| OSCILLATOR
DESIGN REFERENCES
These
References are highly recommended for those that want to learn
more about Crystals & Oscillator design.
1/
Crystal Quartz Resonators Ham Radio Magazine February 1986
2/
Crystals Made Clear Part 1 Electronics World September 1999
3/
Crystals Made Clear Part 2, Electronics World October 1999
4/
Solutions For Stable & Precise Microwave Frequency Generation,
VHF Communications 2/2005
5/
Low Noise VHF Crystal Oscillators DK4XP, VHF Communications 4/2000
6/ www.crovencrystals.com
7/ www.vectron.com/products/ocxo/ocxos.htm
8/ www.wenzel.com/documents/tutorial1.htm
9/ www.isotemp.com/resources.html
CALCULATING
STABILITY IN PPM
The
stability of the Oscillator can be calculated by testing the frequency
drift in Hz. Then by entering the figures in to the calculator
on http://www.jittertime.com/resources/ppmcalc.shtml
the stability in PPM can be calculated.
EME175
OSCILLATOR KIT CONSTRUCTORS ALERT
There
are no alerts at this time. For any problems with your kit please
refer to the kit faults & modification section below.
EME177
HEATER KIT CONSTRUCTORS ALERT
There
are no alerts at this time. For any problems with your kit please
refer to the kit faults & modification section below.
CONSTRUCTION
IMAGES OF THE EME175 KIT
The
Current Kits November 2009, use a different Trimmer capacitor
for the frequency adjustment compared to the picture below.
BOTTOM
PCB OVERLAY SHOWING SMD COMPONENTS |
 TOP
PCB OVERLAY SHOWING LEADED COMPONENTS |
The
above image shows the 500MHz Version of the Kit. Take note how
the coils are wound. Many tune up problems can be traced to the
coils being incorrectly wound. The trimmer capacitor settings
show the positions when using a 96MHz crystal for a 576MHz output.
The
above image shows how the components are soldered to the 400MHz
Version of the Kit. This particular board had a 72.375MHz crystal
with a 434.250MHz output. Take note how the coils are wound &
where the trimmer capacitors are set. Many tune up problems can
be traced to the coils being incorrectly wound.
Above
Pictures show the fitting of optional components to the board.
The Crystals case must be connected to the top of the board with
a wire link for stability. The optional PTC Heater is fitted up
against the crystal & a short length of heatshrink is used
to hold it in place against the crystal.
EME175
PROTOTYPE TESTS
We
make no claims that this oscillator is low in phase noise compared
to other designs, but it is quite good. Some spurs that are imaged
either side of the main carrier are due to interference from other
sources in the Lab.
|
The
picture above shows the 288MHz multiplier output from the
Oscillator at -40dB down from the output at 576MHz. |
CONSTRUCTION
IMAGES OF THE EME177 HEATER KIT
MOUNTING
THE EME175 KIT INTO AN ENCLOSURE
The
recommended enclosure is a Hammond 1590S ( 110x82x40mm ), which
allows enough room to line the enclosure in 10mm polystyrene insulation.
The Pink foam shown below should not be used,
as the density is too high so thermally it is not very good. I
highly recommend that you use 10mm low density white polystyrene
foam in the box, & if you fit it very tightly, then it is
a very good insulator. The PC board is mounted to the bottom of
the enclosure using 10mm metal spacers. The board is insulated
from the spacers using plastic stepped transistor insulators to
reduce heat transfer loss from the board to the enclosure. A piece
of polystyrene could also be used across the middle of the box
to reduce the thermal gradient in the box. This may improve the
temperature stability by not having as much air inside the box
around the main oscillator section to keep at a constant temperature.
The dicast enclosures can be obtained from RS Components part
# 3642269. Expensive Feedthrough capacitors have been used for
+12 volts & the Heater output temperature voltage, although
you may want to use something more cost effective.
References:
1/
The theory of Zero
Gradient Crystal Ovens
2/
HP
Model 10811A/B Oscillator
RF
PROBE CONSTRUCTION
 |
| 1.38v |
+13dBm 576MHz |
| 0.956v |
+10dBm |
| 0.843v |
+9dBm |
| 0.742v |
+8dBm |
| 0.652v |
+7dBm |
| 0.572v |
+6dBm |
| 0.497v |
+5dBm |
| 0.435v |
+4dBm |
| 0.380v |
+3dBm |
| 0.330v |
+2dBm |
| 0.288v |
+1dBm |
| 0.248v |
0dBm |
| 0.115v |
-5dBm |
| 0.048v |
-10dBm |
|
If
you don't have any test equipment to tune the oscillator for
maximum RF output, then a simple RF Diode probe can be constructed.
This circuit is so simple that there is no need for a circuit
diagram. The probe uses a SMA Male PCB mount socket with two
100ohm resistors in parallel, a 1N5711 schottky diode, &
10nF filter capacitor. The probe output uses a short length
of figure 8 type cable to connect to a digital multimeter.
The voltages to the left were measured at 576MHz & may
be slightly different with component tolerance& construction.
Typical output @ 576MHz from the EME175 Kit was 0.956v DC
or +10dBm. |
EME175
KIT FAULTS & MODIFICATIONS
1/
Low -10dBm output power. BC849C transistor
TR4 S/C base emitter.
2/
Low output power or cannot tune output trimmers TC4 & TC5.
Check 100pF chip capacitors for either cracks or not soldered
both sides.
NOTES
ON HY-Q CRYSTALS
1/
It has been difficult to test for oscillator frequency stability
using the EME177 Heater due to the Aging of the HY-Q crystals.
Initial frequencies of most Crystals have been around 250Hz high
at the fundamental using a 10pF NPO across the 10pF Murata trimmer
capacitor 1/2 mesh. Most have also been found to Age downwards
in a rapid rate of around 80Hz per day in the first couple weeks,
& still around 40Hz per day after 3 weeks.
2/
Only one Crystal was found to go up in frequency by around 20Hz
before reversing & Aging downwards. It was suspected that
this was due to contamination on the crystal that slowly disappears
with vibration. Frequency jumps were also recorded as the frequency
increased.
3/
It is recommended that you at least run the oscillator for a number
of weeks before even considering testing frequency stability &
tying to accurately align the frequency. When you are sure that
the aging has decreased, then you can try & set the frequency
more accurately. I am still unsure what capacitor value is required
across the 10pF Murata trimmer capacitor, & this could vary
with crystals. Some 96MHz crystals that were tested from the same
manufactured batch, were over 500kHz different in frequency. Initial
tests used a 10pF NPO, but with one that has aged over 3 weeks
has now been changed from 10pF to 4p7 to bring it up onto frequency.
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Wednesday
30th December 09
