Mixing Better than Before (70MHz)

Well, following on from my efforts at a 70MHz "transmit converter" here:

http://g0mgx.blogspot.co.uk/2012/06/even-more-4m-or-70mhz.html

I've made some of the changes I said I would do. The main two things I have done are re-tune the local oscillator for the 60MHz harmonic rather than the 80MHz harmonic that I was using previously and also re-made the board thinking about layout and symmetry where required:

So now I have a 60MHz oscillator mixed with my 10MHz frequency reference I made here:

http://g0mgx.blogspot.co.uk/2011/07/ocxo-oh-you-mean-ocxo.html

to make 60 + 10 = 70MHz signal. As you can see from the output on the spectrum analyser, the signal is now really much cleaner than before, even the 60MHz oscillator is at least 45dB down. I always like to remember here that 6dB is half voltage, so 45dB down from the 70MHz signal is a really good result.

So what I need to do now is swap the 10MHz frequency reference for the DDS from here:

http://g0mgx.blogspot.co.uk/2012/06/its-been-ages.html

and check that the bandpass filter at the output from here is wide enough for the parts of the 4M or 70MHz band that I am interested in.

The theory here is that a 10.0 to 10.5 MHz input signal should give me a 70.0 to 70.5 MHz output signal - if (and it's quite a big if) the bandpass filter will let that range of frequencies through.

Once thats all sorted I need to think about an RF amplifier for the output and an attenuator for the 10MHz input which eventually will come from one of my HF radios. The RF amplifier will be designed around an MRF151 power FET (because I have some), so that's something to start the next round of fiddling with, I also have a neat 48V switched mode PSU that should do nicely, now maybe I can make something for 6M also....
I even managed to be here and work someone on 4M the other day using the modified FT-847; it was SP6GWB, Stainslaw. He's got a really neat site here:

www.sp6gwb.pl

but thanks for the QSO! My first on 4M....

More madness to follow I'm sure, but it's good though, egh?

Even More 4M or 70MHz

I've been fiddling in the shack today with an oscilator I started back here:

http://g0mgx.blogspot.co.uk/2012/06/4m-70mhz-and-counting.html

It's basically an 20MHz crystal oscilator that's multiplied up to 80MHz. I've then fed that into a mixer just like the one used in the BITX here:

http://g0mgx.blogspot.co.uk/2010/12/bitxing.html

And mixed it with the 10MHz reference oscilator I made here:

http://g0mgx.blogspot.co.uk/2011/07/ocxo-oh-you-mean-ocxo.html

The idea being that 80MHz - 10MHz = 70MHz.

There are a couple of amplifier stages in there followed by a (my attempt at) band pass fiter. The iportant thing with mixers is that you get the sum, the difference and all sorts of other harmonic products out, so you have to really filter out the one you want else you just get a real mess!

It all looks rather badly built and like this:

But the output on the spectrum analyser looks like this:

So you can see that the 70MHz signal is the main "feature" in the ouput (It's 10MHz per cm from left to right where the far LHS peak is 0MHz and it's also 10db per cm vertically).

I think I will re-make this with consideration for all the bits and where they need to be; then I'll think about seeing if this will make a transmit converter for 4M.

Good fun, egh?

More WSPRing and QRSSing

Well,

After my last ramblings here:

http://g0mgx.blogspot.co.uk/2012/05/back-to-real-homebrewing.html

I've been doing some travelling with work, so progress is not so good. However, I have entered into a dialogue with a number of folk over my observation that the amplitude of the DSS reduces as the frequency increases. Some suggestions to overcome this mainly include AGC type circuitry or utilising pin 12 of the AD9851 to feedback a voltage to set the output amplitude based on a sample of the output (recursion see recursion). The most interesting feedback that I got, however, was not that the amplitude was dropping with an increase in frequency, but that my scope wasn't reading the signals correctly. The theory being that as the frequency increases my scope sensitivity drops off....

I ended up plotting this graph:

This is implying that my scope starts to take a dive, accuracy wise, after about 1MHz, this is the blue line with the y-axis indicating how many dB my scope is "deaf" by. The yellow line is the same signal but through the on-board low pass filter from the DDS module. This seems to imply that the LPF is rather badly designed also. If this theory is correct I need to subtract my scope deafness from the drop in signal out of the LPF. If at this stage we remember that 6dB is half voltage, this is not insignificant!

All rather confusing? My scope is a 100MHz rated fluke which I thought was supposed to be a good quality instrument. So, is this behaviour typical or is my scope a pile of dingos kidneys? I wonder...

I've made a bit of progress boxing the WSPR and QRSS beacon project:

So far, I have tried a number of output amplifiers, the first based on the circuit in my original QRSS beacon, from back in December 2010:

http://g0mgx.blogspot.co.uk/2010/12/qrss-beacon.html

This gives me a nice clean 4v P-to-P voltage out, but I wanted more than that, so either I add another stage or do something else.

I have also tried an output amp based on the Analogue Devices AD8008, this works OK also. I'm off on my travels again tomorrow, so I'll pick this up again when I get back.

All good fun though, egh?

RadCom Dec 2010 SSB Exciter

I saw the SSB exciter in the Hombrew column of RadCom Dec 2010 and thought I'd have a go at building it. I had a rummage in my components and found a 9MHz Crystal Filter and two suitable oscilator crystals; 8.9985 and 9.0015, so this seemed like a good starting point.

The first item I built was the Mic amplifier:

I built this using vero board; the output seems quite clean and tidy, and I make the gain about 18dB:


I've built the main part of the exciter and I'm seeing OK carrier suppression through the balanced modulator, however I think this can be better. I've used some BAT85 diodes in the modulator, but I didn't match them first, so I think I need to match some more and see if I can suppress the carrier some more. This image is taken from the spectrum analyser with a 1.5KHz tone injected into the mic amplifier. We should expect to see the carrier plus a signal at +1.5KHz and -1.5KHz (they are the upper and lower sidebands). The balanced modulator should be adjusted for minimum carrier at the output:



The Upper and Lower sidebands aren't quite the same amplitude for some reason, but the carrier is some 30dB down in the noise, so that will do for now. The output of the Crystal filter looks like this with USB selected:



And like this with LSB selected:



So, I need to try some different diodes in the balanced modulator, then I need to try a range of audio frequencies to try and figure out where the skirt of the crystal filter is to ensure that the carrier and the unwanted sideband are set in the correct place for maximum rejection.

Here's the board as-is:



So once I'm happy with the SSB exciter I need to mix this RF output with a VFO and try and get a signal on the Air!

I'll post more when I've figured out the best band to aim at.

Mark.