OT mechanical v crystal filters OT

[David Cutter]

Off Topic but with so much expertise flying around I thought I would ask how mechanical filters compare with crystal filters.  

I think they were fairly low in frequency (455kHz?), so, not suitable in the present discussion,  but it would be good for our education at least.

David
G3UNA
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[Brett gazdzinski-2]

Its my understanding that they are not great other than
they do have steep skirts.

The R390 used IF transformers (tuned circuits) while
the R390A had mechanical filters.
The R390 was judged to work better, and the R390A was actually
designed to reduce cost through the use of the mechanical
filters.

Although it is meaningless in an 8 MHz IF radio, I think at
50, 100, 262, or 455 KHz, tuned circuits may be best, but you
likely need to design the circuit parts very carefully, and they
might take up a lot of space, and be hard to adjust.

Brett
N2DTS

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of David Cutter
> Sent: Monday, May 07, 2007 1:43 PM
> To: Elecraft
> Subject: [Elecraft] OT mechanical v crystal filters OT
>
> Off Topic but with so much expertise flying around I thought
> I would ask how mechanical filters compare with crystal filters.  
>
> I think they were fairly low in frequency (455kHz?), so, not
> suitable in the present discussion,  but it would be good for
> our education at least.
>
> David
> G3UNA
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>

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[John Magliacane]

> Off Topic but with so much expertise flying around I thought I would ask how
> mechanical filters compare with crystal filters.  
>
> I think they were fairly low in frequency (455kHz?), so, not suitable in the
> present discussion,  but it would be good for our education at least.
>
> David
> G3UNA

Hi David.

Mechanical filters, which operate in an acoustical domain, may have somewhat
higher insertion loss and lower dynamic range than properly designed crystal
filters.

However, I've heard some glowing on-air comments regarding the performance of
the old Collins mechanical filters, especially with respect to CW reception.

Well-designed direct conversion receivers are often noted for their superior
audio quality compared to highly filtered superhets.

Recently, I've designed a highly filtered (100 Hz bandwidth) quadrature phasing
direct conversion receiver, and was somewhat amazed by the clarity of CW
reception.

>From these observations I have concluded the following:

Narrow-band reception requires sharp selectivity (obviously).  The higher the
frequency at which the selectivity takes place, the higher the required 'Q'.
The higher the 'Q', the greater the amount of ringing and group delay
distortion that will occur.  Ringing and group delay distortion cause listener
fatigue.

In a direct conversion receiver, a 100 Hz bandwidth can be obtained at 1000 Hz
with a 'Q' of only 10. (Very low ringing)

A 100 Hz bandwidth at an IF of 455 kHz, however, requires a 'Q' of 4550. (More
ringing)

A 100 Hz bandwidth at an IF of 5 MHz requires a 'Q' of 50,000.  (An RF "echo
chamber"!)

My K2/100 experiences have lead me to believe that these effects occur with HF
SSB crystal filters as well, but more so at the passband edges rather than in
the center, provided it is essentially flat.  The K2's filter shape is slightly
asymmetric, so the filter edge with the sharpest cut-off is the one where the
greatest ringing occurs.

In order to minimize the effects filter ringing can have on SSB audio quality,
I have found it advantageous to attenuate the audio frequencies that strike the
corners of the sideband filter (prior to the balanced modulator), especially
when the filter operates at HF (rather than lower) frequencies because the 'Q'
is so high at these points.

(This isn't so much of an issue in reception because SSB signals have already
been bandwidth limited in the other person's transmitter.)

Just some greatly over-simplified and otherwise totally random thoughts on the
subject...


73, de John, KD2BD


Visit John on the Web at:

        http://kd2bd.ham.org/
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[Brett gazdzinski-2]

John,
Please describe the quadrature phasing part of your receiver.

On weak signals, I often find narrower filter settings of the K2
make the signal harder to copy.
Part of that might be my bad hearing and the higher pitch
sidetone I like...
The noise and the signal seem to blend into one pitch...

I suppose a receiver with a 50 KHz or 100 KHz IF
frequency would work quite well in regards to ringing.......

 
> Mechanical filters, which operate in an acoustical domain,
> may have somewhat
> higher insertion loss and lower dynamic range than properly
> designed crystal
> filters.

They operate at 455 KHz, I don't think even dogs can hear that high!

>
> However, I've heard some glowing on-air comments regarding
> the performance of
> the old Collins mechanical filters, especially with respect
> to CW reception.
>
> Well-designed direct conversion receivers are often noted for
> their superior
> audio quality compared to highly filtered superhets.
>
> Recently, I've designed a highly filtered (100 Hz bandwidth)
> quadrature phasing
> direct conversion receiver, and was somewhat amazed by the
> clarity of CW
> reception.

Was not the old heathkit hw7 a direct conversion receiver?
I had a lot of fun with that rig...

>
> >From these observations I have concluded the following:
>
> Narrow-band reception requires sharp selectivity (obviously).
>  The higher the
> frequency at which the selectivity takes place, the higher
> the required 'Q'.
> The higher the 'Q', the greater the amount of ringing and group delay
> distortion that will occur.  Ringing and group delay
> distortion cause listener
> fatigue.
>
> In a direct conversion receiver, a 100 Hz bandwidth can be
> obtained at 1000 Hz
> with a 'Q' of only 10. (Very low ringing)
>
> A 100 Hz bandwidth at an IF of 455 kHz, however, requires a
> 'Q' of 4550. (More
> ringing)
>
> A 100 Hz bandwidth at an IF of 5 MHz requires a 'Q' of
> 50,000.  (An RF "echo
> chamber"!)

Brett
N2DTS

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[Ron D'Eau Claire-2]

Brett wrote:
On weak signals, I often find narrower filter settings of the K2 make the
signal harder to copy... The noise and the signal seem to blend into one
pitch...

-----------------------------

Like antennas, filters are a perennial subject of interest to many Hams,
including me. I've learned a lot just reading the mail here on the Elecraft
reflector over the years, especially about the subjective experiences of
Hams trying to hear a signal and what it takes to allow them to copy.

My interest in filters hasn't been so much about the various parameters that
are measured in the lab, but more about how well I can hear and copy signals
under various conditions. I'm not sure the two are as directly related as
many hope!

Like Brett, I've never much liked a narrow CW filter. I almost never use
less than a 500 Hz filter and generally stay with a filter at 1 kHz or wider
for CW copy. One's first reaction might be, "Oh no! That's backwards! A
wider filter lets more noise power through so the signal-to-noise ratio is
worse with a wide filter."

Measuring the audio voltage at the output of a receiver using a wide filter
might well prove that out: the total audio power my be much the same when
the distant transmitter key is down as it is with the key up. That is, the
S/N ratio might be very poor.

It's not only about power levels. Frequency plays a huge role in determining
how easily we can copy a signal. In this case, the signal we want to copy
occupies essentially one frequency in the audio spectrum while the noise
occupies all the frequencies allowed through by the filter in use. Our
brains can be extremely adept at focusing on the one keyed tone while
ignoring the wide spectrum noise, even when the signal is somewhat weaker
than the noise itself.

Something bad happens when we use a narrow filter to isolate the signal. As
we center a weak signal in the bandpass, the noise frequencies off to the
sides of the signal frequency are attenuated. That's the job of the filter!
The more narrow the filter, the fewer range of frequencies we hear until, at
the extreme, both the noise and the keyed signal have essentially the same
frequency. By eliminating other frequencies, the filer stretches out the
noise pulses at the filter's frequency. At the extreme we call it "ringing"
because a pulse of noise will produce a rather distinct and long tone just
like striking a bell makes it ring. Even if a filter avoids this extreme
phenomena, the pulse stretching always occurs. It's what a filter does. As
that stretching happens we find ourselves listening to what is essentially a
tone right on the same frequency as the signal we're trying to copy.

Of course, as we make the filter passband narrower the total energy in the
noise is reduced as well. That should make the signal to noise ratio better,
even though they are both producing essentially the same audio frequency in
the speaker or phones.

There seems to be some ratio that is easiest to copy between isolating only
one frequency, and having the noise and signal producing the same audio
frequency in the speaker or 'phones, and not isolating one frequency so that
the operator hears noise covering a wide range of audio frequencies
surrounding the signal on a single frequency. To say it another way, some
people seem to be able to copy a signal better when there's a steady weak
signal (the noise) directly on its frequency and others can copy a weak
signal buried in broadband noise. For most of us there seems to be some
optimum ration that works best to copy a really weak signal. Maybe that
ratio varies a lot with individuals. It seems to do with how our brains
process audio information.

There's another reason for filters. The most common reason cited for using
filters is to eliminate QRM from other signals, not noise. Comments here
indicate that's a requirement that varies widely from one operator to
another. Some can focus on one signal in a passband filled with signals
while others are distracted and unable to copy if only two signals are
present simultaneously. I suspect that I'm pretty typical: I can copy one
signal among several in the passband, but not if one of the unwanted signals
is very, very loud. To me, that's like listening to someone talk in a
crowded party. The other voices are just babble that I can "tune out" unless
someone starts shouting in my ear! If that happens I have to do something to
cut down his "signal strength" if I'm going to hear anything but the
"loudmouth".

When I switch to a narrow filter it's usually for that reason. I need to
attenuate a very loud signal nearby. Often that works, but the ideal
solution for me is to use a notch filter that attenuates one narrow band of
frequencies. Placing the notch on the annoying signal removes - or severely
attenuates it - thereby preserving the desirable broadband characteristic of
the background noise. Of course, in the middle of a contest for a DX pileup,
one might need a basketful of notch filters. That's the tradeoff.

After all, it'd take all the fun out of it if we could copy every signal
from every where under every condition <G>.

Ron AC7AC

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[John Magliacane]

Hi Brett.

> John,
> Please describe the quadrature phasing part of your receiver.

In general terms, the design is very similar to quadrature phasing receivers
used for SSB operation, except that since the bandwidth in this case is very
narrow, the 90 degree audio phase shifting network becomes ridiculously simple
(a single all-pass filter).

The receiver uses a pair of switching mixers (I and Q), an L.O. network with 0
and 90 degree outputs, a 90 degree audio phase shifter in one of the audio
channels (I or Q), a summing network, and three stages of active audio
filtering/amplification overall.

The phasing circuitry is necessary to produce "single-signal reception" (no
annoying image response on the other side of zero beat).

Tuning is a bit critical as it is possible to tune across a CW signal in
between the "dits" and "dahs", and not even know a signal was there.

> On weak signals, I often find narrower filter settings of the K2
> make the signal harder to copy.
> Part of that might be my bad hearing and the higher pitch
> sidetone I like...
> The noise and the signal seem to blend into one pitch...

I agree.  In narrow bandwidths, the noise-induced ringing in an HF crystal
filter can almost sound like some sort of narrow shift FSK!

> > Mechanical filters, which operate in an acoustical domain,
> > may have somewhat
> > higher insertion loss and lower dynamic range than properly
> > designed crystal
> > filters.
>
> They operate at 455 KHz, I don't think even dogs can hear that high!

Your TV set uses a Surface Acoustical Wave (SAW) IF bandpass filter that
operates at 45 MHz, where the acoustic energy traverses along a lithium niobate
substrate.  The insertion loss is fairly high, but the group delay distortion
is significantly lower than a comparable filter using LC components.

> Was not the old heathkit hw7 a direct conversion receiver?
> I had a lot of fun with that rig...

I believe the HW-7 was direct conversion...


73, de John, KD2BD


Visit John on the Web at:

        http://kd2bd.ham.org/
.
.
.
.


 
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