Modified Updated Universal Transmitter (designed by W7ZOI)
The transmitter is finally finished (05/10/07)!
The schematic below details the modifications that I made to W7ZOI's Updated Universal Transmitter. It shows the PA section only. Q1 and Q2 are Class A amplifiers and Q3 operates in Class C. Basically I made three modifications:
- Instead of using a crystal oscillator for the signal source, I'm routing the I channel of the VFO into a buffer which increases the signal level.
- I'm keying both the buffer and the first stage of the transmitter.
- And I installed a step up bifilar transformer on the output of the final transistor to match the low pass filters which are 50 ohms input.
- (Many thanks to W7ZOI and W3TS for helping me think through these mods.)
Power Amplifier Board
Next is a photo of the power amplifier board for the transmitter. The buffer was built using SMT parts while the rest of the board was constructed of primarily leaded components. The board traces were cut using a dremel and a diamond burr. I also substituted a 2sc1307 transistor (which I had in my junk box) for the two final stages. A primary difference is that they have metal tabs which are continuous with the collector pin of the transistor. Hence, they needed to be insulated from ground and this was accomplished by cutting two large pads on the board. They can be easily replaced if they don't work as substitutes. (Additional Note: While these transistors worked fairly well, I ended up using the 2sc5739 units as per W7ZOI's original design in the final version of the transmitter. The 2sc5739's can be seen in the last photo on this page on the TX board.)
The Low Pass Filters
The low pass filters are those printed in the IQPro VFO manual. They will be switched in and out via latching relays through the control circuit of the VFO. The first photo is of the filters installed on the board without the relays. As you can see, I dremeled three pads for the common connections: the relay coils, the transmitter and the antenna.
Here is a brief description and a photo of the completed GCR2 transmitter.
- The transmitter board is located just above the mini-R2 board.
- The TR circuit is a modified portion of W7ZOI's Updated Universal Transmitter's TR circuitry which uses a relay to switch between sidetone audio and RX audio. I chose this approach because in previous mini-R2's I had problems with key transients being heard in the audio in the form of clicks.
- The sidetone oscillator is a filtered LM555 sidetone oscillator.
- The relay-controlled lowpass filter board was built according to AA0ZZ's design as detailed in the IQPro VFO manual. I used five of the same kind of latching relay that is used on the IQPro board itself. The lowpass switching is now working very well; however, I needed some help from W3TS to help me sort out a few wiring errors I made with the relays.
- Below the photo is a chart listing the output of the transmitter on the various bands. As you can see the output is not very linear, but this was expected. I optimized the TX for the lower bands. In the future I may modify the transmitter to work better on the upper bands but I've decided to leave it this way for now.
Transmitter Output Per Band
1.8 MHz to 14 MHz = 4.5-5 watts
18 Mhz = 2 watts
21 MHz = 1 watt
24 MHz = 300 mW
28 MHz = 100 mW
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