Using Cheap Asian Electronic Modules
By Jim Rowe
Self-Contained
3.8GHz Digital
Attenuator
This digitally programmable RF attenuator module can attenuate RF
signals from 1MHz to 3.8GHz by 0-31dB in 1dB steps. It doesn’t need to be
controlled by an external microcontroller; it has one built in. You control it
using four small pushbutton switches, while a tiny OLED screen shows the
current setting.
I
reviewed one of the simpler
digitally programmable RF attenuator modules back in the June 2019
issue of PE. It could be configured
either by a separate microcontroller
unit (MCU) like a Micromite or Arduino, or a six-way DIP switch.
It was based on the Peregrine Semiconductor PE4302 attenuator IC,
mounted in the centre of a 33 x 24.5mm
PCB without any shielding. Despite
that, it turned out to have quite respectable performance up to about 1.5GHz.
Above that, attenuation errors tended
to grow, but the module was still quite
practical for many applications.
I recently noticed a new digitally
programmed step attenuator for sale. It
is only a little larger, but has a built-in
MCU with a tiny OLED and some small
pushbutton switches for easy attenuation adjustment. I ordered one from
Banggood (ID number 1769385; also
available from AliExpress – search for:
‘HMC472 6-bit Digital Attenuator’).
At the time of writing, it is priced
at about £14 plus £3 for shipping to
the UK. I haven’t been able to find any
information regarding its manufacturer,
but like most of these modules, it is
almost certainly made in China.
This module measures 42 x 32 x
22mm overall, not counting the SMA
connectors at each end for RF input and
38
output. The digital attenuator section
is on a 33 x 22.5mm PCB inside a 42 x
32 x 10mm CNC machined aluminium
block which forms the ‘case’.
Most of the control section is
mounted on a second PCB measuring 42 x 32mm, which forms the top
of the case. The 26mm diagonal (38 x
12.5mm) OLED is mounted on top of
the second PCB.
The PE4302 digital attenuator chip
used in the earlier attenuator module
was made obsolete in 2018 and is no
longer available. This new module uses
the HMC472 from Hittite Microwave
Corporation, a company acquired by
Analog Devices in 2014.
The HMC472 is similar to the
PE4302 in many ways. It is described
as a 6-bit digital step attenuator using
GaAs MMIC technology, and can provide attenuation from 0dB to 31.5dB in
0.5dB steps for DC to 3.8GHz signals.
It comes in a 24-lead Lead Frame SMD
package measuring 4 x 4mm. Unlike
the PE4302, it runs from 5V DC rather
than 3.3V.
The insertion loss at the 0dB setting
is rated at 1.1-1.2dB below 350MHz,
1.5dB at 2GHz and 1.9dB at 4GHz.
I wasn’t able to find a complete circuit for the new module, but I worked
out a basic block diagram for it, shown
in Fig.1. The HMC472’s RF1 input pin
is coupled to the SMA connector via a
1nF capacitor, with its RF2 output pin
configured similarly. Apart from various bypass capacitors, that is the whole
attenuator section.
The control section is based on an
STM32F103C8T6 microcontroller. You
may have noticed that it controls only
five of the six programming lines of
the HMC472: V1 to V5. The unused V6
line is the one that controls the 0.5dB
attenuator stage inside the HMC472,
which explains why this module only
provides 1dB steps.
Presumably, the module designers decided that given the attenuation
error rating of the HMC472, ±(0.35dB
+ 5%), and the difficulty in avoiding
further frequency-related errors due to,
for example, PCB layout, there wasn’t
much point in providing 0.5dB steps.
The user determines the attenuation
setting using the three small pushbutton switches (S1-S3), and the current
attenuation setting is shown on the
OLED module. The MCU drives this via
a standard I2C serial interface.
When power is first applied, the
MCU sets the attenuation to 0dB. To
increase the attenuation, you first press
S2 (the OK button) and then press S1 (+)
until the display shows the attenuation
setting you want. Then, if you press S2
again, this will be the new setting. To
Practical Electronics | October | 2022
Fig.1: block diagram
for the programmable
RF attenuator. This
module, like many of the
others we’ve discussed
recently, is controlled by
a popular STM32 ARM
microcontroller.
reduce the attenuation, you press S2
once more, then press S3 (−) until the
OLED shows the new setting you want,
and then press S2 again.
It’s pretty straightforward, although
the tiny pushbuttons used for S1-S3 do
have a small amount of contact bounce.
This can sometimes force you to press
the + or − button again to correct any
accidental ‘overshoot’ before pressing
S2 to finalise the change.
The CH340E USB-serial interface
chip shown in Fig.1 has been provided
to allow the attenuation setting to be
programmed from a computer. So the
mini USB socket is not just for feeding power to the module (controlled
by power switch S4), but also to allow
external control.
There’s not a great deal of information provided on external control, but
I found a very brief explanation in the
‘Customer Q&As’ section of the Banggood info on the module:
The protocol is simple 9600 Baud
serial: ‘wvXXYYn’ sets the attenuator to XX.YYdB. ‘rn’ returns the
model number. Search GitHub using
ATT6000 or emptemp for python
code (https://github.com/emptemp/
att6000_control).
I tried this, but I didn’t get very far.
Using the serial terminal app Tera Term
(V4.105) with the virtual com port
driver set for 9600 baud, I tried all of
the possibilities I could think of to try
and get the attenuator module to ‘listen’
to a command like ‘wv1300n’.
I tried sending the command in
uppercase instead of lowercase, with
and without the ‘n’ at the end, followed
by an LF or a CR or a CR-LF, using 8-bit
coding or 7-bit coding
and so on. But there
was no response or
reaction from the module, whatever I tried. It
stayed stubbornly set
at 0dB of attenuation.
So I’m not sure
about how to control
the module from a PC
or MCU.
Fig.2: a graph of the module’s performance at an
attenuation setting of 0dB. This was measured
between 100kHz and 4.4GHz, using Signal Hound’s
Spike software with a USB-SA44B spectrum analyser
and a matching USB-TG44A tracking generator.
Practical Electronics | October | 2022
Checking it out
I measured the performance of the new
attenuator module using my Signal Hound USBSA44B HF-UHF spectrum analyser and its
matching USB-TG44A
tracking generator,
both controlled by
the latest version of
Signal Hound’s Spike
software (V3.5.15) in its SNA (scalar
network analysis) mode.
I checked the performance of the
module at a number of different attenuation settings: 0dB (to see its insertion loss), −5dB, −10dB, −15dB, −20dB,
−25dB and −30dB, to get an idea of the
module’s overall performance. After
examining the results I then checked
the response at three further settings:
−1dB, −18dB and −31dB.
During each of these tests, I saved an
image of Spike’s plot of the test results.
The first of these (the one for a setting
of 0dB) is shown in Fig.2.
Spike cannot combine multiple
results into a single composite plot,
so I assembled one by importing them
into CorelDraw and tracing each plot.
The result is shown in Fig.3.
The uppermost 0dB plot shows the
insertion loss of the module over the
entire frequency range. It is less than
2dB (as claimed) up to about 1.5GHz,
but then wobbles around a bit until it
reaches about −2.5dB at 2.64GHz and
then −3.5dB at around 2.95GHz. Essentially, the insertion loss remains under
2.0dB over much of the frequency
range, apart from some deviations
between 1.5GHz and 3.8GHz.
Most of the lower plots in Fig.3 have
a shape almost identical to that of the
top 0dB plot, just separated by the chosen attenuation setting. This is also true
for the uppermost blue line, with the
attenuator set at −1dB.
But notice that above about 2.2GHz,
the higher attenuation plots (−20dB
and greater) develop a small number
of minor bumps and dips. These are
very apparent in the −30dB plot, for
example, and also in the blue −31dB
plot just below it. I suspect that many
of these minor variations are due to
small resonances inside the HMC472
chip and its surrounding tracks on the
attenuator PCB. There might also be
standing waves inside the attenuator
box at specific frequencies.
These plots reveal that the attenuator’s performance relative to its insertion loss is quite respectable, at least
for frequencies up to about 1.5GHz and
settings up to around −20dB. But the
errors increase above 1.5GHz and with
levels above −20dB.
This attenuator would still have
many practical uses above 1.5GHz and
settings over −20dB, either if the exact
amount of attenuation at a given frequency is not critical, or if you use the
plots of Fig.3 to correct for the errors.
The middle blue plot in Fig.3, for a
setting of −18dB, was to see if setting
the attenuator to −18dB would give an
actual attenuation of −20dB over as
much of the frequency range as possible. This worked for frequencies around
39
The side view of the attenuator module shows the control switches, and the
very tight spacing between the RF and USB power connectors on the right. This
photo is shown at approximately 150% scale for clarity.
880MHz and 3.8GHz, but the overall
shape of the plot was unchanged and
still gave significant deviations both
less than and greater than the desired
−20dB figure.
Conclusions
Overall, this new attenuator module
is reasonably good value for money.
It is suitable for a fairly wide range of
applications, especially if you use the
curves of Fig.3 to correct for the inevitable attenuation errors.
But it does have a few shortcomings.
For example, the module might not
be controllable from a PC, Arduino or
Micromite.
Another is that the RF output SMA
connector and the mini USB power
connector are too close together, so you
have to unplug the USB cable to connect or disconnect an SMA cable to the
RF output just below it.
You also have to adjust the SMA connector’s outer hex sleeve so that a flat
is uppermost; otherwise, you won’t be
able to reconnect the USB cable.
Before I finished writing this review,
I went onto the Banggood website to
check on the price of this module. I
was surprised to see that a larger and
apparently better module had become
available (ID 1648810; https://bit.ly/
pe-oct22-att).
This newer attenuator module is
advertised as having a frequency range
of LF to 6GHz, an attenuation range of
0-31.75dB in steps of 0.25dB and an
insertion loss of less than 1.5dB (but
with the qualification that ‘it will be a
little larger’ at the high-frequency end).
Currently, they are advertising it for
around £25, plus postage to the UK. So
it costs nearly double that of the module I’ve reviewed here, but it might turn
out to be worth it.
I suspect it is based on the Analog
Devices HMC1119, which has a range
of 100MHz to 6.0GHz, seven control
bits to give a setting range from 0 to
31.75dB in 0.25dB steps and a specified insertion loss of 1.3dB at 2.0GHz.
I am planning to order one of these and
write it up when it arrives.
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
www.poscope.com/epe
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PoScope Mega1+
PoScope Mega50
Fig.3: following on from Fig.2, this is the combined plot of testing the module
at various attenuation settings from 0dB to −30dB in 5dB steps, and then three
extra tests at −1dB, −18dB and −31dB.
40
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- resolution up to 12bit
- Lowest power consumption
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- 7 in 1: Oscilloscope, FFT, X/Y,
Recorder, Logic Analyzer, Protocol
decoder, Signal generator
Practical Electronics | October | 2022
