EMRFD Message Archive 6562
Message Date From Subject 6562 2011-08-22 16:13:43 ka3j12 Shielding in SSB Xmtrs I've read some comments indicating that interstage shielding is more critical in SSB xmtrs relative to CW xmtrs. I don't recall seeing this addressed in EMRFD and I'm not totally sure why this would be the case if adequate bypassing and suitable grounding and other good layout practices are used. Also, I'm aware of some rigs (HF handhelds) that appear to have little or no shielding. Can someone shed some light and give their views on this matter? Thanks
6564 2011-08-23 06:15:41 Pete N6QW Re: Shielding in SSB Xmtrs Hi Ron,
I did not see any responses to your inquiry and my response is more of how to do it versus a detailed rigorous tecnical dissertation on why it must be done. I recently completed a QRP SSB Transceiver that was built in modules housed in homebrew copper boxes. With but few exceptions all interfaces are at 50 ohms and the shielded modules give you great flexibility in the how the circuits are arranged in the final form. The module approach also enables swapping modules as new circuits are tried out and needless to say any troubleshooting can be done at the module level.
You can see a youtube of the project here:
I have prepared a tutorial on how to build the copper boxes used in the above project and it can be seen here:
A frequent contributor to this forum Allison KB1GMX shared an idea with me that she used in her design and construction of a 6 Meter QRP SSB XCVR. She used a skeletal framework made from Copper PC Board and within that framework were isolated compartments to house the various circuits. I used that same approach in a pocket sized QRP 20M SSB XCVR that is 2X4X2 inches. Were it not for the shielded compartmnets, I am certain that circuit interaction would be a problem.
Both these projects used many of the circuits found in EMRFD. The most recent one that can be seen in the video uses the HyCas IF/AGC module and I have one word to describe its performance --Stunning!
To repeat, my answer is more of sharing ideas on how it can be done versus the orignal questi
6565 2011-08-23 06:46:48 Tim Re: Shielding in SSB Xmtrs I don't think it's so much CW vs SSB transmitters.
I think it's more "straight through" vs "hetrodyne" vs "multiplier" frequency/signal generation schemes, and the amount of gain done at any one frequency. These issues are discussed in EMRFD, e.g. Fig 6.18/6.19 and accompanying text. And while pages 8.9-8.11 are discussing shielding and gain distribution for direct conversion receivers, the principles work exactly the same for transmitters.
The most sensitive-to-lack-of-interstage-shielding transmitters are CW ones where the oscillator is on the same frequency as the final transmitted signal. What happens is that the transmitted signal (watts to hundreds of watts) leaks back into the oscillator which has a tank on the same (or nearly the same before pulling) frequency. The phase shift between oscillator and antenna adds on to the phase shift in the "standalone" oscillator and pulls the frequency resulting in chirp.
Similar in sensitive-to-lack-of-interstage-shielding transmitters are SSB ones where the last mixer is operating at the microwatt level and there's a lot of gain between the mixer and PA output all done at the same frequency.
The least sensitive-to-lack-of-interstage-shielding transmitters are ones where the gain is distributed such that gain and the profound buffering effect (quoting chapter 6) of frequency multiplicati
6568 2011-08-23 13:23:09 kb1gmx Re: Shielding in SSB Xmtrs 6575 2011-08-23 19:36:40 Ashhar Farhan Re: Shielding in SSB Xmtrs I've found that if one used air coils, it was almost mandatory to
shield them. For instance, the bfo would leak over the modulator and
reduce the carrier suppression to about 20db.
As all our current day work is at low impedances, accidental coupling
between various stages of a tx is less of a problem.
On 8/24/11, kb1gmx <firstname.lastname@example.org> wrote: