23cm Low Noise Amplifiers

I was quietly collecting parts to build a 23cm transceiver for local terrestrial use. I had a NE32684 and loading the S parameters into RFSim99 and using similar matching as other designs produced the following:

Circuit, Gain and Input return loss.

But I wasn't entirely happy with it. While existing designs had enjoyed great success, I was concerned about the stability. It's hard to convey in words but sweeping the frequency slider in RFSim99 showed that while the input was unconditionally stable, the potential existed for sub 1GHz oscillations depending on the load.

RFSim99 - about as easy as it gets to test. Here showing potential for instability with the output load.

Since I don't have a lot of 23cm test gear I decided to see how negative feedback would help. I was surprised to see that even with just a little resistive feedback the potential for oscillation disappeared and input matching with LC networks could be used. That gave me the following circuit:

This would not be optimum from a noise figure perspective, since we are both matching to 50 ohms instead of using a deliberate mis-match for lower noise figure, and we have used resistive feedback which generates noise.

However, on balance I was happy that my front end filter would see a reasonable termination at the image rejection frequency and this would prove to be suitable.

Then I chanced upon two cellular base stations: one at 900MHz and one at 2100MHz. Pulling htem apart and trawling the internet showed they had low noise front ends using ATF54143 and ATF58143 transistors. Both had comparable noise figures to the NE32684 at 23cm, but not as good at 10GHz.

So I decided to spin the wheel again (and again) to see what these transistors could deliver. With a bit of fiddling in RFSim99 I found that the ATF54143 gave me a useful front end RF amplifier:

Yes, it looks wrong.  With feedback between the gate and the capacitor you get quite a different response. What I have here is something with a good return loss and gain from LF to 23cm.  The ATF58143 was not vastly superior at first blush so I put that to one side.

Now I have something that works like a mmic. I know there are a couple of low noise mmics that have good return loss and better noise figure than this "theoretical" circuit, but I didn't have one to hand. So I fired up another software package to see what the noise figure might be. The simulation results were near identical, always a comfort, and the overall predicted noise figure is 1.09dB. I have yet to try to see if this could be improved.

It's time to build a board and add this part of the radio to the front end filter that is now in place. Nothing like building something to prove if the maths was correct.


Update:
I decided not to pursue this. Firstly, it looked wrong and when I checked it still had stability issues. Secondly, I found something better. A GPS LNA which I will write up soon. Board layout for that device is progressing as time permits. It looks like the best device yet.


73's

Solder - further testing of Chinese brands

Well, I tried a few more brands of Chinese solder. One was actually very good but I haven't time tonight to track down a picture for the summary.. I will though, because I will be buying more of it.

In the meantime I do not recommend this brand:

For a moment this looked like a surprisingly good solder.

I was wrong. What appeared to be good joints too often turned out to be dry joints.And this was a thick 0.8mm solder wire. I shudder to think how poor a thinner wire from this manufacturer would have been.

Solder - The Good, Bad and Ugly of Chinese Solders

So I've been really busy for too long now on work matters. One aspect of that was hand soldering SMD boards. As I looked at my dwindling spools of solder I thought it would be useful to order some more. But yikes, the cost was surprising. So I thought I would test a few varieties from Chinese sources.

The results were generally disappointing. Here then is a brief summary of what I found:

 I do wonder how the Chinese can manufacturer so many electronic products without good quality solder. It must exist, but so far I have not found anything that matches what the manufacturers must be using.

I will try a few more rolls from different manufacturers in due course. If you have found something that works, I'd like to hear about it as would anyone else reading this.

73's