Hi all,

I'm in the midst of designing a homebrew transceiver. Some time back I
bought a couple of DDS kits with the intention of using them for this
purpose. I also have some ADE-1+ double-balanced mixers.

Now the DDS kit I'm using[1] uses a Analog Devices AD9851 which is fed
into a couple of high-speed D-flipflops, producing I and Q signals
between 0 and 30MHz.

Sounds perfect for HF. I also have a Raspberry Pi PC, and a STM32F103RE
microcontroller; which has 3 12-bit ADCs and 2 12-bit DACs (apparently
good to 1Msps).

I read somewhere that someone had ported a FFT transform library to the
GPU of the Raspberry Pi, so I thought that I could use the STM32's ADCs
and DACs interfaced to the Raspberry Pi using I²S, then use the Pi's GPU
as a poor man's DSP to de-modulate the signal to baseband.

My plan is something along these lines:

Receive:

ANT - [BPF] - [AMP] - [Mixer (LO=AD9851)]- ...
... [LPF] - [STM32 MCU] - [RPi]

I've omitted the quadrature part here; basically everything between the
front-end buffer amplifier and the STM32 would be duplicated.

The LPF would be set for the sample rate of the STM32; somewhere in the
realm of 250-500kHz low-pass. It'll depend on what I can achieve.

The sticking point right now is in the handling of the output from the
LO. The I/Q DDS kit produces a square wave output. Initially this
concerned me a bit, as from my university days I understand square waves
to be rather harmonically rich.

A bit of reading up, apparently these things thrive on such LO signals,
they need to be driven to saturation. This still leaves me with a
concern. Suppose I had a 1MHz LO.

In reality, 1MHz square wave is in practice, a 1MHz sine + a weak 2MHz
sine + a stronger 3MHz sine … ad-nauseum. There's all the harmonics to
contend with. What effect does this have on a mixer?

Does it see the harmonics and if so, how do people deal with them? I'm
guessing it does and that the answer is a liberal set of overlapping BPF
filters that can be switched in to cover the range of the DDS VFO. In
my case, a selection of filters that cover 0-30MHz.

Something like this I guess:
Filt Start Stop BW Min Max
0 0.00 250,000.00 250,000.00 0.00 240,000.00
1 240,000.00 470,000.00 230,000.00 240,000.00 420,000.00
2 420,000.00 830,000.00 410,000.00 420,000.00 780,000.00
3 780,000.00 1,550,000.00 770,000.00 780,000.00 1,500,000.00
4 1,500,000.00 2,990,000.00 1,490,000.00 1,500,000.00 2,940,000.00
5 2,940,000.00 5,870,000.00 2,930,000.00 2,940,000.00 5,820,000.00
6 5,820,000.00 11,630,000.00 5,810,000.00 5,820,000.00 11,580,000.00
7 11,580,000.00 23,150,000.00 11,570,000.00 11,580,000.00 23,100,000.00
8 23,100,000.00 46,190,000.00 23,090,000.00 23,100,000.00 30,000,000.00

I'd have five GPIOs select them somehow from the STM32. How do people
go about switching between them? I'd imagine relays; in fact the
FT-897D I have, as I tune up I hear relays clicking as I pass in and out
of each band. So I'm guessing this is one of many solutions. I'm just
concerned about how many one might need.

Thanks in advance.
Regards,
Stuart Longland

1.
http://www.minikits.com.au/electroni...-AD9851-IQ-DDS