I was really pleased when I got my Koolertron Programmable DDS-based Signal Generator and
it had a reasonably accurate programmable output down to 2mV peak, from audio range to 60 MHz.
It also had a reasonably stable output impedance (50-ohms), such that when I set it for, say 10mV
peak (open circuit), I could see on the oscilloscope via X10 probe that it was 10mV-pk, and then
when I connected it via 50-Ohm cable to a 50-ohm termination on my o'scope that it had dropped to
half, 5mV-pk.
This made me very excited, because this meant that if I set the signal generator to 6mV-pk (on the
sig gen) and connected it to 50-ohms, I would have 3mV-pk, and that would be equivalent to
2.12 mV-rms (sine) or let's just call it 2mV-rms. Now if I connect four 20-db inline attenuators (I
actually have 4), that would give me 2mV/10000 = 0.2uV-rms. And having a sub-microvolt signal
means I could do some pretty decent receiver testing.
What would make it even better is if I could hit anywhere in that range from 0.2uV to 2mV just by
changing the attenuation in 1 dB steps. A 1dB step (in attenuation) is about 90% of the previous
value, or looking at it another way, about a 10% step. So if I wanted to, and I had the right
combination of inline attenuators (and I actually do), I could set my signal gen to 3mV-pk, and
using 50-ohm inline attenuators, generate any value I wanted between 0.2uV and 2mV-rms, in 1 dB
steps. Very useful for receiver testing.
And inline attenuators can be very useful, but alas, not very practical for receiver testing because
of the rather painstaking process of swapping the inline attenuators in-and-out by hand. What would
really make it bettah, practically speaking, is to have a box with knobs that I could turn to any value
I wanted from 0-to-80dB attenuation. Homebrew step attenuators with 1 dB steps can be done with
slide or toggle switches, but the practical limit is about 60 or 70 dB before homebrew techniques
start to lose accuracy or start to exhibit "blow-by". And they are generally limited to 50-60 MHz
at the high end (if you're careful).
On the surplus or used market, meaning, eBay, rotary step attenuators good from DC to 1 GHz or more
are easy to come by, and if you get them from a reputable seller (who reclaims them from old test
equipment and then tests them to make sure they're not burned out), you won't have to spend a ton
of money. I found a perfect combination, an Alan 50V10 (0-10dB in 1 dB steps) and an Alan 50V70
(0-70dB in 10dB steps). $35/ea! Cable them in series (using short SMA patch cables) and you now have
0-to-80dB in 1 dB steps! The Alan 'V' series rotary attenuators are good from DC-to-2 GHz, with
excellent accuracy (0.5dB or better in the HF/VHF/UHF range) and power-handling capability of 1/2W
or more in that same range. You can find a copy of the spec sheet - here -. This is PERFECT for use
with our signal generator!
Here are some pix of the Alan Rotary Attenuators (the knob on the 0-10dB version is not original):
(click on either picture above to see larger version)
Now, package that in a small bench-top enclosure, and wire it up with BNC connectors (which is
mainly what I use, and we'll be in business!
For the enclosure, I picked the metal/plastic Zulkit 6.7" x 4.3" x 3.1" box Amazon ASIN B08BS243RC.
While this is metal and definitely sturdy, it is not really shielded, because the frame is a 3-D
printed plastic. But it is sturdy, attractive, and cheap. And because the rotary attenuators are
themselves completely shielded, this box will do just dandy.
Assembly is fairly straightforward. Three holes are required on the panel for each rotary attenuator,
two for the mounting screws, and one for the shaft. We picked BNC connectors as interface for
the front panel. The rotary attenuators themselves use SMA jacks, so small 50-Ohm SMA patch cords
from attenuators to front panel BNC Bulkhead connectors will finish it off. I ordered prefab cables
(be sure to get 50-Ohm RG-188 or RG-316) from Seller: wifi_expert (on eBay) who can make
RF cables cheaper than I can buy the parts for them.
(click on picture above to see larger version)
(click on picture above to see larger version)
(click on picture above to see larger version)
This Attenuator ensemble is useful and accurate from DC to 2 GHz. It has rugged detents, and is very
reliable. Keep a calculator or chart handy for converting dB to uV when you're doing receiver testing.
73,
Monty N5ESE
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