MITCH
LEE RECEIVER
RE-VISITED
What
follows here are three edited emails between Steve Ratzlaff and myself
about some questions he had concerning what he perceived as a noise
problem in the ML IF strip design. Steve Ratzlaff has been
the contact
between the designer, Mitch Lee and myself. Steve is the
individual
who put the PDF form and schematic together and obtained permission to
post it here.
*********************
From: Steve
Ratzlaff.
I got a letter today from Mitch Lee; he's too busy to work on any ML
radio mods right now, but he said to say congrats on your successful ML
radio construction and usage.What are those nice cabinets you used for
yours? They don't look like LMB cabinets. Enclosures are so expensive
these days. Do you notice much drift when the temp changes in the room?
I've built 3 IF strips now, and they all drift 8-10Hz, enough to go out
of the peaked narrow passband, if tuned up in the afternoon when the
room is warm, and checked the next morning when the temp is 10deg
colder.
Of course you have to wait
until the DDS clock
oscillator stabilizes, perhaps 10-15 minutes, before checking for any
IF strip drift.The Elecraft NE602 product detector and simple AGC did
not work for me. All the static crashes kept the gain severely reduced,
and it took 5 seconds or more to very slowly recover each time. Seemed
like a good circuit to try, though.
But the main IF strip problem
is the grainy,
staticky sound the IF strip makes--all 3 of mine have the same sound
that sounds like low level static picked up by the antenna, but with
nothing connected, it's still there. That's the main problem I see with
this ML IF strip idea. Using my reference setup, the R75 with 10Hz
599zx DSP, which is very quiet and has no low level of anything heard,
the ML IF strip is much noisier and quite annoying to listen to for any
length of time.
After awhile you imagine you're
hearing idents in
the static, just like when listening to real antenna-received static.
I've tried 2 different circuit variations now, from the original one.
All have the same problem of the grainy, low level static noise
constantly heard.
Unless that and the drifty passband gets fixed, this ML idea probably
is not very reproducible for mass-construction. I've built up 3
different IF strips now, with no parts reused in any of them.
Originally I
thought maybe I had some bad parts, but all 3 sound the same.
One can
certainly hear beacons with the narrow passband, but it would be much
nicer to have a quiet receiver, limited by the external signals.
Steve
*********************
MY REPLY:
Good evening Steve,
I was thinking about writing you and asking how your receiver was
coming along. After extensive use, here are my observations
on the
set. I hope this answers your questions.
1. In "real" use I haven't noticed any problem of changes
with
temperature, though a response check like the one I did in preliminary
testing may be worthwhile when I have time. My radio room has
it's own
window air conditioner now, and while "slight" changes in temperature
as it cycles
drastically affects the Hallicrafters SX-71 when listening to SSB or
CW, it doesn't appear to affect the ML set in any way. Of
course the
ML set with all it's mass of copper shielding, lack of ventilation etc
isn't likely to change temperature much, unless the change is large and
sustained. (It's actually rather heavy for it's size).
2. The cabinets: They come from Radio Shack and are/were
their "deluxe
steel cabinet", same as the one your preamp you got from me
has. They
were a little more expensive than the plastic stuff, but I got these
for 9.95 each. One of them had a slight amount of damage in
one corner
I was able to straighten out ok. They must have had poor
quality
control though. Of the three cabinets, all three are slightly
different shades of paint, either the ivory or the black.
3. The Static or grainy sound that you hear "fake CW"
in. I call that
"CW Ghosties" as it sounds like Ghost CW. It is due to random
noise
causing the extremely narrow analog filters to "Ring" slightly, and I
don't think there is ANY way you can get around it, not with analog
anyway. I have heard EXACTLY the same thing when I've used
VERY narrow
analog filters like the Autek QF-1A cranked way down on conventional
receivers.
Bill Hohenstein once explained
it to me that it's
an indication you're working very near the theoretical limits of
sensitivity/selectivity. Apparently the "ring time" is
affected
somewhat by the actual narrowness of the analog filter. Some
notes:
A. IF I turn off the
RF pre-amp, switch the antenna off etc so
that I'm listening to ONLY the IF strip noise, I have to turn the Audio
Gain up VERY high and then I just barely hear "Ghosties" down in the AF
hiss. The RF gain setting (input potentiometer between mixer and input)
naturally has NO effect on this. To me this indicates the IF
strip is
actually pretty quiet!
B. Turning the RF
preamp on, the Ghosties rise some, indicating
that the RF pre-amp raises the noise floor of the receiver.
The RF
gain pot DOES have an effect on this. This I would expect.
C. Connecting the
Antenna with no signal again raises the level
of the Ghosties, and again I expect this, all indicating that the
INCOMING noise is the limiting factor, not IF strip noise!
D. If you have the
antenna connected and the RF gain set TOO HIGH
the "Ghosties" can get pretty bad! There is probably more
front end
gain in my receiver than is really needed. I find that
OPTIMUM setting
for the RF gain varies across the band and with conditions, but
typically doesn't run more than about 50% up. That's with a
5K pot at
the output of the RF amplifier and an "assumed" 50 Ohm load in the
mixer.
I find that there is better
sensitivity when the
LPF at the input to the mixer (RF amplifier has one by default) is
switched OUT, but on very weak signals the S/N seems a little better
with it switched IN. I have to presume that's because any 2
MHz noise
generated in the wideband RF amplifier is filtered out before hitting
the mixer.
4. The NE602 product detector/AGC: I've not had any problem
with the
AGC "blocking" the set. In fact, I'm not at all sure the AGC
is really
doing anything in my set. I say this because using a
Voltmeter I
haven't been able to see the transistor "turn on". Possibly
the diode
I used for AF rectification wasn't suitable. Next time I get
in the
set I'm going to examine that area closer. Perhaps a
germanium diode
is called for there.
5. AF filtering: I have a brute force RC filter between the
Product
detector's AF chip and the following output stage (volume control
between). This to eliminate any AF
"hiss". This was chosen
empirically and was about .1uF. It is connected where the 1
Hz Notch
filter will be tied in. In the Notch filter I've added an
active 425
Hz LPF BEFORE the Notch filter to remove all above 425 Hz (I'm tuned
for 400 Hz beatnote). Following the Notch filter will be
another
buffer with a 425 Hz active LPF to take out any AF Hiss generated in
the filter or earlier stages. I find any AF hiss is FAR more
distracting than the "Ghosties" ever are.
6. THE TEST FOR GHOSTIES: You probably know all this, but
I'll reprise
it for you. Here is what I am constantly doing ANYTIME I'm
using very
narrow bandwidths as "Ghosties" can emulate nearly any callsign in your
mind.
A. FIRST, TUNE up
and down a little. If it's REAL it will change
pitch. A "Ghostie" NEVER changes pitch. Note, I
said "Pitch", not
volume. Ghosties can occasionally be aggravated by nearby
strong
signals, especially DGPS, but will never have their pitch
changed. The
pitch of the Ghostie is determined ONLY by the center frequency of the
filter in relation to the BFO and Ghosties get louder in relation to
band noise.
B. Change Bandwidth:
Ghosties are afraid of wide bandwidths and
will usually disappear into the noise when things are "opened
up". For
example, in my ML receiver, the gains are about equal, but switching in
the WIDER
bandwidth make the Ghosties nearly disappear, though if you listen
closely there is a hint that they're there, much weaker and slightly
lower in "pitch" (the wide Bandwidth is offset slightly from 2
MC). A
REAL signal won't change pitch with bandwidth (assuming you have more
than one to select).
C. You probably
won’t hear Ghosties on a receiver like the R-75
with outboard DSP simply because the IF bandwidth is too wide and the
DSP even when cranked down narrow generally doesn't "ring".
Now, if
you had a "cranked down" Autek QF-1A on the R-75 they’d
probably be
really bad.
PROPER USE OF THE SET: I've found that this is one set that requires a
LOT more finesse to use than the average receiver on which you just
crank the tuning until you hear what you're looking
for! Turn the RF
gain up too high and the Ghosties and noise will drown out any weak
signals. In fact, turn the RF gain up too high and the set
produces
some "VERY RUDE" noises in the headset on the least little noise
burst. I believe this is due to the NE602 being overdriven,
though I
haven't dug into it much yet. Turn the RF gain too
low and you're not
going to hear that weak one. I leave the AF gain set about
"midway"
and "ride" the RF gain.
HOWEVER, properly used I
believe that it is very
difficult to find ANY receiver that will outperform it, especially
considering the cost. Not counting the DDS (which
can be used
elsewhere too since it's in a separate cabinet) the set cost me less
than a GOOD quality CW filter for any name brand receiver.
IT PROBABLY TOOK ME AT LEAST A WEEK OF STEADY USE TO GET THE IDEA, LESS
RF GAIN IS "BETTER" in the use of this receiver. You
mentioned I'd
probably not need the RF amplifier. Not true. Without it I
only hear
the stronger
beacons, most likely because my antenna isn't a good match for 50
Ohms. This has proven to be true in every receiver I've had
here.
This set is probably better suited for taking time to ferret out the
tougher beacons than it is "cruising". Punching in for
different
tuning steps and the narrow bandwidth slows "Cruising" down
greatly.
But I've NEVER found a receiver that was able to pull ADK out of the
BCB junk on 530 KC. A DSP trying to process the audio after
it's
detected has never been able to do it, at least not here where there
are 3 Highway Info stations on 530 all the time. This one did
it!
I'm also considering tapping the 2 MC IF prior to the high selectivity
stages so that the Hallicrafters SX-71 can be connected for when I want
a much wider bandwidth, such as for things like DGPS, high T-storm
noise conditions etc. That way I'd still have my digital
accuracy and
front end. Probably would tap it after the first Xtal stage
as that
way it's "buffered" from the mixer. That first stage is common to both
bandwidths and is centered on 2MC.
I guess you would have to say that my receiver is only "partially" a
Mitch Lee receiver, the IF section (which of course is it's
"core").
The rest is my own creation or robbed from other circuits.
Well, guess it's time to go look for beacons. Have fun.
73 de Phil, KO6BB
*********************
STEVE’S REPLY BACK:
Hi Phil,
Thanks for the detailed reply. I've read it a couple times now. Very
good information from your use of the ML radio. I now think I'm hearing
what's normal for this type of circuit in my own IF strip. I may still
have a drift problem. I'm using plastic trim caps; I notice from the
pictures you have ceramic ones; Mitch says he has ceramic too. My
Mouser order said ceramic on the ad, but when they came they were
plastic. I'll have to get some ceramics and try them.
It sounds like the gain
distribution is a very
carefully adjusted parameter in this circuit, and I don't seem to have
quite gotten it adjusted quite right yet. Mitch says the Mouser ECS
crystals are simple room temp microprocessor crystals, not spec'd for
operation at elevated temps (like in an oven), thus should be fairly
stable at room temp.
Now I think I'm trying to make too much out of this circuit. It does
amazingly well, for what it is, as you've found out with your ADK
reception out of the TIS/HAR jumble. I'm used to hearing a quieter
receiving situation, dominated by antenna noise. I need to properly
adjust the gain after the IF strip plus the gain ahead of it, now.
That original DDS article you sent me on the JPEG format, I stumbled
across FAR circuits, the guy who does all the PCBs for the ham magazine
articles--he has done a full PCB for that article/DDS. It's also been
upgraded to the v.3a PIC software that you're using, plus he offers
programmed PICs with the PCB if you want. The AD9850 plus the PIC are
all on one small PCB--complete DDS and Controller--no need to wait and
wait and wait for amqrp.org to come out with a new batch of DDS
Daughtercards and hope you're among the lucky to get in on the batch.
So I've ordered the PCB and the PIC from FAR. (The PIC should be the
same as the two you programmed for me, but I bought his just to make
sure, too.) It just arrived--it's a beautiful board, about 3 x 3
inches. Course the AD9850 still has to be soldered on. I
don't have an
iron with a tip that small, or any small solder either.
I guess I'll have to invest in
a new iron. I
haven't bought one in 30 years, and try to solder the DDS chip on
myself. So, assuming I can solder the chip on when my new iron arrives,
I ought to be able to make a functional DDS/controller board finally,
and not have to use my signal generators on this ML project anymore.
All the external parts still are needed-rotary encoder, switches, LCD,
etc.
I think a complete DDS section can be built for just under $100. That
seems to be the most expensive part of the ML setup. I told
Mitch you
didn't have any spur problems; he could hardly believe it.
His DDS
uses a 50Mc clock, which I suspect is the main problem for all the
spurs he reports finding. Ours uses the 100Mc clock--I think that makes
a major difference for spurs at the low freq the ML radio works at for
the synthesizer. Incidentally I could only find one source in my usual
parts catalogs for a 100Mc clock oscillator, from Jameco. Mouser and
DigiKey don't list any that high. I ordered that from Jameco, too. I
have the LCD and rotary encoder already on hand, and of course the
AD9850 from the one you sent. And I figured out how to register with
Analog Devices, and ordered the sample AD9850 from them, too. Hopefully
they'll send me two samples.
Amqrp.org
says you have to get the DDS chip as a sample from Analog Devices.
Apparently they don't mind sending out a lot of them to
non-company/non-engineering use of them, though the request had you
answer a bunch of questions on your application of the sample. Kind of
hard to make it sound you weren't a hobbyist wanting the sample just
for a hobby project, not for a real developmental commercial product.
There must be a distributor that will sell them to you in small
quantities, somewhere.
Anyway, thanks for the thoughts on using your ML radio--that seems to
clear things up for me. Maybe sometime you could tell me again what
your circuits before and after the IF strip are. Sounded like you're
using an additional audio amp after the LM386 amp of the Elecraft
product detector/amp. And those RS cabinets sound like a bargain, even
if they are steel and not easily worked aluminum.
see ya.
Steve
Melt
Some Solder!
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