On 30/05/17 03:16, Jeff Liebermann wrote:
On Mon, 29 May 2017 17:29:44 +1000, Clifford Heath
wrote:
I don't know what frequency the PIC counter input is capable of,
but I know that the AVR counter is clocked; so you can only count
at half the CPU clock frequency. Bah, humbug.
The clock crystal is 4MHz on the old version. I can't read the
numbers on the schematic of the new version.
But I don't think the PIC has a clocked counter anyhow.
I assume that the AVR does it to (sometimes) avoid the
need for a low-pass filter.
The xtal on mine is marked "SCK451C" and "TC,A.426",
whatever that means. It was about 15ppm slow, but seemed
quite stable, based on measurements taken with an HP5386A.
Quite a few people have patched in a TCXO to these units.
It also shows that the gates of the DG MOSFET are
NOT tied together.
I don't understand why they used a DG MOSFET, nor why, since they
did use one, they didn't use the upper gate for gain control.
It seems they're feeding the signal into the upper gate, so
won't get the best bandwidth from the cascode behaviour.
The units I have have both pairs of protection diodes, and the inputs
are joined only at the connector. I cut the trace and soldered a bit
of co-ax onto the prescaler input capacitor.
Oh swell. So the PCB wiring might not follow the schematic.
I haven't found the schematic of the current-manufacture.
I was having nightmares last night from thinking about this counter.
Maybe I should give up while I'm still sane?
I think it's fixable, perhaps with an additional front-end.
It would still be easier and cheaper than building from scratch.
I don't have a good RF source (yet - currently building, see
https://github.com/cjheath/AD9851LCD) so I can't evaluate the
sensitivity.
Before you reinvent the wheel, there are AD9851 based DDS generators
available on eBay.
Who do you think designed those? People like me
I have a bit
of that Jedi "build your own light sabre" thing going on.
Plus there's no accessible used test equipment market here in
Australia. Whenever nice gear comes up at bargain prices,
merchants buy it up and slap a stupid price on it.
http://www.ebay.com/sch/i.html?_nkw=dds+generator+ad9851
That's exactly what I'm using for development. They have all
copied a flaw in the output filter design, leading to very low
output at higher frequencies. Some impedance problem, it's not
designed to drive 50ohms. I'll add a buffer.
along with the associated LCD display:
http://www.ebay.com/itm/PIC16f-Controller-for-the-AD9851-DDS-Signal-Generator-Module-/182593721953
I loathe and detest both PICs and those 16x2 displays.
I'm building one with 320x240 colour touch screen.
The Arduino also has TTL-level RS232, so add a $2 USB module
and you have USB control.
However, those only go up to about 70MHz and the output looks
distorted above 30MHz. If you're going to test the counter all the
way to its rated maximum frequency (2.4GHz), you're going to need a
better generator. DDS has benefits for a function generator and
arbitrary waveform generator, but is limited to lower frequencies.
I expect to incorporate an ADF4351 also, and possibly two AD9851's,
to give quadrature (but still cheaper than AD9854 or whatever
the multi-channel DDS chip is).
E.g.
https://www.aliexpress.com/item/35Mhz-to-4-4GHz-4400mhz-PLL-RF-Signal-Source-Frequency-Synthesizer-ADF4351-Development-Board/32757566484.html
The ADF351's have the same problem as most of those VCO synthesisers,
that they won't sweep cleanly. Changing the frequency makes them
jump wildly about until they stabilise again.
This looks interesting (and tempting):
http://www.ebay.com/itm/ADF4350-v4-0-137-5MHZ-4-4GHZ-OLED-display-Signal-generator-RF-signal-source-12v-/262688224985
137.5MHz to 4.4GHz signal generator in 10KHz steps.
The Arduino clone and TFT Touchscreen LCD cost me $AU14 all up.
https://www.aliexpress.com/item/2-4-SPI-Serial-TFT-LCD-Touch-Panel-240x320-Dots-5V-3-3V-Module-ILI9341-Driver/32665656357.html
Add the $30 ADF4351, a $17 $AD9851, and USB and you have a nice bundle
for half what the above costs.
Or maybe a real RF generator from HP, TEK, Fluke or others that can
actually be calibrated and trusted. This is the cheapest HP I could
find:
http://www.ebay.com/itm/Hewlett-Packard-hp-8656A-Signal-Generator-1-990MHz-rf-signal-generator-04-/252950700229
"Does not ship to Australia"
I have an HP 8656A but prefer to use an HP 8540B. Top right:
http://802.11junk.com/jeffl/pics/home/slides/test-equip-mess.html
I have to give this HP5386A back, but not in a hurry - my friend also
has mountains of test equipment. He worked in sat-comms, so has contacts
who call him before dealers get there - but he loves to hoard it all
Clifford Heath.