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Author Topic: SDR - DC Receivers?  (Read 990 times)
K3UIM
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Posts: 388




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« on: May 23, 2019, 08:38:18 AM »

I continue seeing SDR mentioned in association with DC receivers.
Without having to dig through all my literature, can someone
enlighten me to the meaning of SDR receivers? Is there a relationship
of the two?
TIA,
Charlie, K3UIM
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Where I am: You will be!
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N8AUC
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« Reply #1 on: May 23, 2019, 10:16:06 AM »

Interesting question. Depends on the receiver, I suppose.

You can heterodyne the frequency range of interest and use SDR techniques on the IF.
Or you can directly digitize everything at the antenna port, and use SDR techniques on that.

Of those two choices, and I may be wrong on this, I'd consider the latter to be "direct conversion".
Yet both are SDR.

So I suppose, an SDR receiver can be direct conversion... or not.
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N2DTS
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« Reply #2 on: May 23, 2019, 10:32:50 AM »

I think a direct conversion receiver is like the HW7 and HW8 rigs where the rf is converted directly to audio.
You get both sides of sideband and CW and they tend to be microphonic and pick up hum.
A very basic RX design where low parts count is the priority.

SDR's can be direct sample (everything converted to digital at the antenna (or after preamp/attenuation), or the RF can be mixed down to an IF (9 mhz or audio frequencies and then digitized.

A direct conversion receiver is a bit better then a regen in most cases, and can work well in many situations, but unless you want something REALLY simple, a superhet with a xtal filter does so much better and only takes a couple of ne602 chips and some cheap crystals as a filter.
The peak performance of that would be something like the old wilderness radio sierra (pre Elecraft Elecraft).
Simple but great performance with a very low parts count.
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K3UIM
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Posts: 388




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« Reply #3 on: May 23, 2019, 11:28:17 AM »

Many thanks, guys. (I assume, as some call letters wear skirts. LOL)
Judas Priest! Am I showing my age!!! In today's world, is there any difference any more?? LOL
I'm too old to start learning any more than what I just learned from you'ns.
"One foot in the grave" Charlie, K3UIM
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W6RZ
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Posts: 362




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« Reply #4 on: May 23, 2019, 03:26:52 PM »

It's a very common architecture with wide band (many GHz) SDR's. Here's a "prosumer" SDR that uses direct conversion, covers 70 MHz to 6 GHz, is transmit capable and costs $745.

https://www.ettus.com/all-products/ub200-kit/

It uses a transceiver chip from Analog Devices.

https://www.analog.com/en/products/ad9361.html

Here are two block diagrams comparing direct conversion and direct sampling architectures.



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K3UIM
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Posts: 388




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« Reply #5 on: May 23, 2019, 04:09:13 PM »

Oy vey! I can see that I'm getting too much information! Hi.
(Been away a quarter of a century and it shows.)

Remembering from my General test in 62 or 63, a straight line pointing north means up.
A straight line pointing south means down.

But I digress:

Thanks though.

Charlie, K3UIM
LOL
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Where I am: You will be!
So be nice to us old fogies!!
N8AUC
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« Reply #6 on: May 23, 2019, 08:17:09 PM »

Many thanks, guys. (I assume, as some call letters wear skirts. LOL)
Judas Priest! Am I showing my age!!! In today's world, is there any difference any more?? LOL
I'm too old to start learning any more than what I just learned from you'ns.
"One foot in the grave" Charlie, K3UIM

Hey Charlie!
If the baseball outfield still looks green from where you're sitting, you're doing fine! Don't sweat it.

73 de N8AUC
Eric

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K3UIM
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Posts: 388




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« Reply #7 on: May 23, 2019, 08:54:05 PM »

Eric, I'd love to go to one of those baseball thingies, but at my age, I'd probably catch one of those hockey pucks in the teeth, so Lois is dead set against my going.

She's even a wee bit fearful of me catching too many of the ham radio electrons coming off my antenna. I told her not to worry because they're all too high in frutons to hurt me. She's ok with that now.

I used to enjoy life better when I was in my 20's.

,,, uhhh … By that I mean.... Oh never mind.

Charlie, K3UIM
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Where I am: You will be!
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KB1GMX
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Posts: 1794




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« Reply #8 on: June 06, 2019, 08:47:12 AM »

For the dated among us,  simplified no complex pictures or horey math.

In most cases DC means direct conversion to audio.  Basically the BFO
part of older radios.    The tech for this is easily back to before the 20s.

Centrury21 a DC RX for CW (Transceiver).  Notable that the same signal appears
both sides of zero beat. Very simple just a BFO and Mixer to get audio and lots
of audio amp maybe filtering. Simple, low power and effective.  There are a whole
class of simple DCRX (rockmite and many others) like this.  VEry common in the late
70 and on and still appears as simple but usable radios.

Then there are direct conversion with image rejection.  They down convert
the signal in two channels (I and Q) and with more analog stuff or sound card
and software to process the baseband signals and essentially reject the
unwanted sideband.  Selectivity via math performed in software or using
analog opamps.  Can offer outstanding performance (KX2, KX3 and K3 for
example).  A simple radio that uses the full analog form is the QCX(QRPlabs)
and tuned up right can put make an adjacent signal more than a few hundred
Hz away go away.  This is also old tech and the basic analog form was the
Central Electronics CE10.  Tubes and old parts made it a complex tuneup
as parts aged and heated.  Now with solistate and high performing opamps
a stable system can generate a better than average signal (TX) or make a
very good RX.

Then there are true digital radio that take the signal directly from the antenna
(some processing) convert it to a high speed stream of digital representations
and grind out the desired signal using complex software.  Most of the software
is a digital synthesis of analog functions like filters.  Can be costly due to the
need for very fast A to D but can be very to extremely high performing.

Whats interesting is the mostly or all analog image reject and non image reject
(vanilla DC like the C21) using direct conversion rely on very low distortion in
the RF and audio channels to work correctly.  As a result of that they have an
uncanny good sound even on a crowded band with overlapping signals.


Allison
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N8AUC
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Posts: 589




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« Reply #9 on: June 06, 2019, 09:25:28 AM »

I had a Century 21 for quite a while.
And you're right. There was a pleasant quality to listening to it.
It was horrible on a crowded band due to only audio filtering for selectivity.
But you could listen to that radio for hours almost fatigue free.
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G4AON
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Posts: 1382




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« Reply #10 on: June 07, 2019, 11:41:06 PM »

The Elecraft KX2 and KX3 transceivers are SDR “direct to audio”, the KX3 optional roofing filter is in fact an audio filter. There is an alignment routine to minimise the unwanted audio image, this appears to be very effective at one spot audio frequency (say 700 Hz), but is poor when a few Hz away. The end result is mostly OK in terms of audio image rejection, especially on SSB, but is poor when compared to a conventional superhet radio.

I’m not particularly knocking the KX2/KX3, I have a KX3 and much prefer it over my K2 and sometimes over my K3.

73 Dave
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