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AN920

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Everything posted by AN920

  1. Here is the follow up posted. Also tested with a BFS22A transistor. Work ok, but output slightly lower (~830mW) as this transistor needs a bit more drive being a 4W device. Overall the 2N4427 is the best choice, giving me 1.3W @ 12V and 0.8W @ 9V with about the same harmonic performance.
  2. This circuit was posted on the other board For all those that were asking for more power, well here is a design. This uses the output of the last circuit I posted at about 100mW. The power transistor can be a 2N3866 or 2N4427 and both work well. The transistor runs quite cool and needs only a small push-on heat sink. The values in brackets are my settings to obtain maximum output. This is quite close to my initial design values. On both 0.7W and 1W out the 2nd harmonic is 20dB below the carrier. This may need more filtering not to cause interference. Mount the 3.3k resistor directly over the 1uH choke. Use responsibly!
  3. You can only pull by about 0.2% of the marked frequency
  4. AN920

    FM Radio

    I see the drawing has been modified now. The anode is connected to +5V through the inductor. This is wrong! The diode will now be conducting when the pot is set low enough or less than 4.5V on the slider. It will not function as a varicap anymore. The original diaram was correct.
  5. AN920

    FM Radio

    The cathode side of D1 is connected to +5V through L2 as it should be or is this already modified?
  6. "If you need more power, use my last circuit but with 2N4401's instead of 2N3904's. It can handle a bit more power. Current will go up to around 84mA and RF power just over 100mW. Just remember that this power can get you into trouble in most countries and the 2nd harmonic, although still 30dB below the carrier is now at about -10dBm enough to cause TV interference. You will need some extra LPF to get the harmonics down more. Picture shows results of the 2N4401 version we tested before and after adding the filter. 29pF indicates variable cap set value."
  7. edit: There is another posting on the same topic:- Also it will draw around 53mA at 10mW and about 55mA at 20mW (black trace) so one of these small 9V batteries won't last very long. If you plan to use it on a power supply make sure it is well regulated. And for those interested, the final modification we played with adding some DC feedback to reduce flicker noise in the oscillation device gives 13dBm (20mW) and another 6dB lower phase noise. Overall improvement 100% RF power, -10dB TV band harmonic and 12dB reduction in phase noise. It should be possible to improve the phase noise some more by lowering the 100k feedback resistor but then 2nd harmonic starts to rise. With 82k feedback simulation results give 2nd harmonic at 2dB higher and lower PN(at)10k (-112dBc), PN(at)100k (-132dBc). So you win some and loose some. It is amazing how well the measured results compared. I have uploaded images to show original to final improvement. Average carrier noise (related phase-noise)was measured on a mod-meter for easy comparison. Close-in noise can also be observed on the analyzer display.
  8. Guys, found this on another board. Looks like some analysis and mod of Audioguru transmitter. I attach the file posted there. "Here is a little simulation I did for a friend on the RF section of this transmitter after he complained of some TV interference after building this. Basically I modified the circuit slightly by taking the feed for the final RF amp from inside the tuned circuit, as this helps filtering out harmonics. The coupling cap was reduced 10 times to minimize loading the tank circuit. The osc feedback cap was increased slightly for reliable oscillations. The result is still the same output level around 10mW but the 2nd harmonic is reduced another 10dB. Also there's about a 6dB improvement in phase noise. The simulated results proofed to be close when measured on the actual circuit after modification. The last variation has another extra emitter choke added, resulting in about 50% more power at the wanted frequency and even more harmonic attenuation. Also lowering the value of the 47k to around 33k it is possible to get up to 20mW while still keeping harmonics low. That is a 100% increase in RF power with much lower noise and harmonics as the original circuit. Just thought it may be helpful to others. "
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