[TenTec] proper caps for Omni V?--> Sanyo Oscon

[Dr. Gerald N. Johnson]

On Wed, 2008-03-05 at 19:30 -0500, Joe Roberts wrote:
> I wrote
> >>
> >> Replacing the stock 1st IF sideband filter with the 2.7 INRAD is also a 
> >> fabulous improvement which goes beyond mere bandwidth considerations. I 
> >> have never seem this adequately explained, but the ears don't lie (and 
> >> even if they do, how can you tell?). Others have commented on this 
> >> effect, so it is not just me.
> 
> to which Jerry responded...
> 
> > You may like that added bandwidth out to 3 KHz audio but I prefer to
> > roll it off at about 2100 Hz with the DSP in my receiver. I find copy
> > of voice signals is more comfortable with the narrower bandwidth.
> > Perhaps that comes from years of using a Collins with a 2.1 KHz filter
> > bandwidth.
> 
> Jerry:
> 
> I believe there is more than bandwidth going on here, given that I also 
> tried the 2.1 INRAD 1st IF filter and that was "cleaner" sounding than 
> the stock TT 2.4. Better definition and clarity with either INRAD over 
> the TT stock job.
> 
> What I think I was hearing with the stock filter(no measurements to 
> confirm) is an anomaly of some nature WITHIN the passband. As I put it 
> above, it was a combination of "edgy" and "muffled". Whether phase 
> anomaly, ringing, or amplitude variation, I can't say.

I think little ringing, and not so much phase anomaly, but could be
amplitude variation emphasizing what comes out as an audio harmonic of
the voice fundamental. The transition from slope to passband is not
rapid in a Tentec filter, but a gentle amplitude transition usually
means a lot less phase anomaly in that transition region.

Its generally considered that our ears are phase sensitive between them
(how we pick direction of an incoming sound) but not sensitive to phase
at different frequencies. So in a linear system we hear the same sound
when the odd harmonics are phased to make a square wave as when they
have the same amplitudes and are phased to make a triangular wave.
Amplifier circuits may pass the square wave unchanged and run into peak
clipping on the triangular wave with the same spectrum and same power.
And I'm pretty sure ham rig filters can change phase that much between
fundamentals and harmonics. Audio stages do and microphones do as do
voices. And sometimes the set of voice, microphone, audio stages, and
transmit SSB filter can have a peaky output or a smooth output depending
on the details of each. And the system and user with the smooth output
will show more average power than the one with the peaky output that
requires lowering the drive to keep from clipping the peaks. I've tried
to select microphones on that basis, but not everyone on VHF (especially
FM with a hand using a 3/4" diameter speaker) likes the extended bass
response of the EV-664. This can happen on receive too since the RF
envelope of SSB voice is not the audio envelope.

The ladder filters used by Tentec are among the easiest to have a smooth
phase response, though some emphasis on steeper skirts can roughen the
phase response. Years ago (probably more than 30) I was working up a
panadapter modification to add a lower frequency IF with more
selectivity in the form of a 4 or 5 crystal ladder filter. If I used
nearly equal value shunt capacitors I got a fairly steep skirted
amplitude response but I had to slow down my sweep generator speed to
keep it from ringing. When I designed it for a Gaussian amplitude and
the associated quite linear phase response, the skirts were very flat,
not steep at all, but I could sweep it 5 times faster than the equal
capacitor filter.

Having used the Collins F455FA21 filter for years, its response has
steep skirts and close in notches. And it rings like a bell. Power line
noise goes in as pulses at 120 per second and comes out as a continuous
unmodulated noise envelope. Lattice crystal filter synthesis may have
improved over the years, but lattice filters have been the products that
INRAD has made for other makers. So it worries me about their phase
response and ringing compared to the Tentec filters. I know the Tentec
filters in my Corsair don't ring from power line noise or from summer
lightning static on 75 meters nearly as much as the Collins filter did.
> 
> Since most SSB signals are less than 2.7 kc wide, one would assume that 
> passband differences between 2.4 and 2.7 filter on an in-the-clear 
> signal would be moot, no?

Yes, but in my world there is QRM and noise and I find signals easier to
listen to when I remove the noise above 2.1 KHz.
> 
> Also, 300Hz wider really isn't *that* big of a difference. I think I 
> have a grip on what that magnitude of change in selectivity should 
> yield. There seems to be something else afoot here.

It holds more than 10% of the bandwidth and so more than 10% of the
noise power. And at audio all of that added noise power is at high
frequencies that add little (if it was all voice harmonics) to ease of
copy but add much irritation.
> 
> 
> The strange thing is that the same specimen of TT 2.4 filter sounds "OK" 
> in other radios. I have never figured this out.

Back to that combination of phase responses of filters, audio stages,
and I should add speakers and headphones. And the filter effects are
modified by where on the filter slope the carrier frequency is set.
Might be that moving the carrier frequency 50 Hz one way or the other
would invert your favorite filter selection.
> 
> Maybe I'm hallucinating but if so I have switched back and forth between 
> the 2.4 and 2.7 in the Omni, weeks apart, and hallucinated in a similar 
> manner every time!
> 
> I have read similar comments from other Omni V users on this list over 
> the years, so maybe this is a mass hallucination!! ;o)
> 
> In any case, I am a fan of the wide INRAD filter. The 2.1 is real good too.
> 
> I eventually put the 2.1 INRAD in my Paragon II NAR position, which 
> indeed sounds remarkably like a Collins with that filter installed. I 
> used 75A4 and S-Line gear for many years. The way that sigs jump out 
> with a good 2.1 filter when tuning around is a blast from the past.

Separating signals from noise and QRM is the whole purpose of receiver
filtering.

And I think (once the ringing of the 2.1 is cured) that bandwidth is a
great way to listen and copy signal with not so great a S/N. One
solution I used to use (before the Corsair II) on 75 was a crystal
converter and a Q-5er (BC-453) with the IF coils set for minimum
coupling and that gave a Gaussian amplitude response that merely clicked
for lightning, it didn't go craaasssssh like the Collins filter did.
Then I dug in and put in the 75S-3B dual time constant (fast attack,
slow decay) AGC filter and it was a fine 75 meter receiver.
> 
> 73 Joe N5KAT

Now on FM I take out the audio DSP 2.1 KHz low pass, because it makes
the voices sound muddy or muffled to me. Same receiver, same speaker.
But the S/N is better due to the quieting effect of FM.

73, Jerry, K0CQ