitude will decrease by 20log(N) while the offset will remain the same, as you noted (N is the VCO Output Divider setting).

In addition, other spur artifacts can occur at offsets of: (Fundamental Integer-N Boundary spur Offset) x 1/N

Shown below are spectral plots of the HMC832 in three configurations, all with the same VCO frequency of 2600.01MHz (50MHz comparison), but with different VCO Output Divider configurations:

Bypassed (Fundamental mode);

VCO Output Divide-by-2;

VCO Output Divide-by-4

(note that the stated carrier frequency on the plot readouts appear different because the references of the DUT and E5052B were not synchronized).

Fundamental Mode (2600.01MHz output) showing Integer-N Boundary spur of -60dBc @10kHz offset:

VCO Output Divide-by-2 (1300.005MHz output) showing Integer-N Boundary spur of -68dBc @ 10kHz offset plus -75dBc spur @ 5kHz offset:

VCO Output Divide-by-4 (650.0025MHz output) showing Integer-N Boundary spur of -72dBc @ 10kHz offset plus-74dBc spur @ 2.5kHz offset: