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Programmable Canned Oscillator Phase
Noise
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)
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The photo below shows the home made crystal oscillator at
left (Colpitts design, with buffer and power amplifier stage), a fixture to hold
canned oscillator modules with the Raco oscillator installed and the Cardinal
programmable oscillator. |
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The test equipment setup is illustrated below. The pads between the oscillator
under test and the mixer input were sized to provide -10 dBm input into
the mixer for each oscillator. The ZP1-MH mixer requires +13 dBm local
oscillator drive, provided by an HP 8640B signal generator operating at +19 dBm
output, with a 6 dB pad. The mixer's output connects to an HP 3562A Dynamic
Signal Analyzer, operating in spectrum analyzer mode.
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.
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The data below is taken at a center frequency of 50 KHz,
corresponding to the 50 KHz beat note between the 8640B and the O.U.T. Since the
oscillator input power into the mixer is approximately the same (±1 dB) the
signal levels may be directly compared. Data is shown for spans of 100 KHz, 10
KHz and 1 KHz, for each oscillator type. Home
Brew Colpitts Oscillator - 21.418 MHz.
100 KHz Span |
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10 KHz Span |
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1 KHz Span |
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Raco Oscillator Module (Crystal, non-programmable) 20
MHz.
100 KHz Span |
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10 KHz Span |
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1 KHz Span |
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Cardinal CPP Programmable
Oscillalator Module 21.412
MHz.
100 KHz Span |
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10 KHz Span |
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1 KHz Span |
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Z90 Digital Panadapter Signal Generator Output
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 |
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10 KHz Span |
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1 KHz Span |
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Conclusions 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.
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The image below stacks the Cardinal programmable oscillator
(red), the Z90's AD9851 DDS (green) and the discrete crystal oscillator (blue).
The data shows that the AD9851's phase noise is not too bad, but its discrete
spurious outputs are much more of concern. (The Z90, along with the DDS-60, uses
an inexpensive 30 MHz crystal time base. A higher quality time base with lower
jitter will improve the AD9851's phase noise, but will not do anything for
discrete spurious responses.) |
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Zero Offset Phase Noise, Volts/Square Root (Hz)
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.
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Z90 Oscillator—AD9851 DDS |
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Discrete Crystal Oscillator |
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Cardinal CPP One-Time Programmable Oscillator |
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Comparison of Three Oscillators |
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Comments 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.
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