EMRFD Message Archive 10044
Message Date From Subject 10044 2014-05-20 19:33:15 Neil Martinsen-Bu... Debugging help: chapter 1 first transmitter low power output I made some small component substitutions in my build because of parts I had on hand. I used a FT37-43 core with 7 turns in place of the 15 uH RFC which should give 17 uH. I also did not have a 200 pF cap for the output network so I used 220 pF. The inductor in my output network is 20 turns on a T50-2.The decoupling resistor (I used 12R instead of the 10R specified) drops only 240 mV for a current draw in Q3 of only 20 mA, obviously not enough to give the 123 mW described in the text with R1=270R. (I used R1=220R.)Greetings all,I have measured the power output of the oscillator and the buffer amplifier and I get a value close to that given in the text, around +11 dBm. I can get this measurement using both the AD8307 meter that I built and using a simple diode detector on the dummy load. When I add the driver amplifier as shown in figure 1.34, I measure only very slightly more power, about 14 dBm. This measurement is also consistent between the two power measuring techniques.
I am getting my start in scratch building with the simple transmitter in Chapter 1 of EMRFD. My problem is with the driver amplifier added to the buffered oscillator in figure 1.34. I am measuring very low power output from the output of this stage, and I don't know where to start figuring out what my problem is.
Another data point is that the output power is *constant* with varying supply voltages. I was using 8xAA NiMH at 10.9 V and thought that might explain my low output, but an SLA at 12.9 V gives the same 14 dBm power output!
I am also building my test equipment as I go, so I don't have an LC meter to check my toroids, nor an oscilloscope. Nor (apparently) do I have any idea where to start tracking down the problem. Can you help me understand what further measurements I can make or what likely points of failure might be that could result in this problem?
Thanks in advance for any help you can offer.-Neil KD0UKC
10045 2014-05-20 23:52:52 Ephemeral Re: Debugging help: chapter 1 first transmitter low power output Maybe you should get an oscilloscope? Second hand dual channel 50MHz scopes arereadily available and not terribly expensive. They are also very educational.Adrian 10046 2014-05-22 16:52:15 Nick Kennedy Re: Debugging help: chapter 1 first transmitter low power output Your component substitutions don't seem out of hand at first glance. One thing -- did you note that R1, the emitter resistor of Q2 changed from 270 ohms in figure 1.33 to 150 ohms in figure 1.34. That will double the stage's gain for a given collector load.73-Nick, WA5BDU 10047 2014-05-22 21:08:58 Neil Martinsen-Bu... Re: Debugging help: chapter 1 first transmitter low power output -Neil KD0UKCThanks again to everyone for the suggestions.Thanks to everyone for their suggestions. While a used 50 MHz 'scope would be wonderful, they still run to $200 on that auction site and "terribly expensive" is in the wallet of the beholder. Thankfully, Wes has done lots of legwork already on this circuit. His write-up at http://w7zoi.net/more_first_xmtr.pdf gives RF probe measurements at various points throughout the circuit.For the RF measurements, everything up to the collector of Q2 looks similar to what he got. In my circuit, the peak voltages on the collector and everything after the transformer are about half of what he shows in that article.
Following his lead, I built an RF probe and duplicated his measurements both DC and with the RF probe. The DC measurements agree very well with what he gives, so I think that means that I probably don't have any shorts or opens in my construction.
Inspired by a recent episode of the SolderSmoke podcast, I wonder if I got the sense of the transformer windings wrong. Could that cause the effect that I am seeing? Having never done this before, can someone explain the direction of the two windings on the transformer and the schematic convention on which end should be which?
10048 2014-05-23 03:06:43 Roelof Bakker Re: Debugging help: chapter 1 first transmitter low power output Hello Neil,
> Could that cause the effect that I am seeing? Having never done thisI assume that you have been winding the transformer with twisted wires.
> before, can someone explain the >direction of the two windings on the
> transformer and the schematic convention on which end should be >which?
Using an ohmmeter, find the two windings.
Connect the end of winding 1 with the start of winding 2.
This is the center tap.
You can also say that the two windings are connected in series.
I hope this helps.
Gemaakt met Opera's e-mailprogramma: http://www.opera.com/mail/
10049 2014-05-23 05:29:40 Nick Kennedy Re: Debugging help: chapter 1 first transmitter low power output No, for the transformer used in this circuit, phasing doesn't matter.73,Nick, WA5BDU 10051 2014-05-23 09:36:18 vasilyivanenko Re: Debugging help: chapter 1 first transmitter low power output
Bill initially goofed on a a VFO coil where transformer phasing was a deal-breaker. Your case is quite different.
From many years of experiments (and failures), I've learned the following.
Proven circuits did not work because:
1. I made a breadboard mistake.
2. I failed to test each stage as soon as I built it. Test both at DC and AC - for AC testing, an RF probe is fine.
3. Bad parts.
Best of luck! V.
10074 2014-05-29 09:17:59 Neil Martinsen-Bu... Re: Debugging help: chapter 1 first transmitter low power output Thanks again everyone for your help on this. The good news is that the circuit is working! I am getting +22 dBm or a little more depending on the source voltage. The tough thing is that I don't know exactly why it wasn't working before. Using the voltage measurements it appeared as if the problem was with something after Q2, so I removed and re-wound the Q2 output transformer being careful to put both windings the same direction around the core. When I re-installed the transformer, the circuit worked roughly as expected.I did have to use a smaller value for R1 to get that level of output and I didn't ever see the over 300 mW that they saw in EMRFD with higher voltages, but I got over the 20 dBm level so I feel I have enough drive to go into the PA stage now.Thanks again to everyone for the help and I hope to either work you on the air with my two rockbound watts, or I will see you again here with more questions.-Neil KD0UKC