© 1999 QRP2000 Design Team
By Jan Verduyn G0BBL/ex PA0VDR
The QRP2000 receiver consists of the following boards shown in the block diagram:
KK7B Receiver PCB
This is the well-known design by Rick Campbell KK7B as described in QST January 1993 and is also described in the ARRL QRP Low Power book. The receiver is a two phase Direct Conversion receiver with 90 Degree Audio phase shift network, a 3 kHz LP audio filter on board and requires two RF Local Oscillator inputs in quadrature. See Links for full circuit description, modifications and sources for PCB's. On our prototype we implemented the W7ZOI audio pre-amplifier modification which improved Sensistivity and overall gain by a few dB and is well worth the effort. Understand that Rick is in the process of updating the R2 design and if you are contemplating building an R2 you may want to wait for Rick's new design.
QRP2000 DDS Synthesizer PCB
Developed by us and consisting of two AD9850 125 mHz Direct Digital Synthesizer chips, each driving a MAR11 MMIC Amplifiers delivering around 5 to 7dBm of Local Oscillator signal from 1 Mhz to 30 Mhz via 30Mhz Low Pass filters. The AD9850 circuitry is almost as per Analog Devices datasheet. Serial programming is used. This sets the same frequency on both Chips however U1 is always set to 0 Degrees whilst U2 is programmed to generate RF LO phaseshifted either +90 Degrees or +270 Degrees depending on USB or LSB by the DDS control board.
DDS reset: To ensure that the two LO signals from the DDS chips are always exactly at 90 degree phaseshift it is essential that both AD9850s are reset and start on the same clockcycle of the 100Mhz clock both at power-up and when changing modes. Considerable work has been done on this and on the prototype receiver Quadrature operations reliably about 95% of the time. Sometimes the two LO's are not in Quadrature ( often out by 11degrees) which manifest itself as poor suppression of the unwanted sideband. The cure is simply to press the mode switch button which will reset the IC's. We intend to investigate this again once pre-production boards are becoming available.
QRP2000 DDS Control PCB
This PCB contains the Microchip 16F84 Microprocessor and interfaces with the DDS synthesizer PCB, the LCD Frequency readout module, a keypad and the RF Preselector board. U2 is a 1 to 16 line decoder which is latched with the top 4 bits (b4 - b7) and selects the desired RF bandpass filter on the Preselector PCB via U3 and U4 Darlington drivers by pulling the band select line from 12V to 0V. At Power up, when pressing MPU reset or when changing mode the AD9850's DDS chips are reset by taking P5 on JP3 high for 150 Miliseconds. U5 provides an extra 8 outputs for future expansion but this is not implemented in the receiver. JP6 also is for future expansion and provides three inputs for the transceiver project, however this is also not supported by the current firmware. Provisions are available to program the PIC 16F84 chip on the board from a PC by setting jumpers JP1 to JP7 in the appropriate position and this allows for development work and firmware upgrades etc. This particular interface supports the PIP02 program which is available on the WEB. (See under firmware programming further down) This worked reliable via the serial port of a Toshiba 20 Mhz 486 laptop and several other Pentium PCs running DOS, however we never could get this to work on a Toshiba laptop running Windows 98. This presumably is because Windows 98 or NT does not allow direct control of the COM port lines from the software.
PRA10 Preselector PCB
This board provides bandpass filters for all nine HF Amateur bands. The 10th bandfilter is an allpass filter which is automatically selected for general coverage purposes for frequencies outside the Amateur bands. See the software description for band limits. In the receiver the PRA10 board is wired so the bandpass filters precede the MAR11 MMIC preamplifier so out of band signals are filtered out prior amplification. For Transceive purposes this board can be wired so the Low Pass filters on the Hands LF9 Transmit filter PCB are used for front end filtering and the PRA10 bandpass filters are placed between the MAR11 RF Amplifier and the R2 receiver PCB. The PRA10 board provides about 8 dB RF gain with the preamplifier switched in and about 4 dB loss with the when the preamplifier switched out.
Construction
"Dead Bug" also known as "ugly construction" was used for the DDS synthesizer circuitry.
A screened box was made from a sheet of double side PCB and all components soldered directly onto the PCB groundplane
where possible. This is a good method for quick development and allows for changes to be made to the design if
needed. The DDS Control board was built on a piece of perforated board from Tandy (Radio Shack) by handwiring the
various islands on the board with 30swg wirewrap wire. It is hoped that PCB's will be available in the near future.
Modifying the Bourns Shaft Encoder
The Bourns EC Series Digital Contacting Encoder, 24 cycles per revolution (Mfr order no ECWJ-B24-AC0024) is readily available in the UK at a reasonable price from Farnell Electronics, however it has a click index mechanism which hardly makes it suitable if you want nice smooth tuning on your Rig. I therefore did not hesitate and just had to take the encoder apart to see if it the click mechanism could be disabled. The answer fortunately is yes and here is how the mod is done. On the prototype we are using this encoder with a 55 year old tuning knob from a BC348 ex-WWII receiver and this gives just the right feel!!! Also it is nice to recycle Vintage radio parts for use in a modern design.
Here is how it is done…….
1) With sharp Hobby knife cut the black plastic cups at the back of the rotary encoder. (Warning!!! Mind your fingers!)
2) Carefully prise open the assembly taking care not to loose the small nylon washer at the end of the shaft.
3) Remove and note how the sliding contact fingers are bent with respect to the main body of the metal washer.
4) Using two small pliers adjust bend the contact fingers of the metal washer to exactly the opposite position.
5) Reassemble but with the metal ring reversed so the click indent finger points towards the shaft making sure that there is sufficient contact pressure.
6) Use instant glue sparingly to glue the 4 black plastic locating pins of the front assembly to the rear part
of the encoder housing.
Firmware programming
There are many programmers available to program the code into the PIC84. David Tait's Webpage is an excellent place to get familiar with the PIC84 resources. We used the PIP-02 program (pip02.zip) from Silicon Studio with the COM84 PINAPI serial port driver which worked fine, however we never could get this to work with Alan's Toshiba Laptop which has a pentium processor. The COM84 driver comes with a gif file showing hardware connections.
Typical code change in the source code would be programmed in as follows:
