The amount of power required by the FPGA depends on the number of logic elements used and the speed of operation. Anything that isn't clocked in the FPGA consumes almost no power so the thermal dissipation is controlled by the logic designer. DCMs require a lot less power than an external DDS and an FPGA would be a good place to put other logic. A $15 FPGA has over a million equivalent gates and the DCMs are 1 or 2 percent of that. Essentially, this eliminates the power consumed by the ROMs and DACs in the DDS chips as there is no need to synthesize sine waves or to vary the frequency. Discrete clock distribution chips also perform the same function but a lot of power and PCB area would be cnsumed to drive them.
73,
John KD6OZH
----- Original Message ----- From: "Alan Bloom" n1al@cds1.net To: "John B. Stephensen" kd6ozh@comcast.net Cc: K3IO@verizon.net; "AMSAT Eagle" Eagle@amsat.org Sent: Sunday, March 25, 2007 05:40 UTC Subject: Re: [eagle] Re: Eagle Microwave Antenna Arrays-- mechanical concepts
On Sat, 2007-03-24 at 00:28, John B. Stephensen wrote:
The only disadvantage of the NCOs is the power required. Analog Devices makes a quad 500 Msps DDS but it consumes up to 80 mW per channel and the upconversion mixers will consume additional power. The MA/COM phase shifters consume less than 50 mW per channel.
80 mW doesn't sound too bad for a signal chain with a 1W power amplifier.
However, NCOs aren't needed for the transmitter. The IF inputs could be fixed-frequency square waves with adjustable time delays. Harmonics could be cleaned up by low-pass filters preceeding the upconversion mixers. Spartan-3 FPGAs have up to 8 digital clock managers with 256-step time delays in 15-60 ps increments. A 15 ns maximum delay would allow a 360-degree phase shift at frequencies as low as 67 MHz and the FPGA can be clocked at 200 MHz. Virtex FPGAs run at 500 MHz and have more DCMs.
But how much power do the FPGAs require? Of course, it depends on what is programmed into them and the clock rate. But I know that in some applications they require a heat sink, so it can be quite a lot (several watts) of power.
Alan