This morning I finished up my beacon power amp by putting in the low pass filter. I had a bit of a struggle with that however, as I had no capacitors that would work for the 5 pole filter in hand. I ended up with some rather significant changes in values in my substituted capacitors. The filter modeling still showed that I would barely meet a 40 dB supression of the harmonics, so some additional work is needed here before I can call this complete.
The final breadboard looks like this:
The Propeller Beacon with it's outboard low pass filter is putting in a nice clean signal to the input.
Checking the output with the spectrum analyzer, I can see that the harmonics are 40 dB down or better (but just barely).
So, hooking this up to the matched antenna and measuring the power output, I got found that I was putting out a solid 2 watts.
This is pushing things quite a bit and the transistor finals were getting quite warm. I backed off the bias on the driver until the power dropped to about 500-600 mW.
It seems happy at this level and the entire amplifier draws about 160 mA at this setting at 12 V.
So, we let a WSPR transmission happen on the next even minute and were immediately rewarded:
So, I am quite pleased with the initial results. I have (subsequently) changed my power setting reported to correctly reflect my new power level. More work to be done to clean up the harmonic supression to have some more margin. I am seeing some fluctuation in SWR that appears to be related to the effects of harmonics, so more work to do, but for now we are on the air at around 27 dBm.
Um... Ok... I lied... It is just plain ugly construction...
But, hey... We are trying to prove a concept, right? :) Well, time to see if I can let the magic smoke out of it... I have not built the output 5 pole filter yet, there will be time for that once things check out a bit.
I hooked up the 12V current limited supply and with the current limit just above the minimum current shutoff, slowly turned up the voltage to 12 VDC and there was no current draw. So far so good.
Next I hooked up a dummy load, scope to the output and my beacon on the input. Here is the lashup:
I had set the bias pot all the way down. With the beacon supplying about 1.2 V of signal to the input, I slowly adjusted the bias until the finals kicked in. There is a noticeable threshold effect from the unbiased output stage, but it kicked in at about 2.1 VDC of bias. I ran it up to around 2.8 VDC and the current limiter kicked in (remember it is set to turn off at a very low current value. For the moment I have bias set a 2.75 VDC and am seeing this on the output. Remember there are no low pass filters yet in place and the bias is just rough set. Pretty ratty looking, rather terrible really but about 5.5 V P-P into 50 ohms. There is no skipping a low pass filter on this device.
This is would represent about 75 mW, given the unfiltered output. There is quite a bit of energy going to harmonics with this waveform and we really have not explored the bias setting or allowed the entire amp to consume more than about 6 mA, so the finals are not even getting slightly warm yet. More testing to come.
So, it's time to build a low pass filter and do some more adjustments and performance tuning.
Note to self: Gorilla glue sucks... Don't use it for breadboarding circuits...
While I have enjoyed the few WSPR spots I have received:
I have received no QRSS or Opera spots at all. There are times when I think (wishfully) that I see my 5 mW QRSS trace on remote grabbers, I have not been able to definatively say that I see my signal. So, I think it is time to bring the power level up to a more respectable 10-20 dBm.
I have been scouting around for some interesting designs that I might actually have parts for. I have a boatload of 2N7000 devices, so my focus has been in this area. After some searching I have decided to try out the following design taken from the Wee Willy DSB rig described over on the "popcorn" site.
Don't know how well this is going to work out, but you never know until you try it, so off I go. I am going to build this dead-bug style with a little Manhatten style thrown in for good measure. I even went out and picked up a fresh super-glue tube. Unfortunately, I got it home only to discover that it is a solid mass. Rats...
So, I had to resort to Gorilla glue, unfortunately. It will elongate my building time a bit as it takes a while to dry. But we are building again after a long hiatus.
So, I will post the build progress and performance results as I go. Should be fun!
