LT Spice Simulations

Playing around with LT Spice the last few days. I ran some simulations for the Rx circuit of the LBS Bilateral Amplifier.

I was mostly interested in the impedance of the input transistor, 2N3904. The red trace is the result of the base current * the base voltage.  The result is around the 21k ohms. Quite a bit higher than a measured value obtained by another ham with his circuit.  I've yet to measure my circuit for a comparison.  I also need to confirm the simulations with my actual circuit.   Hopefully they are close so I can have some confidence with the spice simulations.

I also got a lot of help with the transformer model from the LT Spice Yahoo Group. A lot to learn. Good thing I'm retired.

LBS Update

Spent a huge amount of time working on the pre amplifier. Between reading up on how to calculate impedance's for the input and output of the pre and bilateral amps, along with determining turns on cores to make the transformers and learning LT Spice, it has been quite an exercise. While I have a better idea on matching transformers, I still will need to go back and study this topic a little more.

Re-winding the transformer has made all the difference. Sweeping the original transformer with my SNA, showed a flat response across the 40 meter band, good, but an swr of a little over 3.  The core is a FT37-43 with 16Turns for the primary and 2 Turns for the secondary. The secondary also showed an impedance of 23 ohms.  The new transformer works quite well, so that good. We are on the right track. A sweep showed again a flat response but a little less loss, and the swr went way down into the -350 range.  I never thought about a negative reading for swr.  Considering the performance of the transformer, I guess its a good number.  Testing of the core surprisingly, still has the impedance quite low.  It now measures about 22 ohms.

Here's a video of the pre-amp doing its thing. The station is from the US Virgin Islands. I switched back to the pre amp mode just before the end of the video.

For the audio end of things, I think a different speaker is in order. I have distortion when turning up to a level of volume needed. I also have a small amplifier board ready to go to replace the original audio amp, but I think I'll change the speaker first.  Now just have to find one. 

LBS Pre Amp Update

Quick update, I tried 6 turns and 10 turns for the secondary. Not much difference if anything. I need to think about why there was no difference. Now I'm wondering if its the cheap 310s I have.

I do have 1 smt 310 left and I suppose I could try that, but another day.  Time for a cool one.

LBS Pre Amplifier

Spent all day on this board.  Works much better than the breadboard version. However its still not good enough.

I can hear signals down into the -80dbm area now where as previously I was in the -50 range.  The amp comes up to almost the same level as the existing Rx bilateral amplifier.  So we are getting closer.

The previous booster amp that I used had an output transformer with 10 turns for both the primary and secondary. I'd like to try this out in LT Spice, but I'm having trouble getting a J310 spice model and I'm not sure how true to the 310 is the one model presently in spice.

This amplifier uses 16T for the primary and 2 for the secondary.  So increasing the turns on the secondary should boost the output somewhat and may even be a better match for the next stage.

LBS Pre Amplifier

Have spent the better part of 2 days working on this amplifier.  The only thing that i can say for sure about it is that I let the smoke out of a resistor. To be honest, I can't believe this is the first time I've done it on this project.  Not sure why it fried, although I was driving the amp hard when the smoke released.

My first breadboard circuit was a total loss.  I had bought a "cheap? bread board on eBay and that had lots of space and terminals for power feeds. When I got it I was surprised that the power connections went nowhere but figured for the price of it, not to worry. I never used it until the other day. A vast amount of the wire connectors on the board are not suited to the jumper wires that normally come with breadboard kits.  However I did notice after I took everything apart, that the jumper wires you get that have the ends finished off with molded black plastic fit much better. I got those off eBay when looking at Arduino accessories.


The amplifier contains a 2 transistors one of which is a J310.  Yes I got them from eBay and i have read that there maybe issues with the performance of them. That could be the problem I was having so I switched to the smt version.  I had, note the term had, 3 smt j310's that I acquired a few years ago when building up my on hand supply of smt components. Broke one, lost another, and on the last one.  It didn't change the results I was getting, so I've decided to  etch a board and take all the breadboarding jury rigging variables out of the equation.  Besides, it keeps up my Kicad skills.  If I had time I'd put the circuit in LT Spice first, but maybe later.

Here's a picture of the circuit with the smt version of the 310.  Its located a little lower than the burn't resistor at the top left.   The 310 has 3 red wires soldered to its leads.

LBS Update

The change of pace must have helped a little.  The wort is in the carboy and looking forward to the first pour in about 2 weeks.

I was able to repair a booster amp, I had the phasing incorrect on the transformer, and make a big big improvement in the rx function. It's not good enough though, so another amp is definitely required.  I also can now see that an AGC circuit is also necessary.

Now that I have the rx working, more or less, I think I'm going to spend some time with my scope and see if I can improve my measuring skills.  I'm not ready to blame my tools just yet.  Its the low level signals that I am having trouble with.  I sure hope I don't need another scope, but the colour displays do look nice. hihi

73, Peter