Give me your RF knowledge, FM transmitter design - Page 1
Forum section: electronics / rf-microwave

author: Etesla
author_rank: Regular Contributor
timestamp: February 29, 2020, 09:52:58 pm
content: Hi all,I am trying to design and eventually build a stable 250 mW FM transmitter with an adjustable center frequency around 100 Mhz.So far I have come up with the circuit in the attached LTSpice file, but I have some concerns about how the simulation is acting. Please download the model so that you know what parts I'm referencing throughout the rest of this post.Here are my questions/concerns that I would love to know more about:1.) I use the 2n2222 model for all of my transistors, because I have some 2222's on hand. Do you think this model in ltspice can be trusted to a reasonable extent around 100Mhz, or am I kidding myself?2.) Ideally, I think that only C3, C4, D1, D2, and L1 would affect the frequency that transmitter is operating at. In my circuit, its obvious that L2, C7, C8, and the parasitics from the transistors will also play a role in determining the center frequency. Is the impact L2 C7 C2 and the transistors on the operating frequency something I should worry about / is this circuit destined to have uncontrollable drift?3.) As a side note, even changing values for components in the output stage changes the center frequency by a couple of Mhz. The buffer stage seems to do little to help this. If the stuff I talked about in #2 wasn't bad enough, is this something I need to worry about?4.) Are there any other huge inherent flaws with this design? I really want to understand everything I can about it before moving onto prototyping.5.) It looks like the power supply gives the circuit about 600mW, and my estimated output power is around 250mW, giving me an efficiency around 40%. This is after a lot of ideal tuning to get the efficiency as high as possible. 40% seems pretty bad to me and it makes this circuit unscaleable due to the 2222's transistors overheating. Is there anything holding the efficiency of this circuit back, or is 40% about par for the course?Thank you so much for your time!

author: OwO
author_rank: Super Contributor
timestamp: March 01, 2020, 04:25:03 am
content: 1) Yes, the 2n2222 can operate to 100MHz no problem (with ~10dB gain) and spice models are accurate at frequencies this low.2) Any free running oscillator will have uncontrollable drift with temperature because device capacitances change with temperature. Another source of drift is when you attach an antenna, and a portion of the radiated signal feeds back into the circuit. The only way to deal with this is to lock it to a crystal with a PLL. ADF4113 works well for this. However you will need to design an ultra narrow loop filter to allow audio through.3) That is because you loaded down the oscillator too much. If you use weaker coupling from the oscillator and then make up for it with more gain, it will be less sensitive to load pulling.The efficiency is about right for a 2n2222. The issue is the transistor fT is too low so you won't get any class C/E/F action and it'll be a class B amplifier, plus the Vce voltage drop limits efficiency. If you want to "scale" the power up, you need to add one more power stage using a real LDMOS power amplifier. Almost all cellular band transistors (I use mrfe6s9125n) will work but you need to "guess" at the impedance matching. The transistors can be found refurbished for cheap.

author: OwO
author_rank: Super Contributor
timestamp: March 01, 2020, 04:56:26 am
content: I just looked at the circuit attached and have a few comments:- The buffer stage is not doing anything. The 2n2222 does not have enough gain at 100MHz to actually function in a emitter follower configuration. If you probe the input and output you'll see the output voltage is much lower than the input voltage, while output current is only marginally higher than input current. Replace this with a common emitter amplifier with impedance matching. The input impedance of a 2n2222/2n3904 at 100MHz is around 30ohms I recall, and the output impedance is like 100-200 ohms with a bit of capacitance.- oscillator topology is a common emitter colpitts configuration which gives maximum output power but is not known to be frequency stable or have low phase noise. It is not a problem in a FM transmitter if you lock it to a crystal. It seems in this circuit there is too much excess gain because with less loading the oscillator will oscillate on two unrelated frequencies. That is something to watch out for when building.I've made a few random changes to improve the output stage efficiency, but beware that this now actually operates the output transistor in class C mode and is likely to blow the 2n2222 if the output loading is not correct. Also I changed the varactors to bat54 because I don't have the models; you might want to change this back.

author: Etesla
author_rank: Regular Contributor
timestamp: March 01, 2020, 06:12:23 am
content: Is there a different oscillator configuration I could use that would have better phase noise and frequency stability without resorting to a crystal? I don't have any crystals on hand in the right frequency range.

author: OwO
author_rank: Super Contributor
timestamp: March 01, 2020, 06:27:20 am
content: Common collector colpitts is typically used for low phase noise, but output is weak. Most toy FM transmitters you find on the internet are common base colpitts.But that's not very important; the issue is that your frequency *will* drift if you don't lock it to a crystal. You don't need a crystal at 100MHz; use e.g. a 26MHz crystal, and ADF4113 will divide that down to 100kHz, which your oscillator will be locked to a multiple of. For example for 98.1MHz you would use N=981.

author: David Hess
author_rank: Super Contributor
timestamp: March 02, 2020, 02:39:02 pm
content: Low drift LC oscillators are possible however they require selection of the proper components including capacitors with specific temperature coefficients and temperature compensation. I remember some which included varactor diodes and a compensating bias voltage.