[Lowfer] LOWFER receiver conundrum.

[Ben Hall]

Good morning all,

I'm going to reply to John's message...but I want to thank you for all 
of the replies on my receiver conundrum.  I've been very quiet since the 
replies as I've been doing a *lot* of reading and experimenting based on 
what y'all sent me.  Which...will be pretty obvious in the length of 
this post.  (sorry...people at work accuse me of having keyboard 
diarrhea...which is accurate...especially when I'm discussing a subject 
that interests me.)

On 1/6/2015 6:17 PM, JD wrote:
> It's actually a very common thing, Ben.  The reference _oscillator_ is
> seldom used directly on its own as the reference _frequency_  in the
> phase locked loop.  Most often, the crystal or GPS source is divided
> down to whatever step size you want the PLL to be able to tune in
> multiples of (10 kHz, 1 kHz, or whatever).

This is one very important thing that I missed with PLL's!  And probably 
very handy for what I need here.

> Probably the best route is the good old fashioned printed page,
> and probably the best general experimenter's reference book on PLLs is
> the one by Don Lancaster.

Which book is this?  I did some poking around and I think he addresses 
PLL's in "Active Filter Cookbook" based on a snippet of an obscure 
review found on Google Books.  Used...it seems to be less than $20 
shipped.  :)

> A condensed and somewhat mathematical online tutorial is at Analog Devices:
> http://www.analog.com/static/imported-files/tutorials/MT-086.pdf

This is a nice intro.  The math wasn't bad at all and pretty easy to 
follow.

In other replies there was the suggestion of a DDS.  This suggestion 
took me down a real rabbit hole.  Yes...just about all of these require 
many additional components to provide a working solution.  (IE: 
microprocessor, LCD display, tuning encoder, etc...)  While that adds 
complexity...it adds flexibility too.  I've done some PIC programming in 
the past, so this doesn't eliminate this as an option.

And maybe...I've got something already that I can modify.  I've built 
the IQ-VFO and IQ-PRO VFO by AA0ZZ.  (Both are AD9854-based) 
Plus...somewhere...I've got a PIC-EL board with either the DDS-30 or 
DDS-60 daughterboard.  (The -30 is AD9850 based, the -60 is AD9851, IIRC.)

So I got real excited that perhaps...instead of using the on-board 
crystal oscillator...perhaps I could feed that with either the 10 MHz 
standard (Z3801) or OCXO.

For AD9854...this is an option!  The refclk depending on if the 
multiplier is enabled or disabled can range from DC to 200 MHz.  For the 
IQ-PRO, I beleive the refclk multiplier is enabled, making the lowest 
refclk frequency 20 MHz.  I'll be honest, I didn't find this in the 
IQ-PRO literature, so I may be wrong.  So I can't conduct a quick test 
by disconnecting the crystal and feeding it an external reference 
without changing the firmware on the IQ-PRO.  This, while not difficult, 
will take some time.  And I like instant gratification, ha ha ha.  ;)

For the AD9851...regardless if the refclk multiplier is enabled or 
disabled, a 10 MHz reference can be applied.  (with multiplier disabled 
- 1 to 180 MHz input range, with multiplier enabled - 5 to 30 MHz 
range.)  I believe I can easily disconnect the XTAL on my DDS-60 and 
apply the 10 MHz from the Z3801.  I do need to check voltage levels, I 
may need some interface circuitry.  Bottom line...I believe that with 
the PIC-EL and DDS-60 daughtercard, I can try this out.  ;)  My sole 
fear is that I could due damage to the DDS-60 in my experiments.

But...it gets even better.  Some folks here may be familiar with the 
Arduino microcontrollers.  Turns out there is a well-known, open-source 
Arduino-controller for the DDS-60:

<http://www.theladderline.com/dds-60>

It is possible that the Arduino version may be even easier to program 
than the PIC-EL.  Plus...with some code tweaks, the Arduino version can 
control some of the very inexpensive AD9850 variants found on the e-place:

<http://www.theladderline.com/inexpensive-ad9850-dds-boards-ebay>

So...the combo of the above - easy Arduino plus cheap AD9850 board from 
the e-place...this is quick, easy, and if I fry the silly thing, no big 
cheese.

As to phase noise in a DDS...I really don't know.  It is possible that 
the phase noise will render them useless in a receiver application.  In 
past experimentation with using the IQ-series VFO's with 
direct-conversion receivers in the LOWFER bands was not successful.  I 
never did determine why.  Phase noise, perhaps?  Don't know.

SWITCHING GEARS...another frequency-generation solution used on the 
SoftRocks is the Si570 clock generator.  Yes...same downsides as the 
above (needs an interface) but it is known to be a very low phase noise 
frequency source.  It also cannot accept an external frequency 
reference.  Looking at other SiLabs products, I don't recall finding a 
similar chip with an external frequency reference.

The interfaces are cheap, available, and rather compact.  K5BCQ has a 
kit on their page that is sub-$50.

<http://www.qsl.net/k5bcq/Kits/Kits.html>

I built one some time ago for use in making a frequency-agile softrock 
that didn't need a PC for frequency selection.  Works great.  The issue 
with it is thermal drift - the drift, while fine for most applications, 
is too much for QRSS and other "low and slow" modes.  The reason IMHO, 
is the onboard crystal osc drifts.

While this seems to eliminate the Si570...I found that the Genesis G59 
SDR tranceiver uses an Si570, and includes what they can a "quasi-oven" 
method of stabilizing the Si570 temperature:

<http://www.genesisradio.com.au/G59/phase11.html>

I think I may duplicate this circuit and see how well it controls the 
temp (and drift) of the Si570.  Perhaps it is stable enough for "low and 
slow" modes.

I'll keep the list informed as to the progress of my experimentations.

thanks much and 73,
ben, kd5byb