Why/how is this voltage divider increasing the higher frequency gain ? - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: November 16, 2023, 07:54:09 am
content: I'm making a 5.2MHz low power AM TX, and from the oscillator, to the diff-amp modulator output, its working at it's best so far, with -6dBV at 5.2MHz, and -45dBV at 15MHz, 10MHz is at -47dBV. So that's as much separation as I've been able to get. And it modulates pretty well, with 1.5Vpp carrier, about 1.3V audio gives ~2.85Vpp, 1.4Vpp audio looks a bit too much.That part is soldered up on a protoboard. I was using the breadboard to see what worked best for coupling between stages.And I just notice that if I put the output signal above, through any size cap, into a resistive divider like 100k then 1k to GND, or 10k then 10R to GND.....at the divider point, my 5.2MHz drops from -6-7dBV still on the other side, to say -32dBV, but the 10.4MHz is there at 30dBV, and 15/20MHz are now upto -38dBV.I can't get LTSpice to do the same thing, I didn't try adding GND probe inductance, I'm guessing at 10-20MHz it's still no big deal ?But what's going on? The little bit of improvement in the circuit I got from adding filters, all undone by a voltage divider with a probe and some breadboard capacitance ?

author: moffy
author_rank: Super Contributor
timestamp: November 16, 2023, 08:11:20 am
content: You could have some parasitic capacitance bypassing the 10k resistor hence the increase in higher frequencies.

author: MathWizard
author_rank: Super Contributor
timestamp: November 16, 2023, 08:37:13 am
content: Hmmm I still have a parallel LC shunt circuit on the breadboard, a few inches away. The breadboard only has 1 gnd wire hookup up to a bus.That tank was between the RF amp and the diff-amp. When I couple that with only a cap, then the 10k:10R output of the divider is -47dBV at 31MHz, -50dBV down to -60dBV for the rest. Ok maybe some other time I'd want that effect, but not here.Now matter what I do, I can't seem to get under -60 -65dBV of anything on this circuit. I should try it off a battery.I tried some calculations for different L/pi/Tmatching networks, that was fun, but I think my BJT amp's are way over powered, or my signals are way too big from the osc, for what the amps could do.Just the most basic all 100nF cap coupling, was around -10dBV/-45dBV for 5 and 10MHz, and that was as good or better, than any of the the filters I tried, for an already strong signal. But they did work for 20mVpp signals from the sig. gen. They matched power properly for small signals in LTSpice too.In the end I just used some caps, and 2 BP filters, between the buffer/amp/diff-amp. And didn't use any calculations besides the tank circuits.

author: moffy
author_rank: Super Contributor
timestamp: November 16, 2023, 09:21:32 am
content: Quote from: MathWizard on November 16, 2023, 08:37:13 amIf it's one of those plug in breadboards they have high parasitic capacitance. I've given up using them, ended up tracing more problems introduced by the breadboard than the actual circuit. The supply lines also tend to be inductive, lots of ringing even for moderate switching currents. Then there are all the connection issues ...

author: RoGeorge
author_rank: Super Contributor
timestamp: November 16, 2023, 09:31:36 am
content: In practice, electronic components are never pure resistors, pure capacitors, etc. Each component has a parasitic inductance, parasitic capacitance, thermal noise, nonlinearities, etc. All these effects vary with frequency. Even worst, for big enough frequency, the parasitic properties of a component may become predominant.Here is a random example for the equivalent schematic of a resistorhttps://electronics.stackexchange.com/questions/472217/equivalent-circuit-of-a-non-ideal-resistoror a capacitorhttps://www.researchgate.net/figure/aFull-equivalent-circuit-of-a-real-capacitor-and-b-simplified-measurement-equivalent_fig3_3050663Depending on which behavior a model wants to catch, they can be modeled many ways, not only like seen in the schematics from those links. There is no definite model to perfectly describe a component. There are only 'good enough' approximations. Nothing is ideal. Reality is always messy, and not only in electronics, but in everything.For example, apart from the intrinsic parasitic properties, there are also parasitic capacitances between nearby components, parasitic inductance in wires and component terminals, stray voltages from the surroundings, etc. All these internal and external parasitics are distributed like a "mud" inside the physical circuit and vicinity.In schematics we draw only lumped components, but in reality nothing is a pure/ideal component concentrated in a single place.In LTspice, there are options to attach parasitic properties to components, either in the component model, or by drawing the parasitic components separately.As for the real life measurements, stray signals can be induced in the probes, or travel through the ground loops or through air as electromagnetic fields, and that may induce unexpected values in the measuring instrument, or in the test circuit.Usually all these parasitic effects are small enough, but the higher the working frequency, the more visible they become, up to the point where the normal functionality of a circuit is overwhelmed by the unwanted parasitic effects.

author: jonpaul
author_rank: Super Contributor
timestamp: November 16, 2023, 10:15:35 am
content: Never use breadboard with metal strips for RF work. High shunt C and coross coupling.We used to make a fiberglass perf board with solid wire or copper tape.Had good resulst to 10..50 MHz.See ham radio sites, ARRL.ORG, QST, etc.Simulations are GIGO: Garbage in = garbage out.Pls show pix of the built ckt and exactly how you will measure/probe the in/out/nodes.J

author: MathWizard
author_rank: Super Contributor
timestamp: November 16, 2023, 02:41:19 pm
content: Well dealing with just a few RLC's is bad enough, so far I don't include any RLC parasitics (except DCR of inductors), nor do I know them all yet. So it seems a bit like magic. Removing that tank from the circuit, does change the output in the simulator too, soon I'll see if it's ok to use on the protoboard, I'm thinking it's still going to happen tho.Only lately do I include some ESR in some very small caps in LTSpice, so stuff like the output voltage from my oscillator isn't so unrealistically big.I have Copper clad board. Lately I bought some dremel bits, to try making more stuff on those. But I never bothered with it yet.I see the few other circuits I built on cheap C-C-board, have already started to corrode. And I remember seeing lots of pic's of homebrew RF circuits on bare Copper clad, and used for shielding too. IDK if they put some clear coat on them, or it's thicker copper, or what.