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AN920

Micro FM TX (Micromitter)

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A good friend of mine in Australia bought one of these kits. He was having some problems getting it to work properly. He posted it to me to look at. Here are my findings and some modifications I did. I will address it point by point:

1) Move this cap closer to pin 12. The PLL Vcc is very sensitive to any noise. (see pcb layout below)

2) Run the VCO tuning supply from a higher voltage than 5V. It must be regulated. This can be another small 78L09 or suitable zener. This will help locking at bottom and top ends of the band without readjusting L1.

3) Reduce the 5.1k to about 2.7k (It will allow faster tuning). The 22k On the base of Q1 can also be made slightly smaller with not much effect on performance.

4) Try a variable capacitor there to alter the phase of the pilot for best stereo separation. You can replace it with a fixed value after measuring on a LCR meter.

5) Try to get the ratio of the (22pF, 33pF and VC1 to 10pF) as high as possible. This means the total capacitance by the 3 elements in the circle should be as large as possible while the series cap (10pF in this case) should be as small as possible. L1 should be as small as possible. This may take some experimentation to get values that will provide lock at the top and bottom end. If you operate on a single frequency this is not a problem. This will give lowest carrier noise sensitivity for this clapp oscillator.

6,7) I found some slight loop instability (hunting) when tuned to the high end of the band. This sounded like a faint whistle on the audio. I think it is caused by some of the 100 kHz comp reference still getting into the VCO tuning line from the output of the charge pump. I solved this by using 2 separate transistors for Q1. This gave me access to the base of the second transistor. I removed the 47nF and placed a smaller 10nF (make this 22nF, see next post) as shown. This cured the instability and noise problem.

When you set up your transmitter at the low end and your voltage at TP1 reads lower than about 0.7V or is stuck at 0.6V then the VCO can

post-9230-14279143550766_thumb.gif

post-9230-1427914355122_thumb.gif

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Hi AN920,

Thanks for such nice review and critical observations. In fact i have made the PCBs locally and just abt to assemble. -your comments are just useful.
the 47pF cap across collector and base of darlington , if it could be shifted to base to earth of the same device, will it suffice  without need to discretizuing the darlington?
please comment.
Sarma

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Hi Sarma,

No, that won't help as I will show below.

If you look at the first plot, you will see that with the 47nF connected between the base-collector of the MPSA13 the gain at 100 KHz is about 6.5dB. This is the problem I suspected.

In the second plot, by moving the 47nF to the base-emitter of the MPSA13 you make the problem much worse. Now you will have 14.5dB at 100 kHz. No good.

The last plot shows putting the capacitor at the 2nd base of the pair of 2N3904's. I increased the value from my previous suggestion after doing a simulation to 22nF. Now the gain at 100 KHz is zero. With my original value of 10nF there was still about 1dB gain at 100 kHz.

post-9230-14279143550297_thumb.gif

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Here are some pictures showing the 100 kHz noise pulses into the VCO tuning line.

In the first picture it is not easy to spot the problem. The scale is 1mV/div

The next picture shows the pulses after I averaged it 64 times. The 100 kHz pulses can be clearly seen.

These pulses are very narrow. You won't even see them on a 20 MHz BW scope.

post-9230-1427914355063_thumb.jpg

post-9230-14279143550688_thumb.jpg

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Hi AN920,

Yes you definitely rocked the design.

As i have  finished making few PCBs,  i may have to modd them manually.

One observation -  you may  replace the modified schematic by changing the term Vcc on the collector of the modified darlington to +9V Dc .. this would be more appropriate after such nice improvements suggested.

Sarma

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