EMRFD Message Archive 10629
Message Date From Subject 10629 2014-12-18 09:20:54 zx97lite January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF
Hi Folks,The January 2015 issue of QST has an interesting article entitled "Hybrid HF Transmitter". The design uses a 455 kHz IF. It states the design is SSB.Here is the same basic design that the author posted on the Internet:In this version he says it's a DSB architecture (page 2):"the transmitter operates in a DSB mode"The Jan QST block diagram also shows the receiver which uses a SA605 and a SA602 for a product detector, again with an IF of 455 kHz.By using a single conversion architecture with a low IF frequency like 455 kHz one would think any interfering image would be received almost as well as the desired frequency.Sure does make for a very simple SSB transceiver though!Would appreciate anyones thoughts on this.Thanks!
10630 2014-12-18 11:22:49 Kirk Kleinschmidt Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF That design really raised an eyebrow for me, too. I don't want to pick on the author, cause I respect any and all builders. Seeing this in QST is what seems weird to me.1. Low IF, as mentioned, which raises the question of images, etc.2. Lotsa effort to build a radio based on low dynamic range RX parts.3. LOTSA extra effort to use tubes in the TX final -- for a power level of only 30 W PEP. This power level is much easier to obtain with a pair of 99-cent FETs...no high-voltage supporting hardware required.There are other oddities, such as the clipped, terse, editing. You can always tell when the editors don't really have room for stuff, but they cram it in anyway, forcing the rest onto the "in depth" website.I could go on :) But the radio seems to highlight the least-desirable qualities of each respective "hybrid" : Low-dynamic range RX with a questionable IF + the use of tubes (complexity, cost, high voltage, etc) at a power level where tubes seem a bit out of place. I don't have any beef with tubes, having used them on many occasions. They just seemed really out of place with this design. If you happen to have them on hand, might as well use 'em. But...Lotta work for little payback in terms of performance and usability...plus a very questionable use of precious space in a membership journal :)One of the other construction projects in the same issue is for a dc to daylight linear amplifier with a parts cost of $5,000! That means that nobody will ever likely build it...but at least the article references useful pieces and parts and shows how the designer combined separate amplifier modules (HF and 2 meters) in one chassis.Okay. Raised eyebrow coming down...73,--Kirk, NT0ZMy book, "Stealth Amateur Radio," is now available from
www.stealthamateur.com and on the Amazon Kindle (soon)
10631 2014-12-18 11:49:05 Dana Myers Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF 10632 2014-12-18 12:09:28 John Marshall Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF In reference to the receiver description and schematic from the web, the image rejection is certainly inadequate. When receiving on 14.200 MHz, the image is at 13.290 and the frequency response graph, Figure 3 (b), shows that this response will only be down about 7 dB. The RF image problem also applies to the transmitter shown in QST.
I don't think the low IF rules out use at 14 MHz, it just demands a tighter filter. It would be an interesting challenge to design one.
Referring again to the web article, if you thought the receiver had an image problem, check out the transmitter. It has no filtering at all following the mixer. It needs an image-stripping filter and a harmonic filter at the output.
10633 2014-12-18 12:39:07 blumu Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF ----- Original Message ----- From: Dana Myers
...it's the homebrew equivalent of porn, I suppose.
Brilliant lateral thinking, hopefully not identified by
our "guardian angels" :-)
We surely all benefit from designs of all progenity and
in constructive spirit go forth to multiply many more.
Many of us are active, others might be impeded,
but enjoy the exchange and inspiration altruistically.
I look forward to further constructive comments on
this design and its potential outgrowths. My
initial thought was similar : alternative ICs allowing
use of a higher IF.
10635 2014-12-18 14:02:55 zx97lite Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF Hi Kirk,
Thanks for your response. I have to admit I was very leery to post my thoughts as I did not want to appear as casting dispersions for the author. Yosef 4Z1VC is the Dean of the Engineering School at Ariel University. These are impressive credentials that I have much respect for so I defer to his best judgment..
I like you see no reason to make a version of his design with tubes for all the reasons you listed (but if you have the parts it could add "personality" to the rig :-). In fact Prof Pinhasi has made several version using solid state devices for the Driver & Final.
I'm no expert but from what I can tell the published rig is pretty darn close to being a DSBSC (Double Sideband Suppress Carrier) transmitter (which is fine as long as folks understand it limitations). The simple receiver really lacks opposite sideband suppression (again, which is fine knowing the limitations). I keep thinking that I'm missing something here so if I'm incorrect folks *please* let me know :-)
I guess what I'm trying to say is I've been scratching my head as why one would go to all this effort, when with some modest changes it could become a very worthwhile project.
I'm sort of flummoxed,
10636 2014-12-18 14:22:00 blumu Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF My assumption was that this design was intended for
lateral thinking. I kinda like the ATLAS 210X or FT101
style of modular designs. A few of us don't have
the full array of instrumentation...
My first, impulsive thought was : I like basic,
reproducible designs. Could be re-sketched for
newer chips allowing for a higher IF, but KISS ...
----- Original Message -----
I guess what I'm trying to say is I've been scratching my head as
why one would go to all this effort, when with some modest changes
it could become a very worthwhile project.
10639 2014-12-18 17:24:54 Dennis Czelusniak Re: January 2015 QST "Hybrid HF Transmitter" Using a 455 kHz IF Looks like an exercise for a student. Fast easy construction covering many bases, not intended for anything more than a hands on project.