R1 and R2 have a 50 Ohm value. The
potentiometer is a 2-gang logarithmic type which is
connected in such a way that while one section increases in
value, the other section decreases. The maximum attenuation
you get depends on the potentiometer value, being about 30
dB for 1 kilo-Ohm. This was also the smallest value I could
get from the shop.
So what can we expect? These type of potentiometers are
intended for low frequency use, how will they behave at
higher frequencies? But there is something else: are they
setup and measurement of logarithmic potentiometer
As you can see from the graph: relationship between scale
(0-10) and resistance value is not truly logarithmic. To
reduce costs, these type of potentiometers are built with 2
different values of resistance with linear behavior, which
produce a logarithmic approximation. In fact, we have 2
straight and combined lines in one graph. Next, what kind of
losses can we expect?
attenuation (left) and VSWR (right)
Measurements were done at a frequency of
10 MHz. As you can see from the left graph, there is a
nice relationship between attenuation and scale up to
90%, after that the value increases rapidly. Insertion
loss measured is -0.36 dB, which is about 8%. The right
graph shows a maximum VSWR above 2.2, another loss in
power of 13%.
In conclusion, this type of attenuator is not
intended for accurate setups, but it might still be
useful at driver level or at the input of a receiver
where loss is not critical and we have plenty of signal.
Losses are caused by the potentiometer design: they are
not suited for high frequencies and not exactly
logarithmic. Thats why good variable attenuators are so
expensive: they are difficult to make!
I have used an ancient Hewlett Packard (now Keysight)
8753A network analyzer to do my measurements.