On Jan 5, 3:10*pm, "Joel Koltner" wrote:
"K7ITM" wrote in message

...

As you think about all this stuff, it becomes easy to see why nobody
has yet built the perfect receiver. *;-)

Hey, I was at a conference some five years or so ago now where some high-level
muckety-muck from Intel got up there and claimed that within a few years we're
be connecting antennas straight to ADCs and radios would henceforth be 100%
digital... :-)

Of course, it is a bit easier to build a good radio when you're operating in,
e.g., the cell phone bands and by law you control the spectrum (no big
intereferes), you manage the power of all the transmitters dynamically
(limited self-interference), etc.!

Well, I guess it was about four years ago now we introduced a 100kHz
to ~35MHz receiver that practically does that. There are gain stages,
attenuators and some selectable filtering in front of the ADC, but our
customers want to be able to "listen" to the whole band at once, so
you can switch the filtering all out if you want. I did most of the
hardware, up to the signal processing. We've gotten feedback from
some customers that it's the best receiver (for that purpose) that
they can buy. It's particularly spur/residual-free: I added some
copper tape to one to see just how good I could make it, and the worst
residual is about -144dBm at the switching frequency of one of the
switching POL regulators (around 600kHz). Worst residual above 1MHz
is about -154dBm. I think it's fair to say that's pretty hard to do
with a general-coverage superhet design. But neither the hardware nor
the software that goes with it are inexpensive enough to worry that
it's going to replace other ways to do it any time soon--and the
performance doesn't quite equal what you can do with a really good
single-signal superhet design. Check in again in a few years,
assuming that there's enough economic incentive for ADC designers to
give us a little bit better parts. There are some claims out there
for really stellar ADC performance with Josephson junctions, but they
require cryogenics, and from what I've heard not all the claims are
substantiated...

By the way, the gain that's available in front of the ADC in this
design is mostly there because customers expect to need the gain,
based on previous experience. In a perfect world where they really
understood what's needed, I could have gotten by with maybe 10dB
maximum available voltage gain between the antenna and the ADC. I'm
more worried about how to gracefully handle big signals--much more
worried. What do you do about the plethora of short wave broadcast
signals, several of which can each be up to perhaps 0dBm out of your
antenna, or the fellow just down the street (or on the same ship,
etc.) who keys up a transmitter and feeds +20dBm to your receiver --
WHILE you want to keep listening to the signal that's only -110dBm at
your receiver?

Cheers,
Tom

Hi Joel. In my experiance with diode ring mixers they almost always require specific impeadance terminations on all ports. The input should look at 50 ohms, the L.O the same. The I.F port is the most important to terminate properly if the desired switching or mixing products are to be produced. In many receivers designed for high dynamic range the I.F of the diode ring is terminated into the base circuit of a very strong bipolor transistor stage using feedback many times. Because of the feedback used and particular biasing this stage can be designed tolook like near 50 ohms to the I.F port of the diode ring. This is not enough many times though. This same transistor amp stage then many times drives a crytal I.F filter that can have the correct 50 ohm term if designed that way but this term will only be 50 ohms very near the passband of the filter and frequencies that depart from this 50 ohm passband away from this freq causing this hi SWR to reflect the incorect impeadance back through the output to input of the amp missmatching the diode ring. Amp.s that use heavy feedback are even worse in this respect. In cases like this the designer many times hooks a 50 ohm pad (such as 6 db) to force a decent 50 ohm load back through the amp to the mixer. This mixer termination stuff is quite important if the mixer is to work as it should.In situations where lower dynamic range is desired for battery drain, etc, this termination MUST be provided for in other ways. The same thing holds true for direct conversion or zero I.F receive schemes, the audio preamp is biased and designed for a 50 ohm term for the diode ring. Much can be learned in literature such about this topic from Solidstate Design for the Radio Amateur, Experimental Methods in Radio Design, both by coauthor Wes Hayward, W7ZOI and much can be learned from publications by Minicircuit Lab's, the builders of many high quality DBM's for industry. Check the web, much can be learned. 73's, Doug,

"K7ITM" wrote in message
...
On Jan 5, 3:10 pm, "Joel Koltner" wrote:
What do you do about the plethora of short wave broadcast
signals, several of which can each be up to perhaps 0dBm out of your
antenna, or the fellow just down the street (or on the same ship,
etc.) who keys up a transmitter and feeds +20dBm to your receiver --
WHILE you want to keep listening to the signal that's only -110dBm at
your receiver?

At work one of the products we sell to the military consists of a handful of
electronically adjustable notch filters for precisely this purpose -- the
output is fed to (someone else's) SDR.

....although our dynamic range isn't the 130dB+ that you'd need for your later
example... (at least in some reasonable bandwidth...)

Your almost-all-digital HF receiver there sounds quite impressive. Do you
give the user the option to adjust the switcher's frequency away from 600kHz
if they happen to really want the best sensitivity right there? (...this
seems to be the common approach with many a ham HF rig...)

---Joel

Thank you Doug; I'll go grab Wes's book from the shelf and do a bit more
Googling.

---Joel

On Jan 6, 9:47*am, "Joel Koltner" wrote:
"K7ITM" wrote in message

...
On Jan 5, 3:10 pm, "Joel Koltner" wrote:

What do you do about the plethora of short wave broadcast
signals, several of which can each be up to perhaps 0dBm out of your
antenna, or the fellow just down the street (or on the same ship,
etc.) who keys up a transmitter and feeds +20dBm to your receiver --
WHILE you want to keep listening to the signal that's only -110dBm at
your receiver?

At work one of the products we sell to the military consists of a handful of
electronically adjustable notch filters for precisely this purpose -- the
output is fed to (someone else's) SDR.

...although our dynamic range isn't the 130dB+ that you'd need for your later
example... (at least in some reasonable bandwidth...)

Your almost-all-digital HF receiver there sounds quite impressive. *Do you
give the user the option to adjust the switcher's frequency away from 600kHz
if they happen to really want the best sensitivity right there? *(...this
seems to be the common approach with many a ham HF rig...)

---Joel

Hi Joel,

I'm curious about the tunable notch filters. Is there a data sheet I
can find somewhere?

There's really no need to get the ~600kHz any lower. Any decent
antenna at that frequency will pick up way more atmospheric noise than
the level of the switcher residual. After all, -144dBm is only about
14 nanovolts RMS at 50 ohms. I think it's fair to say that any of our
customers looking for little signals will be using good antennas for
the job. Admittedly, a stock unit won't do that good, but it will be
in the neighborhood. Of course, the high noise level at ~1MHz is why
we get by with such crumby antennas for our portable and car AM
radios.

Cheers,
Tom