On Sun, 09 Feb 2014 15:44:26 +1000, Stuart Longland VK4MSL wrote:

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.

You're probably being ambitious with what can be done. While the ADC can
sample that fast, the processor -- even running at 48MHz or whatever it
can do -- may not be able to keep up.

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?

Diode ring mixers basically end up acting like switches anyway. Sending
them square waves just means that the transition regions are smaller,
which should do no harm and may lead to less phase noise.

Does it see the harmonics and if so, how do people deal with them?

Yes it 'sees' the harmonics. Basically -- even if you're exciting it
with a sine wave -- you should plan on the mixer's input being mixed
against a square wave.

In fact, some microwave equipment used to take advantage of this, to keep
the LO frequency down. It may still -- it's been a long time since I've
really paid attention to radio technology.

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.

Yup.

This is why modern radios (or at least radios as of 25 years ago, when I
was last paying attention) had a first IF above the band of interest --
it put the most significant frequencies that could be spuriously mixed
well above the desired reception frequency.

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.

Why not 12? Surely you have lots of spare pins to play with.

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...zer/basic-dds/
Basic-AD9851-IQ-DDS





--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com