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Wednesday, 22 June 2011

A most Diplexing Mystery

Well, finally a day when I can spend some time at play here in the shack; the next part of the project has to be the beginnings of the receiver front end. So, in true form, and trying to stay faithful to the articles in RadCom Homebrew, I've moved on to the 1st RF mixer and the diplexer that follows it. The basic concept of a diplexer is really a band pass filter. Post the RF mixer there will be a mix of the RF input (through a bandpass filter from an antenna) and the LO. My LO isn't yet built but I have a DDS VFO here which will do just nicely. My IF is already set at 9MHZ, but the RadCom articles are assuming an IF of 10.7 MHz so I have some maths to do.
There are loads of on-line references to diplexer design, but I found this to be a particularly good starting point:

and more specifically, this:

I'm wanting to build a Bridge-Tee RF Diplexer, a la:

and I now understand that I'm looking for a reactance for C1 and L1 of about 250R and for C2 and L2 a reactance of 10R at my chosen frequency of 9MHz.

Using my basic maths, and also this great on-line reactance calculator:

I've come up with values of C1 = 70.5pF, L1 = 4.42uH, C2 = 1760pF and L2 = 177nH.

When I wind coils I always tend to measure them out using my AADE LC meter:

So here I have wound 4.5uH when the theory suggested that 33 turns would give me 4.4uH, but it's close enough. I've built the diplexer here, and included some trimmers hopefully to peak the resonance:

And as per design, it peaks nicely at 9MHz. So post 1st RF mixer we should be able to bin all the other junk thats generated and have a clean (ish) signal. This is my homebrew signal generator:

Apart from suffering from the worlds brightest power light, it works very well. I stole the design from here:

I'd like to add a second output at higher amplitude, but that's another project for another day.

As per normal, chief technical assistant has been close to useless today:

Fun, egh?

Monday, 20 June 2011

Not low enough?

I've been testing the Low Level Amplifiers from my last post. There is something very strange going on! The first amp, from the RadCom article, seems to have some pretty good gain:

The input signal is at 14.14 MHz and an amplitude of about 300mV, here the output is clearly looking OK and has an amplitude of about 17V pk-to-pk. That's a gain of 20 log (17/0.3) which is about 35dB - exactly on the nail with the design gain. However, as soon as I hook another scope probe on the input to compare the two signals, the circuit seems to oscillate and some of the gear in the shack goes a bit nuts. The signal generator display starts to ramp up and down frequency as does the voltage display on my bench PSU, I suspect there's just too much RF flying about. This is the ouput signal with no scope probe on the input:

and this is the display with both probes attached:
Looking at the second circuit, the one from here:

The whole thing seems much more stable but I haven't got anything like the gain I expected. Also the transistors are running hot, hot, hot!

The output is only about 3V pk-to-pk which is only 20 log (3/0.3) - 20dB gain.

Here are the two circuits as they sit under test:

Confusing, egh?

Sunday, 19 June 2011

Low Level Amplifiers - how low can you go?

Well there's not been a great deal of progress within the shack recently, chief technical assistant is of little or no use most of the time, and he does insist on sitting on the most sensitive bits of RF engineering. That plus the fact that my job has been getting rather badly in the way of play time means I've made little or no progress recently.

I've been trying to make the Low Level Amplifier as part of my ongoing attempt to replicate the SSB transceiver being built in the Homebrew articles in RadCom. This is the schematic of the LLA, published in Feb 2011:
My build of the circuit is here:
Now, if anyone is actually paying any attention to this blog of mine, or anyone happens to come across this post, I'd be grateful for any comments on my construction pictured above. All I am finding with this LLA is that it oscillates at the slightest whim. I've started to blame my RF signal generator (yes that's homebrew also), and various other aspects including the capacitance in my x10 scope probes. But I've just decided to find an alternative. Now the author of the fantastic homebrew column also has a website, Ed - EI9GQ has an alternative circuit on his site here:

So I've built that one also:

I'm planning to apply power to this tomorrow so will keep my blog updated with progress.

I keep trying!