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Monday, 3 March 2014

More Mixer Musings


Following my total confusion over the KISS mixer, I decided to go back to basics and make certain my very little brain:

was up to speed and understanding the diode ring mixer as a starting point.

So, I made one of these:

Please accept my apologies for the simply appalling quality of the image above.

Now, the LO looks like this into 50R:

I make that 1.25V peak to peak which is 5.85dBm.

The RF input looks like this into 50R:

thats a whopping 2.36V peak to peak which is 11.44dBm (that's way too big!).

The output of the mixer looks like this (after a lot of fiddling with the 'scope):

So hopefully it's clear that ever time the LO (we could call it a switching signal) is negative the RF input is inverted on the IF output. If you then apply a filter to the IF output we can find a nice clean signal at the wanted frequency.

Now, if I keep the amplitudes as they are above, this is the output of the diode mixer on the SA:

I've put some markers on some notable signals; the 5.85dBm LO signal is visible at the output port at -32dBm. The RF input signal at 11.44dBm is at the output port at -20dBm and my wanted signals are at 1.7dBm. I also placed a marker on the harmonic at 40MHz - that's at -38.5dBm.

If I now add 30dB of attenuation to the RF signal, the output looks like this:

So now, the 5.85dBm LO signal is visible at the output port at -32dBm (no change), but my RF input signal now at -18.5dBm is visible at the output at -42dBm. My wanted signals are at -23dBm and my random harmonic is at -50dBm.

So, back to the KISS mixer confusion:

Let's look at exactly the same input conditions with the KISS mixer configured as per the schematic:

So, here we have the LO signal at 5.85dBm, the RF at 11.44dBm, with these input signals the bias pot has no effect at all that I can see:

and here, the LO signal remains at 5.85dBm but the RF signal is at -18.5dBm, with these signal levels I can make a slight change to the amplitude of the LO signal in the output with the bias pot - I have set it for minimum LO:

What does all this tell me? Primarily that I need to go out more!

I'm going to let those far cleverer than I draw the conclusions.

Interesting, egh?

Friday, 28 February 2014

KISS Mixer Musings


Part of the Minima transceiver I made back here:

was the so called KISS mixer:

Above I have stolen the mixer part from the Minima schematic and labelled the ports. In my test setup I have the LO connected to a test crystal oscillator I had lying about, it's running at 20MHz (ish), the RF port is connected to my bench signal generator and the IF port is connected to the Spectrum Analyser.

Here's what the test setup looks like:

Now, the author of the Minima Farhan gave us some information on the KISS mixer here:

and also a reference here:

Now, here is what I see when I have a play. I have 20MHz LO, 15MHz set into the RF port from my bench sig gen and the spectrum analyser connected to the IF port.

Here's the output spanning 0 through 100MHz with just the LO (20MHz) signal:

This is just my oscillator and it's harmonics - so far so good. Now lets add in the 15MHz signal to the RF port and keep the 100MHz span:

So now we see the 20MHz signal plus harmonics, the 15MHz signal plus harmonics plus all the differences and sums which land at every 5MHz interval. So far so good.

Now, lets zoom in on one of the mixed signals at 35MHz:

Looks good to me.

Now, if I connect my x10 'scope probes to the gates of the FETs, here is what we see:

The 'scope struggles to trigger on anything here, because there is all sorts of frequencies in the signal, however, I think we can safely say that the two signals are the same but out of phase with each other as we would expect.

Now, if I look at the drains (my sources are connected together and the bias supply), we see this:

and if I add the LPF in the scope to bandwidth restrict the inputs:

If we replace the SA on the IF port with a 50R load, then look at the signals on the drains with the SA, here's drain one:

and here is drain two:

Now, in all of the tests above I have tried altering the bias pot and I can see no difference at all under any circumstances. If I look at the voltage at the wiper of the pot I can see 0-5V as expected. However the voltage at the sources read from -0.12 to -0.15V. That isn't what I expected either! If I remove the +5V supply the output doesn't change nor does it if I connect the sources to ground.

The FETs in the KISS mixer effectively replace the diodes in a more traditional diode ring mixer; this chops the RF signal thus providing the IF signal at the centre tap of the RF transformer.

So I replaced the single bias pot with one on each FET, under these circumstances I can alter the amplitude of the unwanted signals in the output but not make any difference to the wanted signals.

In my case the wanted signal is the 20 + 15MHz, here is the mixer output with the two bias pots adjusted to minimise the unwanted signals:

However, if I just remove the 5V line, the bias pots and just ground the two sources on the FETs, here's the output:

And these are the signals on the drains with no bias (the sources grounded):

Any volunteers to explain?

Thursday, 27 February 2014

BITX40 RX Test


I hooked up the bits and bobs needed to test the receiver of my new BITX40 V3B kit, here's what I heard:

Good egh?

Friday, 21 February 2014

There's a whole lot of BITXing goin' on


I've been fiddling some more with my BITX40. I've nearly completed the exciter board and have been playing with the set up of the BFO and the crystal filter. This is where a spectrum analyser really comes into it's own.

Initially I modified the BFO and removed the inductor - the purpose of the inductor is to lower the BFO below the crystal frequency - this is needed in the BITX20 as we need to retain the USB. As I want to keep the LSB in my 7MHz radio, I need the BFO to be higher than in an USB rig.

Here's the carrier or BFO output (in purple) on top of the save I did of the crystal filter sweep - now I don't much like the dip in the top of the Filter sweep, but I can come back to that later.

Now, here the BFO adjustment is at it's lowest frequency and I looked very much to me like this wasn't low enough. Here is the same thing with 1KHz of audio mixed in too:

So, to move the BFO down a bit I have added in the 15pf capacitor in the original design which is in parallel with the trimmer adjustment of the carrier:

Now, this looks close to perfect to me.

So I adjusted the balanced modulator to get as much carrier suppression as I could:

I think this could be way better if I actually matched some diodes, but I don't think it's going to matter a whole pile. Now if I look at the signal on the output side of the crystal filter with all these adjustments made it looks like this:

Which I reckon is as near to perfect as I am going to get.

Here's a recent picture of the board:

So now I need to move and test further into the circuit. We now need to take this 10MHz output of the filter and mix it with my VFO to make the 7MHz LSB signal I am after. So here is the output of the Band Pass Filter in TX:

and here on the 'scope:

Looking like a very nice clean signal at 7MHz to me!

Good, egh?

Wednesday, 19 February 2014

More from my BITX40


I've started to fiddle with my BITX40 that I am making from the V3B kit I mentioned back here:

I've also uploaded a high res pdf schematic of the 14MHz V3B here:

For my version which I am aiming at 40M, I need to change the VFO. Mine is going to tune from 2.8MHz to 3MHz. I did some sums and calculated that I needed about 22uH of inductance. Now that's quite a lot!

I ended up winding 62 turns onto a T68-2. Here's the VFO part of the circuit built:

Now, the FLL/counter comes already built. It connects to the output of Q17 - when I connect there I get no results from the counter, but when I connect to Q6 it works fine. The output of Q17 looks like this on the 'scope:

The waveform at Q6 is the same, just slightly larger amplitude - strange - almost like there isn't enough drive at the FLL/Counter connector. I've decided to worry about this later and move on.

This is the FLL/Counter looking at the VFO output at Q6:

I've programmed the counter to display the IF - VFO frequency.

I've also made the Band Pass filter as I discussed back here:

and the 1st RF amplifier is built too:

So far, so good.

Fun, egh?

** UPDATE **

I found that I had the voltage regulator in the wrong way round! I've corrected that and the output at the FLL/Counter point is now slightly higher amplitude and is driving the counter OK. Odd?

Monday, 17 February 2014

Turn back the clock!


I started a project a while back here:

but then I became a touch distracted by the Minima!

So, when I was fiddling with the DDS VFO at 17-17.2MHz I ordered a BITX V3B kit from Sunil in India:

Now, what arrived in the post has simply astounded me:

So I have a case, a microphone, a complete FLL stabiliser/frequency counter, a PCB set, all the components, wires, interconnects, meter, plus the other bits I can't remember!

This cost me the sum of $125 plus $45 shipping - quite incredible.

There are some good pictures of the finished article here:

So, as I have a FLL stabilised VFO with the kit, I could move that to 2.8-3MHz and use it with a 10MHz IF to get to the 40M radio I am wanting instead of the DDS I was planning....

Much to fiddle with!

Good, egh?