74HCT240 transmitter

Discussion in 'Homebrew and Kit Projects' started by SA7CNG, May 31, 2018.

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  1. SA7CNG

    SA7CNG Ham Member QRZ Page

    Im working on a new QRP transmitter based on the 74HCT240 octal buffer, the circuit has been described on many places, even in ARRLs books. http://www.qsl.net/5z4ft/74hc240qrp.html

    I am getting about 150mW on 20m, and about 225mW on 40m, but i was hoping for a bit more. People have reportedly gotten approx 500mW out of these circuits. I am using the HCT i stead of the HC(problem finding the HC model), but thats the only difference i could fint.

    I have left some room on the board for a low pass filter, and a microprocessor to control the transmitter.

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    Last edited: May 31, 2018
    N0TZU, N1OOQ and VK4HAT like this.
  2. SA7CNG

    SA7CNG Ham Member QRZ Page

    As you can see on the graph, driving the 74HCT240 with more than 10V results in little less than a lot of heat, http://www.ti.com/lit/ds/symlink/cd74hct240.pdf The HC variant is supposedly able to handle 0,5V more than the HCT, but i cant find any other significant difference.
     
    VK4HAT likes this.
  3. VK4HAT

    VK4HAT XML Subscriber QRZ Page

    Drop an extra stage after it and you will get close to 10w on 40m. Needs 7th order LPF to get the harmonics to legal, not the 5th order shown in the schemaic

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    N4NYK likes this.
  4. K6BSU

    K6BSU Ham Member QRZ Page

    Is there a reason for the 8 volt regulator? For max power, I would go for max voltage.
     
  5. K7JEM

    K7JEM Ham Member QRZ Page

    Those chips are spec'd at lower voltage than 8v, but maybe works OK anyway.
     
  6. SA7CNG

    SA7CNG Ham Member QRZ Page

    Thanx VK4FFAB, that was exactly what i was looking for!

    I got about 2,5W out of the 20m version, and 5,5W from the 40m version. Output isnt very beautiful, it will probably need a good lowpass filter just as you said.

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  7. VK4HAT

    VK4HAT XML Subscriber QRZ Page

    Good job :) If you can push the voltage up on the FET to about 24v + you should see 5w or more on 20m. With 1/2w of drive into a IRF530 on 40m i am getting 10w out at 13.8v, but it does need a lot of low pass filter to make it clean.

    Because you are only getting 0.25w drive, you might want to add in another drive stage to get that up to 1/2w ish, will help to drive the FET better.
     
  8. SA7CNG

    SA7CNG Ham Member QRZ Page

    I moved the amplifier to a separate board, and added a lowpass filter, found a calculator online for a "butterworth" filter, and tried my best to wind coils/find caps. The signal is somewhat more sinusoid, and the output went down with about 2,0W so my best guess is that it works.

    I have no equipment to messure inductance or capacitance, will try to find one. Perhaps a signal generator would be helpfull.

    http://www.calculatoredge.com/electronics/bw pi low pass.htm


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  9. SA7CNG

    SA7CNG Ham Member QRZ Page

    I realised that i had used the wrong crystal on the mesurement above, the butterworth filter is LP, 15MHz, and the filtered signal looks like below, alot better. I did a test with an automated keyer, but when i switch out the dummy load for my antenna, the amplifier shorts out completely. I even tried to use my antenna tuner, but with no luck.

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  10. SA7CNG

    SA7CNG Ham Member QRZ Page

    I solved the problem with my PSU shorting out, by adding a 100nf capacitor in line with the filter. Now im waiting for some more parts to try and increase the power output some more.

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