siliconchip.com.au Australia’s electronics magazine July 2019 81
Fig.13: measured performance of the high-pass filter
comprising inductors L1-L3 and four small ceramic
capacitors. As you can see, the response is pretty much
flat from 70MHz to 400MHz, but signals from 0-40MHz are
attenuated by 60dB. The transition is smooth and quick, at
around 75dB/octave, or 2dB/MHz.
trum analyser. See the figure captions for details.
Fig.13 demonstrates how effective the high-pass filter
is, despite being made from self-wound air-cored induc-
tors. This shows that the filter provides 60dB of attenu-
ation for signals below 40MHz with a virtually flat pass-
band from 70MHz up. The filter roll-off is quite steep at
around 75dB/octave (the span from 40MHz to 70MHz is
about 0.8 octaves).
Future possibilities
It is possible to add further features to the software.
With the supplied software, less than 30% of IC1’s pro-
gram memory is used.
For example, RF output levelling would be possible, by
using the pin 11 PWM output which drives the RSET pin
of the AD9850 module (currently used to provide AM)
to offset the sinX/X roll-off for frequencies up to about
50MHz, at the cost of a reduced maximum output level at
lower frequencies.
Extended frequency coverage also appears possible
through the use of alternative high-pass filters and/or by
replacing the AD9850 module with one based on the pin-
compatible AD9851.
Some minor additional software changes would be re-
quired to permit the AD9851 to be used. The AD9851
can be clocked at up to 180MHz, which may allow the
generator to operate up to 100MHz in a single range, and
possibly up to 300MHz with a modified HPF. Suitable
AD9851 modules are available from the same sources as
the AD9850-based module.
Adding other modulation modes such as FSK and BPSK
is also feasible, but adding QPSK, for example, may be be-
yond the reach of this design.
Moving to an even more advanced DDS device, such as
one based on the more modern AD99xx series chips,could
be done. However, this would substantially increase the
overall cost and complexity of the device.
It is also possible to replace the basic passive output
variable attenuator network with a more elegant PIN di-
ode based system.
This involves using components that are more difficult
to obtain, but sufficient space has been left in this area of
the PCB for such an addition.
Finally, you could consider adding a numeric keypad
on the front panel to permit the direct entry of frequen-
cies, tuning step and scan settings, plus you could add a
settings memory for frequently used configuration.
However, this would likely require a processor change,
or potentially even an additional microcontroller for han-
dling keypad entry, to obtain the necessary spare I/O pins.
Having said all that, the design as presented is a good
compromise between low complexity and cost, while still
having a useful frequency range and a good set of features.
It makes a great entry-level RF signal generator – a “must”
for anyone interested in radio at any level!Fig.11: a “narrow band” 1.75kHz frequency modulated
signal with a 10MHz carrier and a 20kHz span. The iconic
equi-spaced 1kHz sidebands of a standard FM signal are
clearly visible.Fig.12: “wideband” or broadcast radio style FM, again with
the carrier at 10MHz, this time captured with a 500kHz
frequency span. This clearly illustrates that most of the
signal energy falls within the 200kHz channel bandwidth
permitted for broadcast FM signals.SC