I'm not new to electronics but to vhf comm. I want to build small circuits: LNA, power amp to 10 watts, etc. At the moment between 10MegHz and 150MegHz.
What are the standard parts, cheap but effective, preferable with spice model availability (I'm doing much with LTspice)? Transistors, ICs, coil-suppliers, suppliers in general for small quantities
Please no triodes etc. I like more SMD ;-) Lists somewhere in the net? Japanese parts?
I think such lists will be of interest to others too!
Yes Jim! Im not very experienced there (Digital designs/layout is my profession) - but for example I designed a LNA and got simulations results (even for noise performance) very similar to a real circuit built from a ham with great background (found later on the net). Even a Synchronous Oscillator for 145MegHz seem to work with LTspice. I work on it today trying to modify it to an fsk modulated power oscillator to keep component count to the lowest.
Why you ask?
- Henry
"RST Engineering (jw)" schrieb im Newsbeitrag news: snipped-for-privacy@corp.supernews.com...
VHF traditionally begins at around 6 meters, but design techniques there are not much different from those used at 10 meters. The point where you need to begin being more careful regarding circuit layout is around 2 meters. Many devices that operate on the lower shortwave bands still function reasonably well at this region. So... the same analog circuits will work, but WATCH OUT for circuit layout.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)
Never take a laxative and a sleeping pill at the same time!!
Ok Jim. You're asked about the stray components hidden in the real parts/circuit. If you take the values from the datasheets for die and/or package or give it a guess if not available, and add values for the pcb lines, you have a good simulation result. The pcb by itself is not of much interest if you simulate not much over 100MegHz. Even starting a oscillator with thermal noise is possible! Sure, you must know what you're doing :-)
Spice by itself can simulate almost all if you have the right models included - even if you wanna simulate a mechanical system. There is no frequency limit at least to 100GHz.
Do you had bad result doing Spice?
To come back to your question: Spice will give you better results than your real circuit! You can connect a probe with no interaction to the circuit! Try this with your real parts! Don't forget the component variations in real circuits coming from the manufacturing processes of the parts.
- Henry
"RST Engineering" schrieb im Newsbeitrag news: snipped-for-privacy@corp.supernews.com...
There is nothing hidden at all. Strays are a part of the real life at VHF and above. Sometimes strays have more effect than the part itself.
If you take the values from the datasheets for die and/or
Horsefeathers. What "values" do you add for the pcb lines?
The pcb by itself is not of much
As with anything in the world.
The problem is getting the right models, which is an art in and of itself.
Again I ask the question: What does Spice say about a 1000 pf capacitor with 1" total lead length at 150 MHz.? Don't give me the BS about Spice being "better than a real circuit". Until you can give me a ferrite slug on one end of a toothpick and a brass slug on the other end to increase or decrease inductance in Spice, the physical circuit is the "real" circuit.
You have to put the strays into the spice model if you want an accurate result, just like you would have to if you work it out with a pencil and paper.
You can work out the inductance, resistance (including skin effect) and capacitance of arbitrary structures using the free programs FastCap and FastHenry, though these consider inductive and capacitive effects separately so you have to break the problem into several parts sometimes, e.g. if the structure is a significant fraction of a wavelength in size. Constructing the model files can be tedious, and you may end up having to write some code to automate it. You can view the model files using a viewer program which used to be and hopefully still is available from
formatting link
You could also buy one of the commercial full-wave solvers if you have more spare money than time.
Right but you have to do that one way or another anyway even if the model is inside your head, unless you design purely by trial and error.
The real advantage of spice is in situations where trial and error is more expensive than getting it right the first time (where you can justify spending a long time making good models). He does also have a point about being able to probe components inside a circuit that you could never probe on a real one due to loading effects of the real probe. It seems to me that your real objection is not with computer simulation of circuits, but rather with poor models for components. Fair enough, garbage in garbage out, but I would consider using bad models to be a form of 'user error'.
How about dielectric absorption ("soak")? I first encountered this in an electrolytic capacitor being used for generating a sweep of about a minute duration. The capacitor had to be kept shorted for several seconds between sweeps in order for the charge to adequately empty, an equivalent frequency of less than one Hz.
For some of the university work I've done, some of our reference papers are from people modeling power lines as full-fledged transmission lines that are many wavelengths long at 50Hz. No simple equivalent circuit model works for them either!
OK, I did get a little over-excited there -- it should say "that are significant fractions of a wavelength long," (e.g., more than 1/6th) such that you can't use a simple circuit model for them.
When they matter, just poke in the appropriate series L or a bit of transmission line. I use Spice now and then to sim picosecond stuff. Even if the simulation isn't highly accurate, it helps train your instincts, shows you which parasitics will have which effects, so when you build the real thing you have a jump on the complexity.
That's interesting -- SPICE has models for (lossless) transmission lines, and lines of that length can also be adequately modeled with a few LC sections. I can imagine line loss could be tricky, though. I've modeled skin effect loss in the process of designing time-domain circuits to compensate for delay line loss over a wide time range (and therefore broad frequency range). But loss due to interaction of the fields with ground underneath the wires might be more difficult. At least you've only got one frequency to deal with -- unless harmonic content is high enough to worry about. Perhaps you've also got to deal with loss due to radiation?
Agreed, John, 150 MHz is damned near DC for a lot of us, but as yet I have no answer from the "spice" folks for the 1 nf capacitor question. A lot of fancydancing but no answers.
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