EMRFD Message Archive 3977

Message Date From Subject
3977 2010-01-14 08:56:36 Pete N6QW Hybrid (Filter + Phasing) Transceivers
First the dis-claimer I have only had my copy of EMRFD for about two weeks and have mostly skimmed cicruit diagrams and read bits and pieces here and there.

In looking at Chapter 9, specifically figure 9-20, would it be possible to use 4.9152 MHz as the crystal frequency and in lieu of the tuned antenna network run that output/input through a homebrew 4 pole Crystal filter with a Cf of 4.9152 Mhz and 2.5 KHz BW. (With appropriate matching to the filter.) From there I would use conventional circuits commonly used with filter type single conversion transceivers for signal conversion and transmit chain.

Any minefeilds with such an approach?

Pete N6QW
3978 2010-01-14 16:59:24 KK7B Re: Hybrid (Filter + Phasing) Transceivers
Hi Pete,

There is a very subtle mine field, involving the rapidly changing impedances across the passband and stopband of a crystal filter. Please read the text on pages 9.9 and 9.10 and study circuits 9.10 through 9.13 for a discussion of this topic.

Hybrid techniques are excellent, but it is important to have some broadband isolati
3979 2010-01-14 18:08:21 Pete N6QW Re: Hybrid (Filter + Phasing) Transceivers
Hi Rick,

Thanks for the info and I will read and review the sections you suggested.
Pete N6QW

3980 2010-01-14 18:43:02 ajparent1 Re: Hybrid (Filter + Phasing) Transceivers
The pay off is if you attend to the impedance needs (buffering and gain) you get a great radio. I have done this using miniR2/T2
for a 6M down converting receiver with high IF (36mhz).

The filter adds about 20db to the opposite sideband rejection
both RX and TX and the single conversion makes for a clean
transceiver (no audible spurs).


3981 2010-01-14 21:40:38 KK7B Re: Hybrid (Filter + Phasing) Transceivers
Another subtle detail is that you can use phasing and audio filtering for very steep skirts, and design the crystal filter for a bit wider bandwidth and graceful impulse response. The combination is quite amazing--I expect that's the way Allison's rig works.

This is an old technique. ARRL handbooks were suggesting it when I was in grade school. As early as 1950 there were phasing systems added to the tail end of narrow 455 kc IF systems, and the technique was covered in mathematical and practical detail in the IRE special issue on SSB--December 1956 if I remember correctly.

Those of us who play with phasing systems are constantly humbled by the accomplishments of radio engineers half a century ago, before IC Op-Amps or any of the computer simulati
3983 2010-01-15 03:47:12 Vojtech Re: Hybrid (Filter + Phasing) Transceivers
> Another subtle detail is that you can use phasing and audio filtering for very steep skirts, and design the crystal filter for a bit wider bandwidth and graceful impulse response. The combination is quite amazing--I expect that's the way Allison's rig works.

I believe both Elecraft K3 and Eton E1XM are using this trick.

73, Vojtech OK1IAK
3985 2010-01-15 07:39:47 Robert Cerreto Re: Hybrid (Filter + Phasing) Transceivers
If you really want to blow your mind, read about the research that the Bell System (Labs) did over the years before "electronics". What they accomplished in the area of telephone transmission without electronics is equally amazing.
When I started in Telephony 45 years ago I had a head full of solid state stuff and just poo pooed the "antiquated" Bell System stuff. Boy did I get an education after really dug into their work''!
73, Bob WA!FXT

3989 2010-01-15 17:15:34 ajparent1 Re: Hybrid (Filter + Phasing) Transceivers

3990 2010-01-15 21:20:57 KK7B Re: Hybrid (Filter + Phasing) Transceivers
Yes, amazing stuff. The Labs were formed in New York City in 1925, the pre-electronics era was before that. Transmission line equations predate Maxwell's equations, and the entire telephone system predates electronic amplification, so there was a lot of fundamental work before Marconi et. al. I have books from that era that I use as modern references
3993 2010-01-16 07:34:25 aa7hq@comcast.net Re: Hybrid (Filter + Phasing) Transceivers
how is the convolution of a "graceful impulse response" different from one more gauche?

[Non-text portions of this message have been removed]
3996 2010-01-16 09:30:13 ajparent1 Re: Hybrid (Filter + Phasing) Transceivers
3998 2010-01-16 19:19:41 Tayloe Dan-P26412 Re: Hybrid (Filter + Phasing) Transceivers
This is something engineering does not teach us. Our "grey matter" does
not seem to like the sound of signals with sudden, sharp changes in
phase or delay variation. If you look at the phase changes or the delay
variation near the edge of a sharp cut off filter, there can be a very
large, abrupt spike.

I remember in the dark ages of my youth a HiFi amplifier company called
"phase linear". I think they understood this concept well.

I think it is possible to get the best of both worlds (step cutoff,
better sounding audio with gentler phase response) by ganging two
filters in series. The gentler filter can be earlier in the gain chain,
with the steep one being later on. If the cutoff the the steep filter
is place somewhat down the skirts of the gentle filter (-10 dB?), I
think listenability can be greatly improved while still having the steep
roll off.

I tried to do this with the NC2030 design which used a more gradual 7
pole Q=3 max R/C active filter early on as the receiver "brick wall",
and a sharp SCAF at the end of the audio chain as a comfort filter to
completely remove residual energy that could still be heard on the
skirts of the R/C active filter. The SCAF was normally offset so that
its breakpoint was a bit higher than that of the active R/C filter. The
folks that have heard the resulting audio loved the sound.

- Dan, N7VE


4000 2010-01-17 12:07:48 victor Re: Hybrid (Filter + Phasing) Transceivers

The equivalent filter to the scenario that you describe is "Gaussian to 6dB" or "Gaussian to 12 dB".
You can find them and their design parameter in the filters "Bible": Zverev.
Many years ago I designed a FM receiver for a large company, and we used such filters to get good (lower) group delay variation (which means that it is linear phase) for achieving better "eye pattern" when receiving data.
Using two filters as you suggest will expose the amplifiers between the filters to higher bandwidth and they will generate more distortion products than when using one filter.

Victor - 4Z4ME