Home

 

February 2021 - 10dB (ish) Attenuators.

This is an example of a really simple homebrew test equipment project. For the price of a few resistors and perfboard - plus some SMA plugs and sockets, you can build something that will be useful for years to come.

While Nick G8INE and I were having fun building the N6QW Direct conversion receiver (from the Spring 2021 Sprat ) we suspected that perhaps the mixer injection from the Si5351 local oscillator was perhaps a little high (in our examples) and causing undue internal noise?
But we needed to confirm that. One way to have tested the idea was to have altered Pete's code for a lower output from the Si5351 - but we thought that simply attenuating it a little would be just as good.

So we needed some attenuators.

At the early stage of building I was also using my Chinese MHS-5200A Signal generator as the VFO.
  ....and it is not designed to produce an output a signal below 200mV.

That seemed too high a level for the mixer that I was using. In addition when we got to the RF amplifier stage, a more realistic (low) signal level was needed to test the input. So here then was yet another need for some handy attenuation.

As is often the way when we  collaborate on something, Nick and I go off on a tangent and spend time there before returning to the project. Of course, you can easily buy attenuators for around £15 on Amazon/Ebay but that requires patience for the order to arrive - and where is the fun in that? I already had a pretty nifty digital attenuator - so I tried the idea and indeed it did seem that the receiver sounded just slightly less noisy - but perhaps not so much that it mattered greatly for this simple receiver.


Just a quick mention of my cheap signal generator. It has been very useful and I do not regret buying it, It is built down to a price of course and it cost about £50 a couple of years ago. The menu interface is a bit weird - but you get used to that and the sine wave output only goes up to 25MHz - but that covers most of my needs. The output is really awful with the level turned up as high as six volts. Turned down low, the ouput is acceptable.

You will rarely ever need output as high as 6 volts with radio work. So how low would it be useful to go?
This web page shows a chart of signal levels versus "S" reading.



An "S9+40dB" signal would be measured as 5mV (into a 50 ohm load). Good luck measuring voltages that low with a hobby oscilloscope. As stated, my signal generator can only be set as low as 200mV but I can measure that. if I then add a known attenuator I can calculate what level of signal is coming from the output.
With all three attenuators (described below) a 200mV signal has 7.35mV coming out of the attenuator.
That would be a signal just over "S9+40dB". So you can see how we can get low level signals easily into the ballpark that we might want.

As we were writing up the building of Pete's project it occurred to us that a variable digital attenuator might be overkill for other constructors to have on the bench. So we went in two directions. Nick produced a nice homebrew switched attenuator and I built the "10dB ish" attenuators that are the subject of this article.

Hams have been making resistive attenuators for years. Often they have toggle switches to select the desired attenuation. As you go up in frequency, the wires, component leads and switch characteristics cause attenuation and impedance to change and often screening between stages is required.
You can see how Nick handled that with his neat 80dB four stage attenuator. He can switch in 20dB stages. He also added good screening between stages and he even used surface mount resistors to get a superior HF performance. The gerber files are available here.

I went for a simple fixed attenuator (or three of them) and having terminated them in SMA connectors, they can be plugged in when needed. (see photo below).
I tested mine using a NanoVNA (Vector Network Analyser). Even if you do not have one, you probably know someone that does - and once tested you know exactly what attenuation they offer.

Here is all three of mine connected together.


Using an online calculator I chose standard resistors that would get me close to 10dB for each. I designed them to be 50 ohms in and out. Then using small pieces of perfboard I built them as shown.

Once the attenuators were built I tested them using a NanoVNA and of course, they weren't exactly round numbers. Hence "10dB ish". But they performed pretty well across the part of the HF spectrum that interests me. They would be virtually the same "10dB ish" value wherever I needed them to be in the HF spectrum.

Once I had the output of my signal generator turned down to 200mV and added one attenuator to the output and I measured the output at around 64 milliVolts

I created a small spreadsheet to calculate the output from an attenuator for a given input. The result was close enough to the measured voltage. I have tested the spreadseet in an old copy of Excel and the latest LibreOffice.



So do we need an attenuator to be an exact whole number? I suppose the answer relies on what you want to achieve. If you need lab grade measurment then you probably need lab grade test equipment - but as a home constructor you can get away with far simpler and inexpensive test gear.



 

[RSS]