Question about truncation phase in NCO

[faye_hongdou]

I saw ddt694's post, which mentioned:

you can realize a accumulator like this

process(clk) begin
if rising_edge(clk) then
acc(23 downto 0) <= acc(23 downto 0) + input;
end if; end process;

then, you can address the lookup table like this

process(clk) begin
if rising_edge(clk) then
nco_out(7 downto 0) <= rom(conv_integer(acc(23 downto 14));
end if; end process;

In the lookup table, the address is "conv_integer(acc(23 downto 14))". I do not understand that why the address is conv_integer(acc(23 downto 14)), but not conv_integer(acc(23 downto 0)).

 

[freeinthewind]

Question about NCO.

it is so because it takes truncation of phase.

 

[faye_hongdou]

Re: Question about NCO.

But why should i take the phase truncation?

Is there any fundamental for NCO?

 

[echo47]

Question about NCO.

That's so you can use a simple 1K word ROM. Otherwise you'd need a 16 megaword ROM.

 

[Black Jack]

Re: Question about NCO.

I saw ddt694's post, which mentioned:

you can realize a accumulator like this

process(clk) begin
if rising_edge(clk) then
acc(23 downto 0) <= acc(23 downto 0) + input;
end if; end process;

then, you can address the lookup table like this

process(clk) begin
if rising_edge(clk) then
nco_out(7 downto 0) <= rom(conv_integer(acc(23 downto 14));
end if; end process;

In the lookup table, the address is "conv_integer(acc(23 downto 14))". I do not understand that why the address is conv_integer(acc(23 downto 14)), but not conv_integer(acc(23 downto 0)).

Try to find this book.
"Digital Frequency Synthesis Demystified DDS and Fractional-N PLLs
Bar-Giora Goldberg"

Let me know and I sent you this book by e-mail (If you can`t find it)

Other good source www.analog.com
search for DDS tutorial

 

[nanostallmann]

Re: Question about NCO.

Try to find this book.
"Digital Frequency Synthesis Demystified DDS and Fractional-N PLLs
Bar-Giora Goldberg"

Let me know and I sent you this book by e-mail (If you can`t find it)

Other good source www.analog.com
search for DDS tutorial

Hi, I'm searching this document since two day and nothing...
ideas??
thank you

- - - Updated - - -

Hi, I'm searching this document since two day and nothing...
ideas??
thank you

Ok I found it...