Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com

I've been intrigued with the small custom frequency programmed oscillator modules, such as Cardinal Components' CPP series units. http://www.cardinalxtal.com/docs/series/CPP.pdf. The CPP series is a one-time programmable module, with 1 Hz steps and is modestly priced at $8.18 each from DigiKey, programmed to your frequency.

If, and it's a big if, these modules have acceptable phase noise they would be extremely handy for BFO or 2nd oscillators in one-off or low volume designs. A few months ago, I purchased several of these oscillators for 21.4 MHz and was disappointed at their broadband noise output. This page provides phase noise comparisons of three oscillators:

• Cardinal CPPT1-H5RT one-time programmable oscillator at 21.412 MHz. 
• Raco 20 MHz crystal controlled canned oscillator module.
• Home brew crystal oscillator at 21.418 MHz.
• Z90 Digital Panadapter in signal generator mode (AD9851 DDS, similar in design to the DDS-60)

The 8640B's frequency was set to 50 KHz above the oscillator under test's frequency to provide a 50 KHz beat note output into the 3562A Dynamic Signal Analyzer.

This method of phase noise measurement, of course, actually measures the combined noise of the HP 8640B and the O.U.T. Although the HP 8640B is a low phase noise signal source, its phase noise is not zero. Hence, don't take the crystal controlled oscillator data as representing the oscillator noise only. The Cardinal programmable oscillator's noise is sufficiently large, compared with the 8640B, that its noise is the dominant factor in the 3562A's display, however.

At 50 KHz offset, we may expect the 8640B's phase noise to be in the -150 dBc/Hz range. John Grebenkemper, KI6WX, has provided phase noise measurements of his HP 8640B, as well as his Elecraft K2 transceiver, at http://home.pacbell.net/johngreb/k2phasenoise.pdf.
 

Home Brew Colpitts Oscillator - 21.418 MHz.
 

100 KHz Span

100 KHz Span

100 KHz Span

My Z90/91 digital panadapter has an AD9851 DDS local oscillator. The AD9851 design is similar to the DDS-60 and should have similar phase noise response. The Z90 includes an auxiliary signal generation mode and the following plots show its phase noise performance at 21 MHz using the same test setup as the other tests.

100 KHz Span

My home brew Colpitts oscillator, designed without paying particular attention to low phase noise, has slightly lower phase noise than the Raco oscillator module, most noticeable when viewed with 1 and 10 KHz spans. The Z90's AD9851 DDS-based oscillator is not too bad for close-in phase noise, but exhibits the typical spurious responses found in this chip, most noticeable in the 100 KHz view. Slight changes in frequency result in changes in spurious levels so these results are only examples of one particular operation mode.

The Cardinal one-time programmable oscillator has much inferior phase noise, viewed at all spans. At 100 KHz span, we see broadband noise approximately 25 dB above either crystal controlled oscillator. When viewed close-in, at 1 KHz, the one-time programmable oscillator does not look as bad, relative to the crystal oscillators, but still shows increased noise levels, of perhaps 8 - 10 dB over either crystal oscillator's performance.

The image below shows three oscillators, 100 KHz span, stacked onto one picture. The stack order is: Cardinal : Raco : Home brew oscillator. The image shows there is a small but perceptible difference between the Raco and home brew crystal oscillators and that the Cardinal one-time programmable oscillator has a huge broadband noise disadvantage.
 

Traditionally, phase noise plots are presented with a zero offset, and the vertical axis in volts / square root Hz, scaled in dBc, i.e., dB below the carrier level. The plots below are in that format, but the vertical reference point is not scaled to 0 dB = carrier level. However, since the mixer input level is approximately the same in all plots, the relative phase noise levels are correct.

One further point—the data is presented log frequency scale from 1 Hz to 100 KHz. Since there is some drift in both the 8640B signal generator (even though it is phase locked) and the oscillator under test, data below about 10 Hz is suspect and should be disregarded.

The data is the average of 16 sweeps.

The plot caption Y axis reads "V/   Hz." The plotting program does not reproduce the square root symbol and the Y axis should read V/Sqrt(Hz).

And, of course, as with all the data on this page, the phase noise presented is the composite of the oscillator under  test and my HP 8640B signal generator.
 

As with the other data, the Cardinal one-time programmable oscillator has significantly greater phase noise, particularly in the range > 1 KHz carrier offset.

The AD9851's phase noise is not bad, but has a number of discrete spurious outputs.

Caveats:

• The data presents the composite phase noise of the O.U.T. and my HP8640B signal generator.
• Data below 10 to 20 Hz should be disregarded as it reflects short-term frequency drift as well as phase noise.
• The Y axis 0 dB reference point does not represent 0 dBc.