Hi,
I would like to design a custom board based on the Spartan-3AN
FPGA, based on the Spartan-3AN FPGA Starter Kit, but would like to build
my own RF front-end.
If I look at the AD9874 IF-to-Bits RX subsystem component
it contains an ADC and a whole bunch of other stuff.
Could someone help me by pointing out what other components or parts I
should be looking at to build the RF receiver front-end to start with,
assuming that I am going to learn from the USRP2 design, and the fact
that it already has the Spartan-3AN FPGA, and I’m going to build my own
RF front-end?
Could someone help me by pointing out what other components or parts I should be looking at to build the RF receiver front-end to start with, assuming that I am going to learn from the USRP2 design, and the fact that it already has the Spartan-3AN FPGA, and I’m going to build my own RF front-end?
Designing an RF frontend from scratch sounds like a challenge, and the
AD9874 requires a fair amount of support circuitry. Have you considered
trying something a bit more integrated? How about this:
While that project isn’t getting a lot of support lately, there are
schematics and supporting materials available and the part that it’s
based on is highly integrated and fairly inexpensive:
An RF Front-end can take many forms, depending upon the application. For
use with the USRP or similar hardware, a direct-conversion approach is
probably adequate. This type of front-end typically consists of low
noise amplifier (LNA), mixer, and perhaps a baseband/IF amplifier or
automatic-gain control (AGC) amplifier. Determing the requirements for
amplifier gains will be based on mixer conversion loss and the receiver
sensitivity desired.
Designing the RF front-end requires knowledge of RF and microwave
circuits and perhaps impedance matching skills and amplifier design
depending upon how low-level you want to get into the design process.
This is not a simple task. That said, however, coaxial components are
available from manufacturers such as Mini-circuits that are relatively
easy to work with provided you are working relatively low frequency (~7
GHz or less).
Also, it is worth pointing out that the ADC and DACs that you choose are
part of the analog front-end, and they have an associated noise figure
and power requirements. As such, if you are looking to design an RF
front-end, choosing an ADC/DAC might not be the way to go.
This type of front-end typically consists of low noise amplifier (LNA), mixer, and perhaps a baseband/IF amplifier or automatic-gain control (AGC) amplifier. Determing the requirements for amplifier gains will be based on mixer conversion loss and the receiver sensitivity desired.
Designing the RF front-end requires knowledge of RF and microwave circuits and perhaps impedance matching skills and amplifier design depending upon how low-level you want to get into the design process. This is not a simple task.
Would it make sense to model and simulate using Matlab/Simulink, for the
amplifiers, mixers and other components using their datasheets?
Matlab/Simulink has a VHDL code generator for Xilinx, so I am wondering
if I could use it initially to model the behaviour or the RF part, and
then later on, use it to realize the baseband processor?
What other tools would I need to complete the process, so that I can
model, simulate, and develop the schematics and PCB layout for the RF
front end? e.g Is Altium Designer better, and does it have all the
required RF modeling and simulation capabilities or should I use pspice?
I would like to have an integrated workflow, from simulation to
prototyping, with as few tools as possible, and then continue to refine
the model and prototypes incrementally.
Hi Jeffrey,
I’m reading a book called “RF and Digital Signal
Processing for Software-Defined Radio - Elsevier Newnes - 2009”. It
seems to have a good coverage of the theory.
On Jun 18, 2010, at 10:36 AM, Jeffrey L. wrote:
I would recommend characterizing all devices yourself with equipment that has been properly and carefully calibrated.
Would this be an oscilloscope and a signal generator, or do I need some
more stuff?
Can you recommend a model that will allow me to characterize devices
that operate in the 80MHz to 2.4GHz range (i.e. FM radio broadcast to
WiFi operation).
I don’t know about Matlab’s capabilities, but the approach seems correct. What do you mean by “baseband processor”?
By baseband processor, I mean the part that does the actual
demodulation/modulation that is implemented in software/firmware using
the FPGA.
Would it make sense to model and simulate using Matlab/Simulink, for
the amplifiers, mixers and other components using their datasheets?
Simulation is helpful, but it cannot replace the design process. The
parameters you will need can potentially come from datasheets, but
manufacturers don’t often provide enough information in the datasheet or
take much care in producing the given results. For receiver design,
s-parameters are sufficient except in the case of amplifier or component
design. This type of data is usually separate from the datasheet if
it’s available at all. I would recommend characterizing all devices
yourself with equipment that has been properly and carefully calibrated.
Matlab/Simulink has a VHDL code generator for Xilinx, so I am
wondering if I could use it initially to model the behaviour or the RF
part, and then later on, use it to realize the baseband processor?
I don’t know about Matlab’s capabilities, but the approach seems
correct. What do you mean by “baseband processor”?
What other tools would I need to complete the process, so that I can
model, simulate, and develop the schematics and PCB layout for the RF
front end? e.g Is Altium Designer better, and does it have all the
required RF modeling and simulation capabilities or should I use pspice?
Access to PCB fabrication equipment is helpful, but not required. Open
source layout tools are available if cost is an issue, but I am not
familiar with any open source RF tools. SPICE is not adequate for high
frequency work; you will need something like Agilent’s Advanced Design
System.
With any hardware design, simulations are good to verify circuit
operation and measurement results, but sometimes there is black magic
involved that can only come from hands on experience. There is a lot of
theory and background that goes into RF design and what you are asking
is not as simple as it might seem.
~Jeff
–
~Jeffrey L., K1VZX
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