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Receive Path Transmit Path Clock Path
The oscillator-filter module is responsible for generating
the RX_CLOCK and TX_CLOCK signals for use by the receiver and exciter modules,
providing RF bandpass and low pass filtering for both receive and transmit
functions, as well as antenna switching. As
designed, this module provides for operation of the transceiver on a single,
crystal controlled channel within the 60 meter allocation.
Modification for use on other frequencies would be possible.
A temperature stabilized crystal oscillator circuit is used for frequency
generation to ensure frequency stability over a wide range of temperatures.
DC supply voltages: 10 – 15 VDC and 4.5 – 5.5 VDC (regulated)
Operating Temperature: 0 – 70 C
Output levels TX_CLK and RX_CLK: 5 volt CMOS logic compatible
Output frequency: 21.614 MHz (4 x operating frequency)
Frequency stability over temperature: +/- 10 ppm
PCB dimensions: 3.9
PCB layout can be seen here.
U1A forms a pierce crystal oscillator which generates a
clock signal at 4x the operating frequency.
The clock signal is then sent to U1C
and U1D, where it is buffered and gated to either the exciter via J2 (TX_CLK)
or J1 (RX_CLK), depending on the logic level of the /REGEN_PTT.
Since the transceiver was intended to be operated in
mobile, portable and home environments, it was felt that some sort of
temperature stabilizing scheme should be included to keep the transceiver as
close to the assigned frequency as possible over the expected environmental
temperature range. During the
winters, outside temperatures routinely drop below 0 C (32 F) and exceed 32 C
(90 F) in the summer. While FCC
97.303 makes no reference to any frequency accuracy specification, it
was simply felt that good operating practice would encourage keeping to the
assigned frequency as closely as possible, without the use of “heroic”
The stabilization scheme chosen was to simply scatter
Positive Temperature Coefficient (PTC) thermistors around the oscillator
section, as well as bonding one to the crystal. PTC thermistors behave like normal resistors in that they
dissipate power (heat) in response to current passing through them.
PTC thermistors exhibit a unique behaviour, in that their
resistance increases as the temperature increases. This temperature to resistance relationship is very
non-linear; in fact, the change becomes very rapid above a temperature known as
the Transition Temperature. This
property can be exploited to make a very simple, self-regulating heater.
When the device is below the transition temperature and voltage is
applied, heat is dissipated in the device.
Heat is conducted out of the thermistor and into the circuit board where
it heats the board and nearby components. When
the thermistor reaches the transition temperature, its resistance rapidly
increases, lowering the power dissipated in the device.
This action causes the thermistor to act as a heat source whose
temperature is maintained at a nearly constant temperature near the transition
temperature. Since heat is being conducted out of the thermistor and into
the circuit board, nearby components are held at relatively stable operating
temperatures, despite varying environmental temperatures.
The thermistors used in this design were chosen to have a
transition temperature of 65 C. Approximately
2 Watts of power (at a supply voltage of 12 VDC) can be dissipated in the
thermistors while below the transition temperature.
This power drops considerably after the temperature has stabilized.
In order to increase the effectiveness of the thermistors,
slots were cut in the PCB material to thermally isolate the oscillator section
from the rest of the board. A small
enclosure was built around the oscillator section to further thermally isolate
the oscillator from the environment. The
enclosure was built from layers of foam board.
Foam board consists of a sheet of some type of foam (Styrofoam?) with
heavy paper bonded to the outer surfaces. The
material is available from art supply stores.
Squares of the material were cut, removing material that would interfere
with the components, and layers built up until the entire oscillator section was
contained. An additional piece was
made to cover the bottom of the oscillator.
The layers of foam were pressfit over #6 nylon hex spacers to hold them
If the transceiver will be operated in a benign environment
where the temperature will be somewhat constant, the heaters may not be
necessary. The heaters may be
controlled by placing a jumper or switch across J9. Shorting J9 turns the heaters on while an open circuit at J9
turns them off. Not using the
heaters will result in a substantial power savings, an important consideration
when operating from battery power.
During the course of debugging of the oscillator, a change
was made to the original trimming capacitor configuration.
The original configuration had C2 populated with a 7-50 pF surface mount
trimmer and C1 not populated. It
was noted that while this configuration did function as intended, it was
difficult to accurately set the frequency with C2.
In the end, both C1 and C2 were removed, and a 3 – 13 pF ceramic
trimmer from the junk box was added across Y1.
This configuration seems to provide enough frequency range to adjust the
frequency as needed, while not being overly touchy.
Another (surprising) property that was discovered during
debugging was the degree of frequency sensitivity due a change in supply
voltage. Varying the supply voltage
from 4 to 5 volts would cause approximately 200 Hz shift in oscillator
frequency. 200 Hz for a 1 volt
change in supply voltage was somewhat more than was expected.
This sensitivity to supply voltage is not a problem in this design as the
oscillator receives regulated +5V from the exciter module, but, should be kept
in mind for future designs.
When the transceiver is in the receive mode, /REGEN_PTT is
logic high. This causes transistor
Q2 to conduct, which drives transistor Q1 into cutoff (no conduction), leaving
relays RL1 and RL2 in their relaxed state.
CR2 is a clamping diode which prevents inductive kick from destroying Q1.
RF from the antenna enters J8 (ANTENNA) and passes through
a halfwave lowpass filter consisting of L4, L3 and their associated capacitors,
to RL2, pins 13 and 4. The signal
exits RL2 via pins 6 and 11, where it then enters RL1 at pin 11.
The signal passes through RL1 and exits at pin 13, where it then enters a
bandpass filter consisting of L2, L1 and associated capacitors.
The filtered RF signal leaves the bandpass filter and comes to RL1, pin
4. The signal exits RL1 at pin 6 and leaves the board via J4 (RX
INPUT) for use by the receiver module.
When the transceiver is in the transmit mode, /REGEN_PTT is
logic low. This causes transistor
Q2 to be in a cutoff condition, driving transistor Q1 into saturation (maximum
conduction), causing relays RL2 and RL1 to pull in.
RF from the exciter module enters the board at J5 (EXCITER
OUTPUT) and enters RL1 at pin 8. The
signal passes through RL1 and exits via pin 4, where it then travels to the
bandpass filter, consisting of L1, L2 and associated capacitors.
The filtered signals returns to RL1 at pin 13, exits at pin 9 and leaves
the board at J6 (PA INPUT).
After leaving the board, the signal enters the RFPA module
and is amplified to a usable power level. The
amplified signal returns to the
board via J7 (PA OUTPUT) and is applied RL2, pins 8 and 9.
The signal exits RL2 at pins 4 and 13, and is sent to the halfwave
lowpass filter consisting of L3, L4 and associated capacitors.
After being lowpass filtered, the signal leaves the board via J8
(ANTENNA) where it is ready to be connected to the external antenna.
For those interested, the calculations for the bandpass filter and the halfwave lowpass filter are available. Click here for an Excel spreadsheet which calculates component values for the Bandpass filter. The spreadsheet can be saved to the user’s computer and used to calculate component values for two-resonator filters of other frequencies, if desired. Please keep in mind these calculations show ideal values; the values used in the final circuit represent combinations of commercially available values that approximate the ideal values.
A simulation of the bandpass filter frequency response can be found here.
A simulation of the halfwave lowpass filter frequency
response can be found here.
It should be noted that the outcomes of these simulations
are calculations based only on the component values used in the simulation.
The simulations do not consider the effects of parasitic resctances and
resistances that may appear in the circuit.
These parasitics may be attributable to the use of real-world
(non-perfect) components and the PCB layout.
During debugging of the board, frequency sweeps of the actual circuits
showed similar to those depicted by the simulations in the frequency range below
100 MHz. Above 100 MHz, the filter attenuation decreases to values
considerably less than those predicted by the simulation.
Present thinking is that there is leakage around the filters through the
PCB groundplane, or around relays RL1 and RL2, causing the loss of attenuation
at the higher frequencies. Plots of
the actual responses will be made available when possible.
The Bill of Material (BOM) is provided in three different formats. Excel format is ready for use directly in Microsoft Excel. HTML format can be viewed directly on the screen if you do not have Excel or other spreadsheet software. CSV format can be used by most spreadsheet software, including Microsoft Excel and others. Here is an explanation of the columns:
|A||NI?||Component Not Installed|
|B||Pattern Name||This is the name of the component pattern used by the layout software|
|C||Ref Des||Component Reference Designator|
|F||Wat||Component Rating in Watts (if applicable)|
|H||Volt||Component Voltage Rating|
|J||PMFR P/N||Primary Manufacturer Part Number|
|K||P Vendor||Primary Vendor (Note: digi = Digikey)|
|L||P Vendor P/N||Part Number Used By Primary Vendor|
|N||SMFR P/N||Secondary Manufacturer Part Number|
|O||S Vendor||Secondary Vendor|
|P||S Vendor P/N||Part Number Used By Secondary Vendor|
The listing of any vendor or manufacturer in this Bill of Materials does not in any way constitute any endorsement of any vendor or manufacturer. Vendors, manufacturers and their part numbers are listed soley for convenience of the builder. The information presented in this Bill of Materials may contain errors; the author assumes no liability for the accuracy of the information contained herein. The user assumes all liability for the use of any information presented in this Bill of Materials.
|Oscillator section||PCB backside|
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Copyright Paul Alexander WB9IPA 2006