Help simulating crystal in RF transmitter circuit?

Thread Starter

procnias

Joined Jun 2, 2016
9
Hello,
I'm a wildlife biologist attempting to understand and modify a "simple" RF transmitter created for use as a small-animal telemetry transmitter (see my previous 2 posts here and here for details of the original design). I have been attempting - unsuccsessfully - (for a number of years) to simulate this circuit to better understand how it works, and to modify it to work as a MCU-controlled transmitter, and could really use some help with troubleshooting the PSPICE model.

I am aware that simulating a crystal oscillator is tricky, but that it can be represented relatively well as a shunt capacitance and RLC circuit in parallel. I have found a forum post about simulating a 27 MHz crystal here, but it is unclear exactly how the R, L, and C values were obtained. Since the circuit I'm trying to simulate has a 48-49.5 MHz crystal (with a filter that emphasizes the third harmonic), I'll need to modify these R, L, and two C values with something moderately realistic for this frequency of crystal (of course, all of these values are not provided by the crystal datasheet; I'm using a 48 MHz NX2520SA). I'm not looking for anything too exact here, I'm just hoping to get the circuit to oscillate and pulse (i.e. RF pulses controlled by the RC circuit of R1 and C1 in the attached diagram) in a way that vaguely resembles what the circuit does in real life.

Any thoughts?

Many thanks!
-Julian

*Note that the values I have for CS, CL, RL, and LL are based on the 27 MHz crystal forum discussion and are therefore wrong for this circuit! As recommended in the referenced forum post CL has been set with an initial condition of 5 kV to "jump-start" it.

**Also, you might notice that I'm using a Q2N2222 NPN BJT as T1 here, not for any reason other than that the RF BJTs I'm using do not have PSPICE models available. If this is likely to interfere with the basic functionality of this simulation this is an issue that will need to be resolved, but I'll otherwise cross this bridge when I get to it.

Tx.jpg
 

MrChips

Joined Oct 2, 2009
27,711
Why bother to simulate?
I have no experience nor confidence with simulating such a circuit. I would build the circuit and take it from there.
Actually, from the looks of it I doubt if it would work.
 

Sensacell

Joined Jun 19, 2012
3,098
Simulation = waste of time here.

Way too much significant parasitic capacitance and inductance from real-world layout for any simulation to mean anything.
With this kind of RF stuff, stay in the real world.
 

Alec_t

Joined Sep 17, 2013
13,232
Much as I enjoy playing about with simulations I agree with Sensacell. With capacitance values of only a few pF and inductance values of nH, these are likly to be swamped by parasitic impedances presented by pcb traces.
 

MrChips

Joined Oct 2, 2009
27,711
You can model the RCL characteristics of the crystal but it still will not oscillate in the simulator.
That is because the model does not take into account the mechanical vibration of the crystal
 

Thread Starter

procnias

Joined Jun 2, 2016
9
Thanks everyone for your input!

It sounds like the consensus is that it's a waste of time to try to simulate this circuit. I agree that real-world parasitic effects are likely to invalidate much (though not all?) of what I would see.

My primary goal (as I've stated in my other forum posts) is to modify this circuit so that I can control the pulses with a microcontroller rather than depend on the RC timing circuit. When I do this in the real world the harmonic filtering stops working (the original circuit produces 80% of radiated power on the third harmonic, 15 and 2% for the 2nd and 4th harmonics, versus a strong emphasis of the 2nd harmonic and almost no radiated power on the 3rd harmonic in my modified circuit). My intended purpose in simulating this circuit is to better understand how the "pi-like" filter of L3, C4, and C5 works (especially how it responds to changes in parasitic capacitance when a low-side MOSFET switch replaces R3 [R3 can have a value of 0]). Replacing C and L values in the filter one-at-a-time has not been fruitful, so I am trying to find a way to understand how the circuit's RF filtering works from either a theoretical or simulation perspective; I have not had great luck with the former approach.

The creator of this design has simulated the circuit successfully in PSPICE (though, frustratingly, no longer has the model). His discussion of the simulation results of the above circuit is below. This excerpt makes me think that simulation could be useful in evaluating a number of things including the sensitivity of the tuning elements, which is essentially what I'm after.

P-Spice® simulations revealed that a simple two-stage design (i.e. an oscillator circuit and a coupled amplifier/multiplier stage, e.g. the design suggested by Kenward, 1987) is very difficult to realize with a minimum number of components. Although all simulations predicted considerable radio frequency (RF) power of up to –17·dBm at 1.55·V for the oscillator stage, a model circuit further improving the radiated power with very simple amplifier stages could not be found. In particular, the tuning of the amplifier stage was so sensitive that the simulation results offered little prospect to realize such designs without accurate tuning. Giving priority to minimum mass and volume of the electronics, we therefore abandoned the evaluation of two-stage designs. The favoured circuit diagram is given in Fig.·1.
Given that I've now tried all three methods at my disposal to understand how this circuit operates (theoretical, empirical, and simulation), I'm pretty much at a loss. I'd appreciate any input about the best way to proceed to get this tag working as a simple OOK transmitter.
@DickCappels, I have seen this application note, but was hoping to avoid having to empirically measure the RLC characteristics of the crystal. That said, I think I could do this with my setup.
 
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