Merge of VK2DX Captures

Early on in my membership to the 600m Yahoo Group I was introduced to the idea of merging multiple spectrogram captures together to sharpen the received signals. With VK2DX now having transmitted for about 24 hours on 2200m I thought I'd try my hand at it.

I simply used about 6 captures and combined them in layers in GIMP and set the layers up as 'multiply'. It was a bit of a rush job, but I'm happy with the result. I recommend having a play yourself. GIMP is free and so are all the capturing tools, so all you need is a signal and some time. ūüėČ

(Image 1 is before, 2 is after the merging.)

Spectran-capt-0011Spectran-capt-0011-layers

WSPR 101 Presentation

Recently the local club CRARC asked if I'd be interested in giving an 'Introduction to WSPR' presentation. I thought, sure, why not! I enjoy WSPR, so why not share what it's all about with others.

So I gave the presentation tonight, and it seemed of interest, so that's good. ūüôā

Anyway, here's the presentation – feel free to download for your reference.

CW Stage 1 Complete!!

Screen_shot_2011-07-14_at_11

Alrighty, I never thought I'd get there (well, that's not entirely true) but I've apparently committed the morse characters to memory. By that, I mean I've just now completed all 40 of the Koch lessons on LCWO.net. It's taken me a couple of months of almost daily practice (but at least a week off once) and now I'm there.

Originally it starts you at a speed of 20 wpm character speed, but with a character spacing of 10wpm. I tried to stick at this for as long as possible, but eventually had to drop it a little. Then at the last CRARC club meeting where there was a presentation on CW, it was suggested that I should probably reduce more and just focus on getting the alphabet down and then increase. So after this I decided I'd go for 16 wpm character speed and 8wpm spacing.

I held that for awhile, but finished just now on 16wpm character speed and effective speed of 6wpm. So now, I'll start bumping up that effective speed and try and get it to 1:1.

Guess now might finally be time to consider getting a key so I can move on to sending.

Classes of Amplifiers – A thru F

I'm still pondering the output power of my transverter. It's designed around an E class PA in theory, so it was time for me to learn a few more things. For your Advanced license you're only required to be familiar with the basics of A, B and C classes of amplifiers. However, out in the real world people also regularly consider D, E and F.

So before I post up about some findings with my transverter, here's a run down (by a newbie) on the various classes of amplifiers.

First, you can split the six classes into two groups:
Linear amplifiers: A, B and C
Saturation amplifiers: D, E and F

The term linear here is used in a slightly different fashion compared to when people refer to A being linear and B and C being non-linear. Here, were simply meaning linear amplifiers operate for the key part of their time in the linear region of the amplifying device (BJT or FET). Conversely, the opposite to the linear amplifiers are those that focus on operating from cutoff to saturation – sometimes also as a result called switch mode amplifiers.

With linear amplifiers the main differentiation is the duration for which there is current flowing from the output:
A – 100% of the time
B – greater than 50% of the time
C – less than 50% of the time

That's all pretty simple. For saturation amplifiers they're primarily classified based on their output network. But before we get on to that, what exactly do we mean by saturation amplifiers?

Classes D thru F rely on three key things for their input/driving waveform:
1. A square wave (ideally);
2. With minimum voltage just below the active devices threshold – so for an IRF510 FET the Vgs(th) is 4V so commonly the minimum voltage is 3V; and
3. A maximum voltage just past the saturation point of the active device – so again, for an IRF510 FET this is 8V

In this scenario then, you can have a 5V Vpp square wave set with a bias of 3V driving an IRF510 FET and you'll have either a class D, E or F. Which of those it is, depends on the output network:
D – Treated no different to if it was a class C setup (the key here is that the way the amplification device is driven is different to a normal class C);
E – The output is made up of a inductor on the drain and a capacitor in parallel so as to form a tuned network (thereby a low impedance circuit) taking into consideration the FETs output capacitance (Coss), all of which then feeds into a normal LPF;
F – This one replaces the LPF with a combination of tuned circuits to capture at least the 3rd harmonic (and potentially the 5th) and then feed the voltage back in phase with the fundamental – fiddly stuff and understandably a bit hard to get going (but apparently used in commercial kW transmitters).

And a reminder that one of the key drivers for the increasing number of classes of amplifiers is efficiency. A class is at it's theoretical best 50% efficient, but normally in the 40s. When you get to C you can get around 70%. With E and F there are real world examples at 90% efficiency and above.

Of the switchmode amplifiers, E is probably the most popular with amateurs. F is great for commercial setups, and D was originally targeted at HiFi systems with a PWM input source. That said though, with D being so similar to C it is considered a good starting place to start playing with these types of amplifiers.

All one needs to get started with class D, is work on generating a square wave as the input waveform rather than the sine wave we normally use. This can be done with various CMOS and TTL chips.

That should be enough for an overview, and being new to this myself there could be several errors. But, search the internet and you'll find far more authoritative sources. For me, I relied on a document by Paul Harden NA5N Р"The Handiman's Guide to MOSFET 'Switched Mode' Amplifiers" (originally published in the journal "QRPp"). I was also hoping there'd be information in the ARRL book "Experimental Methods in RF Design", but there was only a very very short mention that class D and E amplifiers exist Рneed to check the ARRL handbook yet.

With this in mind, I'll post up some interesting bits on why I think my transverter is producing the low power it is.

2200m Transverter Progress

Finally, some real solid progress on my transverter for 137kHz. It wasn’t by my efforts alone, because at the end of the day this is still my first build of any kind of transmitting device – so there’s a lot to be learnt. Thanks goes out to Owen (VK1OD), Dale (VK1DSH) and Dimitris (VK1SV) for their help and tolerance of my spam.

G3xbm_transverter_v2-001

After completing the build a few weeks back, things just weren’t right. First and foremost, the output from the transverter was not a sine wave.

5-transverter_output

From here I ended up focusing where it all started way back at the LO and the attenuator for the input from the FT-817. Basically, the LO and RF going into the SBL-1 mixer were way high. General guidelines seem to be that for a balanced mixer you want an LO of 7dBm and an RF of 1dBm. In it’s original config RF was way up at 13.75dBm and LO was up around 9.65dBm.

From here I reduced the voltage to the LO module by a bit more and got the LO closer to 7dBm (slightly below) and built a new attenuator to offer 25.5dB as on low power the FT-817ND was actually feeding in 28.3dBm rather than the 27dBm (500mW) that is commonly believed. Even then though, the design only used a ~15dB attenuator.

So using the online pi network calculator and some nice big 1W resistors from Jaycar I had a nice new attenuator:

G3xbm_transverter_v2-003

From here, although things were better at the start fo the chain, the output was only slighly improved. So I started to focus on the PA module and eventually on the load inductor (L4) for the IRF510. Basically, seeing this was acting ultimately as an RFC I was feeling that it was letting too much of the fundamental frequency through and as a result the secondary harmonic (254kHz) was becoming too strong and destorting the output.

The design used 40t on a iron powder core (possibly a T-106-2, but there’s a bit of mis-information on the schematic), but this seemed to yield a rather low impedance at 137kHz. Seeing this was not being used for tuning etc, it was early on pointed out to me that using an iron powedered core could be a waste and that a ferrite would do. Well, now wanting to also increase the inducatance a ferrite core seemed the way to go.

After some trial and error, I settled on using an 18mm Jaycar Ferrite Toroid with 20t. This definitely showed improvement (as did the previous attempt with just 10t) and the result of the output waveform was:

Newfile6
G3xbm_transverter_v2-005G3xbm_transverter_v2-006

After this, my thinking led to was there enough load for the IRF to feed into (which in this design, comes in the form of the low pass filter – LPF) and indeed was the LPF doing a good enough job at filtering. After using a few calculators etc. it seemed that the simple 3 pole LPF didn’t really match the frequency and thereby was probably not providing sufficient filtering nor was providing a suitable load.

The plan then came about to replace the LPF with a 5-pole Chebyshev filter in the hope of getting the filter to bring the harmonics to a suitable level and provide a suitable load (hoping to increase the output power). For this I used this online calculator and came up with a design for a 5-pole cheby with cutoff frequency of 150kHz and hopefully a bit over 30dB of attenuation at the second harmonic.

G3xbm_transverter_v2-002G3xbm_transverter_v2-004

The benefits of this LPF were immediate. The resulting waveform was finally the long sort after smooth sine wave and power output even increased slightly. As a result, I’ve finally got a transverter for 2200m!

Newfile0

As you can see, the power is a bit low at only about 1.6W (not the 8W advertised) but it’s a start. From looking at the PA module it looks like it could definitely do with some work (bit of biasing and coupling/buffering at both the input to the PA and output) but I think now I might move onto trying to setup a usable antenna system for 2200m.