RE: here is how to "get a feel" for a roofing

[I7SWX]

Hi Brett,

your questions are interesting but they requires  many
words to clarify them... I will try to give you simple
answers and may be other friends will add replies.

Message: 22
Date: Wed, 02 May 2007 11:38:44 -0400
From: Brett gazdzinski
<[hidden email]>
Subject: RE: [Elecraft] here is how to "get a feel"
for a roofing
        filter
To: [hidden email]
Message-ID:
<002001c78ccf$f7e09d00$[hidden email]>
Content-Type: text/plain; charset=US-ASCII

Is there no way to put dsp at/as the first filter?

> Yes, more or less. This is High Performance Software
Defined Radio solution where you connect the ADC to
the antenna, then a DSP is handling all the
demodulation of signals, manage the filters and
notches, once this works (software) is completed the
voices or dit dah willa come out from an audio
amplifier and on to the loudspeaker(s).

> K3 is on the hardware and software transmitter and
receiver group.

What are the limitations of the dsp, frequency, filter
quality?
Is there a problem getting dsp to work at say 455 khz?

> Putting the DSP (including ADC and DAC) at 455kHz is
not a problem as the key is the ADC frequency limit.
There will be not too much difference between havin
the IF at 455kHz or 15kHz as the major work will be
done by the software application.

What would be the result of a real good variable
filter
at the 8.215 Mhz IF just after the mixer and NO dsp
for filtering? (K2?)

> The result will be a K2 with a higher IF. DSP is a
powerfull stage (including the software) to generate
filters at audio level. No ringing with narrow filters
and soft to straight attenuation skirt better than
xtal filters.

The 2nd IF of 15 khz is audio, is it not?

> NO. certainly if you have a good ear you can listen
to something... It is an electrical Frequency like the
ones used for submarine communications and other
services.

At what point does the audio spectrum end and
RF begin?

> The RF (IF) ends inside the Analog to Digital
Converter (ADC)... then you have digital signals
handled by the DSP (and SW). The audio start at the
output of the Digital to Analog Converter (DAC)

I notice that the K3 info says the dsp is in the 2nd
IF,
but the 2nd IF is high pitched audio?

> Even if the 15kHz is a frequency that has an
equivalent in the audio, it is a real IF and the
conversion to this second IF is done by a second mixer
converting from 8.250MHz to 15kHz, more or less like
an SSB/CW demodulator.


This modern stuff sure spins my head, the homebrew
receiver
is a single conversion to 455khz, no rf amp, good
filter
just after the mixer, wide IF chain, detector.
It seems to work real well, and is basically the old
tube table top
radio (all American 5?) with a good filter, s meter,
and a digital
frequency readout.

> I know what you mean ... I am an analog chap and do
not like a PC in my radioshack ... but I am fiddling
with it, on and off to learn what SDR, Software
Defined Radio con do ... today we can play with an
analog mixer that converts from HF to audio...yes the
stuff that comes out from the loudspeaker ... and then
you have a BIG DSP ASSY ... the Audio Card (Stereo) a
Big PC and a Software application that may make you
crazy to look at the PC screen ...
Soon all this type of SDR will become obsolete like a
valve receiver because the receiver will be nearly
100% all digital ... the only "analog" stuff will be
the antenna, the bandpass filters and the loudspeaker
... plus a BIGGGG and Powerfull PC.

> At this point you will have to change operating
technique as you will be able to operate with several
stations and your roofing filters will be your
BandPass Filters ... our good friends at INRAD will
retire and we will be able to buy second hand surplus
xtal filters  for a couple of dollars (I hope .. hi)

> I hope I was able to clarify at least partially your
right questions

> BTW, if you can fidlle with a soldering iron and
have a K2 I can send you a working front end mod (1st
mixer replacement) that can improve it to an IP3 of
+27dBm , from a +15dBm value, and the addition of a
xtal homebrew roofing filter at 4.9xx MHz.

73

Gian
I7SWX

Brett
N2DTS




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[Ken N9VV-2]

F.Y.I.

I know of at least two experimental ANTENNA --> DSP designs that are
out there being tested:

HPSDR "Mercury" design
http://hpsdr.org/mercury.html

and Phil Covington N8VB's QuickSilver QS1R/QS1T design
http://pcovington.blogspot.com/2006/12/qucksilver-qs1r-software-defined.html

de ken n9vv

> Is there no way to put dsp at/as the first filter?
>
>> Yes, more or less. This is High Performance Software
> Defined Radio solution where you connect the ADC to
> the antenna, then a DSP is handling all the
> demodulation of signals, manage the filters and
> notches, once this works (software) is completed the
> voices or dit dah willa come out from an audio
> amplifier and on to the loudspeaker(s).
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[Stuart Rohre]

Additional comments on using an A/D at the front end of a radio.

High Speed A/Ds that can handle the wide dynamic range of radio signals are
still an art form.

Any A/D suffers from the need to have filtering in front of it to avoid a
malady known as aliasing.  In A/D theory, without band limiting the signals
into the A/D, it might try to convert signals out of the band you want, as
well as the in band components.  You could have the unwanted freqeuncies be
the same spacing from the converter clock frequency, as the desired signals,
and then they "fold" into the band of desired signals, but would be like
noise or interference.   The clocking of the A/D conversion, (the switching
speed) must be at least twice your highest desired frequency, and preferably
four times the highest in band frequency.  The clock must be stable, and not
have jitter.  For a 30 MHz radio, you might then need a clock of 120 MHz.
This has the aircraft band among others, below the clock frequency, and
there is the FM broadcast band, as well as low band public safety
communications.

In other words, a lot of undesired signals that could "fold" into the
desired HF bands when using an A/D.

For this reason, as an "Anti-aliasing filter" we DO put a high quality
filter in front of precision A/D converters, to limit the signals to those
of interest to us as an HF radio.

Thus, the need for a roofing filter is not restricted to just an analog
radio, but enters in for direct digital conversion as well.

If you Google search for "Analog to Digital Conversion" you will get many
white papers and tutorials on all the faults of A/D processes.  That is why
they are not common in radios yet at front end RF frequencies.  You need
many bits to handle a radio's dynamic range, or you need programmable gain
amplifiers to keep the incoming signal up at the optimum amplitude for the
range of the A/D system.

-Stuart
K5KVH


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