More Mixing to 4M

Well,

Been fiddling some more in the shack today with the 70MHz linear amplifier that I am building. The first total screw up was that I need to make an input attenuator so I can drive with a 10W ish input signal and reduce it to about 2W to the amplifier module.

I used an on-line attenuator calculator to design the pad and purchased some suitably power rated components for the job. I soldered it together and then measured the attenuation with the Spectrum Analyser, here with a single fixed frequency output from the tracking generator:

Now, this looked very much like it was about a mile from my target value of attenuation of 7dB and I seen to have more like 20dB at 70MHz. This didn't make much sense.

So I stuck it on my return loss bridge I made back here:

http://g0mgx.blogspot.co.uk/2013/09/so-whats-happened-to-bridge-then.html

to see what the input match to 50R was. Here's what I saw:

Now, this is telling me that at 50R impedance the input SWR to my attenuator at 70MHz is 23:1 (ish) i.e. a complete load of dingos kidneys.

If I just connect a 20dB attenuator (much lower power rated) I made a while ago, I see this:

This one was only really built with HF In mind, but still the SWR at 70MHz is a respectable 1.2:1.

So - why is this attenuator I have made so pants? I now realise that the resistors I bought to make the attenuator are wire-wound rather than carbon. So we have buckets of inductance as well as resistance. That just won't do at all!

Whilst I was messing about with the attenuator I decided it would be a good idea to box the 70MHz TX converter I started to make here:

http://g0mgx.blogspot.co.uk/2012/07/mixing-better-than-before-70mhz.html

and then revisited recently here:

http://g0mgx.blogspot.co.uk/2013/12/testing-times-for-my-linear-project.html

Well, I made an output amplifier for it to raise the output level a bit and have boxed it with it's own PSU:

Clearly this should be built into a metal case but I only have this plastic one which will have to do. I must be aware of external RF when I am using it that may make the output wobble somewhat:

The output looks OK on the scope:

but if I were to use this as a TX converter and put the signal on-air I clearly need another low pass filter post the output amplifier I added as the harmonics are quite high:

So, another box for the shelf and a failed attenuator build.

You win some, you lose some.

So, lets get a bit smarter

Well,

Been rummaging some more in the shack today and have decided to make a variable attenuator. This is in addition to the fixed attenuators I made here:

http://g0mgx.blogspot.co.uk/2012/11/calibration-complete.html

and the switch attenuator I made more recently here:

http://g0mgx.blogspot.co.uk/2013/09/time-time-time.html

Why? Because I would like to be able to set the output of my newly constructed signal generator from here:

http://g0mgx.blogspot.co.uk/2013/10/so-hows-sig-gen.html

to be exactly 0dBm, how will I be able to set it so accurately? Using the dBm meter I made back here:

http://g0mgx.blogspot.co.uk/2013/01/calibration-finaly-complete-really.html

So, that's the background, here's the variable attenuator:

And now with it attached to a fixed attenuator at the input:

There are two switched input attenuators in this variable thingamabob, lets call them A and B these are then followed by a variable pad to the output port, so, if I connect my new signal generator to the input and my dBm meter to the output I can try and assess what the attenuation of the two fixed pads and also the variable pads are:

and then if I "do the math" and calculate the raw attenuation rather then the power readings:

So it looks like my switch position A is giving me 3.3 dB attenuation and position B is giving me 10.9 (I was aiming for 4 and 10). When combined with the variable pad I am getting 3.3dB to 29dB attenuation in switch position A and 10.9dB to 36dB in switch position B - the fact that the ranges overlap is good enough for me.

Now, for the sake of it I am going  to now look at the return loss of the input and output of this filter, primarily because I am aiming for an impedance of 50R and it will be interesting to see how close to this design goal I have achieved across the HF spectrum.

So, using my return loss bridge from here:

http://g0mgx.blogspot.co.uk/2013/09/so-whats-happened-to-bridge-then.html

I can sweep the HP spectrum and see what the return loss is, firstly lets try the 30dB attenuator I have here:

Now, In the image above, you can see three traces. The purple trace is the normalised return loss of the bridge in a completely unbalanced state i.e. open circuit. The yellow is the return loss with the device under test connected (in my case a port of the 30dB attenuator), and the green is the maths calculated as the delta between the two i.e. the difference between the open bridge and the bridge with the device connected. The table at the bottom then shows the application of the maths to calculate the return loss and VSWR.

So we can see that at 10MHz my 30dB attenuator has a return loss of 33dB which is as close to maximum bananas as makes no odds - thats a VSWR of very close to 1:1. As the frequency increases you can see that the return loss decreases which in turn means that the VSWR increases. At 40MHz the return loss has reduced to 25dB which is a VSWR of 1.1:1. All in all though this attenuator is an excellent match to 50R.

Now, here's the same trace but with the input port of the variable attenuator connected in switch position A:

And now in switch position B:

Interestingly the return loss is flat across the spectrum, but not such a close match to 50R and hence the VSWR is higher. This probably means that the variable attenuator on the output side of the pad is affecting the input impedance and hence the input pad should probably be of a greater attenuation value; the downside of this would be that the device would have a much larger starting attenuation value and therefore not meet my design goal!

Here's the new ham cat Florrie, she seems to be learning from Geddy Cat to assist very well:

Good, egh?

So what can you do with this bridge?

Well,

Now I've got what I think is a reasonable Return Loss Bridge - what can I do with it?

Lets take a very simple example, we have already noted previously that you calculate the Return Loss by:

where:

Open = Peak to Peak voltage with no connection to the "?" port
DUT = Peak to Peak voltage with the Device Under Test connected to the "?" port

So if we were measuring the output with an oscilloscope we would use that equation to calculate the difference in dBm. However, as I have a Spectrum Analyser that reads directly in dBm, in this example I'll cut some corners.

So, initially I have the Spectrum Analyser and the RLB set up like this:

To start with I have the "?" Open and my Spectrum Analyser, at my selected test frequency of 7.1 MHz sees this:

So at 7.1 MHz we have a signal from the Tracking Generator of -31.67dBm. Now, lets couple up the Carolina Windom I made here:

http://g0mgx.blogspot.co.uk/2013/08/so-who-is-caroline-ah-carolina-windom.html

to the "?" port. Now the Spectrum Analyser can see this:

So now, at 7.1 MHz we have a signal strength of -42.74 dBm - that's a difference of 11.07 dBm.

If we now use this equation to calculate the VSWR:

and plugging in the results:

we can calculate that the VSWR is 1.78.

Now, what does my newfangled Antenna Analyser thingamabob think the VSWR is at 7.1 MHz

the RLB works, doesn't it?